This question is referring, specifically, to the “15 degree calorie” which has a value of 4.1855J where 1J is the SI unit for a unit of energy.

A 15 degree calorie is defined as:

The amount of energy required to warm one gram of air-free water from 14.5 to 15.5 °C at standard atmospheric pressure.

[1]

The definition is as it is, precisely because at different temperatures and pressures the amount of energy required to heat 1 g of water by 1 degree changes (it, in fact, reduces) per degree of increased temperature.

This is an empirical fact that was determined by experiment. Given the empirical truth of this fact, it is necessary to specify the reference temperature when discussing the number of calories required to heat a mass of water since the number varies according to how hot the water is.

A thermochemical calorie is exactly 4.184J which is .0015J less than a “15 degree calorie.”. The value of a thermochemical calorie is specified by direct reference to the SI units and not in relation to a mass of water. This ratio is absolute and does not depend on a medium, such as water, whose properties vary according to temperature.

Footnotes

No, I don’t think that any other number would be just as good. I admit up front that I don’t know a better answer for certain; but I can make a guess that you can verify.

What you didn’t say (exactly) is that the definition of the calorie is the amount of heat transferred when a gram of water changes temperature by 1deg. K. Note, a change in temperature in degrees C is the same as a change of temperature in degrees K; but the latter is consistent with all other uses of T in thermodynamics applications. It’s not just some kind of coincidence that one calorie, defined otherwise, happens to be the heat change between 14.5 & 15.5 K.

Because defining the unit of heat is a concept central to the thermodynamics of heat exchange, it’s important that it shall be easy to measure it precisely. The problem with doing this measurement is that the heat capacity of any substance varies with temperature. At super-cryogenic temperatures, heat capacity varies only slightly with temperature, but it is not precisely constant. Consequently, the magnitude of the change in heat accompanying a 1K change of temperature in water will vary with temperature. In other words, the very definition of the calorie would depend on the temperature at which it is measured. Besides this conceptual imprecision, there is the practical problem of how one goes about measuring a calorie precisely. To wit, an error in measuring the temperature at which the heat change is measured will propagate through to an error in measuring the heat change that depends on it.

And so, it behooves scientists to define the calorie over a temperature range in which the heat capacity is least sensitive to temperature. And that would be within a temperature interval that contains a relative maximum in the heat capacity, i.e. where [math]\frac{\Delta C}{\Delta T} \overset {\sim }{=} 0[/math]. This is also true at T=0, where C is at a minimum (0), but we can’t do measurements there. There must be a reference to the dependence of C on T somewhere. Find it, and see how C depends on T over a range of temperatures.

Calorie is simply energy required to increase the temp. Of water by 1°c at standard atmosphere which is approximately 4.185 but there are may definations of calorific value

1)the amount of energy equal to exactly 4.184 joules = thermo chemical calorie

2)the amount of energy required to warm one gram of air-free water from 3.5 to 4.5 °C at standard atmospheric pressure.=4°c calorie

3)the amount of energy required to warm one gram of air-free water from 14.5 to 15.5 °C at standard atmospheric pressure. Experimental values of this calorie ranged from 4.1852 to 4.1858 J. = 15°c calorie.

So, it's standard value which is obtained from experiments.there are several other definitions.

I think you are asking why did they select THAT range of temperature, not why you need to specify any specific temperature range in defining the standard for a calorie. Of course, the heat capacity of water, or any material, changes with temperature, so to define a unit of energy based upon the temperature change of a substance has to reference a specific temperature range. But in the case of the calorie definition why not use so-called Standard Temperature (of STP) of 0C? The problem is, besides the fact that this is the freezing point, that water has anomalous thermal expansion coefficient behavior close to 0C, curved at low temperature and actually negative between 0 and 4C. If you look at the expansion curve of water it becomes a linear function with temperature at a little below the range of 14.5–15.5. One does not want a standard based upon a material’s behavior to be confounded by the anomalous non-linear behavior of that substance.

The energy needed to raise the temperature of a substance by one degree varies slightly based on the starting temperature, so if you’re going to define something in terms of a physical change (like heating water), you need a known starting point.

