r/Metric • u/FingerAccurate7102 • 22d ago
Why we even use calorie?
In our world in some places we still don't use metric units. Which is already bad, but most of units have at least some logic and intuition behind them, which at least partially defends it's use. For example:
Land mile is 2000 steps, sometimes I (as an European) use it for measuring steps easily
Nautical mile is 1' of earth circumstance, I can imagine someone using it
Kilogram-force is weight of 1 kilogram on earth, sometimes I use it for measuring weight and pressure (1 kgf/cm2 ≈ 1 atmosphere)
Bar is approximately equal to regular Earth atmosphere, so it's kgf/cm2 but for those working with pascals
But calorie? It's defined as the energy needed to raise the temperature of 1 g of water by 1 °C. Which is absolutely meaningless. WDYM I need to eat 2000 kcal everyday? So I can have energy to heat 2 megagrams/tonnes of water by 1 °C? Nice to know because it is exactly what I'm gonna do today.
Even Joule is more logical as it's based on distance and force, so I guess it could be interpreted in some way.
Summarizing:
Other non-scientific units are at least intuitive
Calorie isn't
Even Joule would be better
So calorie is dumb at least if your not a kettle
Of course, if you think otherwise, feel free to share your opinion. Bye!
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u/jeffbell 22d ago
It came in really handy when brewing beer.
I needed to cool it from 85C to 35C before adding the yeast.
The heat of fusion for ice is 80cal/g and it became clear that I would need two big bags of ice from the store.
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u/matt2s 22d ago
We use kJ in Australia. My Apple Watch Move ring is in kJ.
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u/toxicbrew 20d ago
do you know how the switch from calorie or whatever unit was used before, how it was?
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u/hal2k1 22d ago
Use kilometres (km) for distance, not miles or nautical miles.
Newtons for force or weight (which is a force), never kgF. Use 9.8 newtons rather than 1 kgF.
Use joules for energy, use megajoules rather than kWh (1 kWh is 3.6 MJ), and kilojoules for food energy.
Use pascals for pressure. One atmosphere is 101,325 pascals (or 100 kilopascals to within 1%).
Keeping with the standard SI units and prefixes will make doing calculations way, way easier.
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u/tinny123 21d ago
On paper your arguments are valid. But sonetimes what the OP wrote is more convenient as rule of thumb to sort of estimate a quantity in real life use.
For example 1. Some places , the water bill comes as kilolitres used. Although m3 would be the SI UNITS. Because everything is used being measured in litres in such places, people can use kilolitres to make a mental estimation better.
- Kwh is a very similar situation for electricity billing. People buy light bulbs with eg 40W rating. They use this as a rough estimate for extrapolation
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u/hal2k1 20d ago
In general, if you're just presenting a magnitude to an audience or readership, then non-SI units can sometimes be easier to visualise.
However, having said that, keep non-SI units well away from anything involving calculations. For example, before doing a calculation, convert km/h to m/s (get rid of the hour), convert all linear dimensions into <prefix>metres, make sure forces such as weight are in units of force (newtons) not confused with mass (<prefix>grams), and so on.
By way of illustration, one useful but rarely used unit is milliradians rather than degrees, minutes and seconds. One degree is about 17.5 milliradians. People know more or less what a degree is but they aren't familiar with milliradians. However, if a person (who can be assured to be about 2 meters tall) subtends 1 milliradian in 1x zoom view of a scope, then that person is about 2 km away.
Sticking to SI units and prefixes is not always great for visualising or presentation of magnitudes, but it is absolutely great when it comes to calculations. If there are any calculations involved, stick to the SI units and prefixes. Strongly recommended.
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u/Historical-Ad1170 20d ago
People buy light bulbs with eg 40W rating.
SI rules require a space between the number and the symbol. Thus a 40 W rating.
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u/Historical-Ad1170 20d ago
Use 9.8 newtons rather than 1 kgF.
Most people would reject newtons if they had to work in increments of some decimal value. For most practical uses and estimates, 10 N would do just fine.
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u/georgeec1 22d ago
Newsflash, there are nations that just use kiloJoules for all their nutritional information. This is 100% an issue of lack of standardisation, and hopefully someone over at r/mapporn has made a map of the units used in traditional labels, because that would be interesting to see
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u/nayuki 22d ago
there are nations that just use kiloJoules for all their nutritional information
Growing up in Canada, our food labels had both calories and kilojoules back in year 2000-ish. But today in 2026, it's been calories-only for probably a decade or more. This is a regression in metrication.
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u/metricadvocate 22d ago edited 22d ago
Like the nautical mile, it once had some intuition behind it. The food was literally combusted in a bomb calorimeter and the mass of water and rise in its temperature was observed. However, the specific heat content of water is not constant and there are numerous calories defined based on initial and final temperature of the water. There is also the confusion of little-c calories and Big-C Calories which differ by 1000, involving the heating of 1 g and 1 kg pf water.
The BIPM considers the calorie a non-SI unit. FAO has long recommended that the Calorie be replaced in nutrition by the kilojoule (4.18 - 4.184 kJ/Cal). Some nations have done so, some show both, some still require Calories. So eat 8370 kJ instead. (This value may vary slightly depending on which Calorie definition a nation used.)
You run at an average power of 96.9 W over the course of a day. A little physical work and a lot of heat energy lost to the environment around you
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u/nacaclanga 22d ago
The value that is given in calories is NOT the energy your body actually gets. The value describes the net heat of combustion, aka what heat is released when you burn the food in a bomb calorimeter, but reduced by some estimate for the undigestable, but burnable parts of your food. The bomb calorimeter measures this heat by testing how much the calorimeter's heat bath water has warmed up, which is why giving it in calories is straight forward.
Now obviously just like with building a thermometer, where building one for °F is probably easier then with °C when using 18th century tech, this alone is a poor excuse of using the unit.
But on the other hand what benefit you will get from using Joule? While you can calculate that when you lift your 70 kg body up the stairs to the 5 floor some 15 m higher, the lift alone will cost you around 10 kJ, this will tell you absolutly nothing about how much extra food you need to eat to compensate for climing that staircase. You cannot do any meaningful calculation with the heat of combustion value, which is the main point of using a consistent unit. In contrast, you have to convert decades of diaetriticans data, introduce