In the past, there were several definitions of the (small) calorie, one of the most popular ones was the “15 degree calorie”, which, as asked, was the energy required to heat one gram of (air-free) water from 14.5C to 15.5C. About 4.1855J. There was also the 4 degree calorie (4.204J), the 20 degree (4.182J), the mean calorie (1/100th of the energy require to heat a gram of water from 0C to 100C - 4.190J), plus a few more. There were also several definitions not based on heating water.

So why 4, 15 or 20 degrees C? Presumably those were convenient for someone at some point, and they caught on.

These days, if calories are used at all, they’re usually just assumed to be the thermochemical calorie (4.184J exactly, which is actually pretty close to the 15 degree calorie), except in some specialized applications.

But in general just say no. Use Joules, and nothing else.

Celsius did not invent or defined a calorie.
Celsius invented a temperature scale.
A scale where water boils at 0 degrees and freezes at 1 degree. Then he used centigrades (of that degree) to get usable units. (centi=1/100)
After his death we made a new scale - based on the centigrade scale: But inverted so that the freezing point are zero and the boiling point: 100. And as a tribute to the man who invented the centigrade scale we named it Celsius. (missed in some languages thou)
The later definition of a calorie needed a temperature span. For a long time it was set as just one degree C. Any degree.
But we found out that it takes different amounts of energy depending on pressure and initial temperature of the water. And now I am taking a guess here: When this happened there where allready other, better defined units of energy and conversion formulas between all of them and calories. So that gave us 14.5 to 15.5.

How are calories in food calculated? How is that number figured out?

They burn it in a device that measures how much heat was generated by burning it. The device, a “bomb calorimeter”, is a tank of water with a smaller chamber (the “bomb” in “bomb calorimeter) in it. They put the stuff they want to measure in the bomb, pump in oxygen, and set the stuff on fire. They measure the temperature change in the water, and that corresponds to the number of calories in the stuff in the bomb. A “calorie”, as far as food is concerned, is the amount of heat needed to raise the temperature of 1 kilogram of water 1 degree Celsius (specifically, if I recall correctly, from 14.5 degrees to 15.5 degrees, when that level of precision is needed).

So if you need to know how many calories are in an ice cream sandwich, or in a bowl of whole wheat cereal, you dump it into a bomb calorimeter, burn it up, and measure the temperature change.

Or at least, that’s the basis for how they calculate it.

Scientists studying how the body works discovered that the digestive system can’t absorb everything that you might throw at it. In fact, it works by breaking the components of food down into small molecules, and then actively absorbing the small molecules it wants. If it can’t break down a large molecule, or a small molecule isn’t what it’s looking for, it is much less likely to be absorbed into the body.

As such, while starch and cellulose are both polymers of glucose, because we can’t break down cellulose into glucose, we can’t absorb it. We can’t get the energy from burning it. While oil and Olestra(tm) are chemically very similar (oil is a tri-ester of fatty acids and glycerol, Olestra is an octo-ester of fatty acids and sucrose), we don’t have enzymes that will split the fatty acids from the sucrose. We can’t break it down, we can’t absorb it, we can’t get the energy from burning it.

Put 50g of starch into a bomb calorimeter, and it’ll tell you it has (about) 200 calories. Put 50g of cellulose into a bomb calorimeter, and it’ll tell you it has (about) 200 calories. In practical terms, the cellulose has 0 calories as far as the body is concerned. Put 50g of vegetable oil into a bomb calorimeter, and it’ll tell you it has (about) 450 calories. Put 50g of Olestra(tm) into a bomb calorimeter, and it’ll tell you it has (about) 450 calories. In practical terms, the cellulose has 0 calories as far as the body is concerned.

Because of this, it is inaccurate to dump an ice cream sundae or a bowl of cereal into a bomb calorimeter. The sundae isn’t too bad, but the cereal naturally contains cellulose and would read too high. The water in both make it hard to burn stuff as well.

But we know that the body absorbs and derives calories from a few classes of components: sugars and starches, proteins, and fats. We can take each of those and put them by themselves in the bomb calorimeter, and see what we get. Sugars and starches burn to give about 4 calories/gram, proteins are similar. Fats are highest at about 9 calories/gram. Alcohol shouldn’t be a significant part of your diet, but they tested that too, and it’s 7 calories/gram.

So you can take your ice cream sundae or your bowl of cereal, and separate out the sugars, starches, proteins, and fats. Take the quantity of each, multiply it by the appropriate calories/gram figure, and add them all up.

But even that is a lot of work, and not something that you can really do at home. Most companies, when they do that, send samples of their products out to specialized labs which can break the food down into its components.

So most food companies go even further with the simplification.

Instead of sending samples of their products to be analyzed, they take advantage that the ingredients they use have already been analyzed. The ice cream sundae is made from cream, milk, sugar, and eggs. According to FoodData Central (the USDA nutrition database), 100 grams of heavy cream has 36.08 grams of fats, 2.84 grams of protein, and 2.92 grams of sugar. The same database has data on whole milk, sugar, and eggs as well. These values can be scaled to match the portion size, added together for all ingredients, and multiplied by the calories per gram of each nutrient.

This is fast and easy, and doesn’t involve a chemistry lab in your kitchen. It is also allowable by law for labeling purposes.

The calorie was a former metric unit, which was based on convenient materials. You could easily calibrate a calorimeter (a laboratory device used to measure the heat of reaction and physical changes). As mentioned elsewhere, it was the energy required to raise the temperature of 1 gram of water by one degree Celsius. It has the symbol cal.

However, as it was a bit small for some purposes, people dealing with food used what they called the big calorie, which should really have been called the kilocalorie, since it is 1000 cal = Cal (with a capital C). However it is very commonly (I would say mostly) given with a small c. This is the unit mentioned in the question, heating one kilogram of water by one degree.

Because the numbers quoted for diet purposes became well-known, it has persisted. Some food labelling quotes both joules and kilocalories for the energy content of products.

The joule was adopted for the SI system because it did not depend on the properties of an arbitrary substance (water, its specific heat) but could be defined based on the fundamental quantities of mass, length and time, so that the SI is what is called a coherent system.

Heat was measurable before it was known as another form of energy. So the Calorie was invented as a measure of heat. The mechanical definition of energy is force times distance. So knowing F=ma, energy must have units of mass times distance squared divided by time squared. In metric, kgm^2/s^2, which is a joule. All components: kilogram, metre, second; had already been defined. But the calorie had already been defined too. They differed by a factor about 4.2. This factor was determined by the simple experiment of measuring the temperature rise from a falling mass stirring a viscous liquid: Mechanical equivalent of heat - Wikipedia

If every calorie in food were the same, you wouldn't expect to see weight-loss differences among people who eat the same number of calories that are doled out in different types of food.

A calorie is a calorie is a calorie, at least from a thermodynamic standpoint. It’s defined as the amount of energy needed to raise the temperature of 1 kilogram of water by 1 degree Celsius (2.2 pounds by 1.8 degrees Fahrenheit).

But when it comes to health and your body’s energy balance, not all calories are equal.

For example, some studies have reported that diets that are high-protein, low-carbohydrate or a combination of the two do yield greater weight loss than diets with other levels of fat, protein and carbs.

If every calorie in food were the same, you wouldn’t expect to see weight-loss differences among people who eat the same number of calories that are doled out in different types of food.

Dietitians like me know there are many factors that influence what a calorie means for your body. Here’s what we understand about calories and nutrition so far.

Energy actually available to your body

In the late 1800s, chemist W.O. Atwater and his colleagues devised a system to figure out how much energy – that is, how many calories – various foods contain. Basically, he burned up food samples and recorded how much energy they released in the form of heat.

Not every bit of energy in food that can combust in the lab is actually available to your body, though. What scientists call metabolizable energy is the difference between the total energy of the food consumed and the energy that passes out of your body, undigested, in feces and urine. For each of the three macronutrients – proteins, carbohydrates and fats – Atwater devised a percentage of the calories they contained that would actually be metabolizable.

Calorie is an energy measurement unit

A calorie (cal) is a pre-SI (pre-System International) measurement unit of energy or heat. ... In 1948, the scientific community decided that since heat (like work) is transferred energy, the SI unit for heat should be the one we use for energy -- namely, the joule.

Calorie, a unit of energy or heat variously defined. ... Thus the “15° calorie” (also called the gram-calorie, or small calorie) was defined as the amount of heat that will raise the temperature of 1 gram of water from 14.5° to 15.5° C—equal to 4.1855 joules.

What do you mean by 1 calorie?

The small calorie or gram calorie (usually denoted cal) is the amount of heat energy needed to raise the temperature of one gram of water by one degree Celsius (or onekelvin).

A calorie isn’t a thing like a ball or an atom - it doesn’t have any physical form.

A calorie is the amount of chemical energy stored in the carbon-hydrogen bonds of a large number of molecules of fat, carbohydrate or protein. Fat, carbohydrate and proteins are things - they have physical form and to the extent that you can visualise a carbon-hydrogen bond, then you can visualise the physical structure that contains a unit of energy, which summed up in truly vast quantities, yield a calorie of stored chemical energy but do realise that if you are doing this you are visualising the arrangement of carbon and hydrogen atoms that contain the units of energy, not the units of energy themselves.

This is like asking: “show me the potential energy in a 1kg weight held 1m off the ground”. You can see the weight. You can see the ground. You can visualise the 1m difference between the weight and and the ground, but you can’t ‘see’ the potential energy associated with this arrangement. If you let go of the weight, you can observe the 1kg weight accelerating and exchanging potential energy for kinetic energy but you can never directly ‘see’ the units of the energy themselves.

Chemical energy doesn’t have physical form, so it doesn’t make sense to ask what it “looks” like. The only question that does make sense to ask is: how much work can it do?

A food calorie (a Calorie or kcal) contains the amount of energy that can raise 1kg of water by 1 deg C (at room temperature). If you have 10 kcal, you can raise 10kg of water by 1 deg C or (roughly) raise 1kg of water by 10 deg C. With ~ 2000 kcal of energy you can power an average human for 1 day without causing them to deplete the energy held in the chemical bonds of the stored fat reserves (the exact amount is determined by the TDEE of the particular humans involved).

The calorie (cal) and the joule (J) are two different units of energy, where 1 calorie is defined as the amount of energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere. On the other hand, the joule is the standard unit of energy in the International System of Units (SI), and it is defined as the amount of energy transferred when a force of one newton acts over a distance of one meter.

The conversion factor between calories and joules is approximately 1 calorie = 4.184 joules. However, the value can sometimes be rounded to 4.18 for simplicity in certain contexts.

This conversion factor is derived from the specific heat capacity of water, which is the amount of energy required to raise the temperature of a substance by a certain amount. Specifically, one gram of water requires 4.184 joules of energy to raise its temperature by one degree Celsius. Hence, the conversion factor between calories and joules is based on this relationship between the energy required to raise the temperature of water and the energy unit defined by the joule.

A calorie is generally the amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius. The exact definition depends on how the measurement is done, and as a result we have, unfortunately, several definitions of “calorie”:

• The international calorie is 4.1868 joules
• The thermochemical calorie is 4.184 joules
• The [math]15^\circ[/math] calorie is 4.1858 joules

and there are several others, but they’re all roughly 4.18 joules. I would use the “international calorie” definition unless you’re told otherwise.

In addition, the “calorie” used in the food industry (sometimes written as “Calorie”) is actually a kilocalorie. I’m not sure which of these definitions is used for the “food calorie” — a food Calorie might be 4186.8 J, or 4184 J, or 4185.8 J, or some other value.

Calorie intake, in terms of medical and medicine, as well as that of nutrition, is taken into consideration of the fact that you must consider the environment in which you consume the food in, the type of food that you’re consuming, and, the fact that consuming the said food is healthy, or, not. If you’re, say, consuming a healthy tuna sandwich with plenty of said protein and that of carbohydrates and, enough, “healthy calories,” which will fill your stomach to satisfy your hunger, then it’s considered to say that, consuming these healthy calories are ok, and they’re not as big as one would think they’d be due to the fact that there are healthy chemicals in tuna, like H2O and other said toppings combined in, as said to be prepared from and in. Now, that is said, when you mix it with mayo in a container after draining the water from it out of the can, plus then placing it in the fridge, while in said sealed container… you may have a healthy, 300 calorie meal after it’s cooled. However, other foods, such as chips and Doritos, are of higher calorie due to the fact that the calories, are tightly packed inside of the food, since there are more carbohydrates and fats. It’s said that comparing this to said tuna, it’s not as healthy as sad tuna fish, when either are prepared in said kitchen environments. All in all, it’s stated, that of, as a matter of fact… of which both meals are completely different from each other, one being healthy and one being non-healthy, due to the fact that these types of sustenance are both different in carbohydrate and chemical, caloric composition, making the tuna healthier than the chips and/or Doritos, during afar.

Some things that are true:

• 1 cal = energy to raise the temperature of 1 g of water by 1 degree Celcius (at a certain starting temperature)
• 1000 cal = energy to raise the temperature of 1 kg of water by 1 degree Celcius
• 1000 cal = 1 kcal (using metric scaling, as designated by the leading k, meaning kilo)
• 1 Calorie = 1 Cal = 1 kcal = 1000 cal (e.g. a so-called “food calorie”)

This can be a little confusing because the scientific unit, “calorie”, is often confused with the unit used in the food and fitness industries, “Calorie”. A big-C Calorie contains 1000 times the energy of a small-c calorie Unfortunately, it is often more convenient to refer to “Calories” as “calories”, so the confusion is perpetual.

The general rule if you are talking about humans, food or fitness is that both “Calorie” and “calorie” means “kcal” (or “Calorie” or “Cal”) unless otherwise specified. In these contexts, f you actually mean to refer to “calories” or “cal” in the scientific sense it is best to use the unit “cal”, but ideally convert it to “kcal” and use the unit “kcal” to make explicit which kind of calorie you are talking about.

The calories in food or for that matter any hydrocarbon are calculated by oxidizing (burning) it completely in a calorimeter. The calorimeter is essentially an instrument to estimate as to how much heat was generated during this oxidation process. The heat produced is measured in calories. One calorie is equivalent to heat required to heat one gram of water by one degree centigrade. Even though the heat produced by food is reported as calories but actually they are Kilo calories or 1000 calories.

The thermodynamics limits as to how much of those calories calculated as above are actually generated in our body. Further there are foods that have calories in them but our body cannot break that food into simpler constituents to make those calories available to us. Example is fiber in the plants or very high molecular weight oils.

In thermodynamics, calory (or little calory) is a way to measure energy.

“In biology and in nutrition the kilocaloria (symbol kcal), or great calorie (symbol Cal), is the energy necessary to raise the temperature of 1 kg of distilled water by 1 ° C at a pressure of 1 atm, and corresponds then to 1000 small calories. It is used to indicate the average energy intake of a certain specified quantity of food (for example a gram, 100 grams or a portion).” Caloria - Wikipedia

So, kcal “is a term most commonly used in spoken language to indicate energy values ​​and nutritional values ​​of foods and diets.

The meaning of kJ, on the other hand, is chilojoule, or the unit of measurement adopted internationally (IS) to indicate energy, still little used in the nutritional field. Each kilocalories (kcal) is equivalent to 4,186 kilojoules (kJ).

Kcal and kJ are both present in the table of nutritional values ​​of foods, or nutritional label ... From a nutritional point of view, however, there is no difference between the two values! It's a bit like expressing a length using kilometers or miles!”

text excerpt © Orogel Surgelati – Sito Ufficiale, wiki

If the food is pre-packaged, then check the label, in the UK it will list the Kcal (calories) per 100 grams along with fat, protein etc content.

There are various sites that list the calorie content of fruit, vegetables and meat, some examples are.

Calories in Fruit.

Calories in Chicken and Meat.

Calories in Vegetables.

Look on the left-hand side of the page, of the above links for extra tips/info.

Some examples of calories in fish is below.

The Healthiest and Unhealthiest Fish to Lose Weight.

Google search is also your friend, just type “how many calories in (insert food here)” and you will get an answer, I used ‘egg’ as an example below.

how many calories in an egg.

I hope this answer is useful to you.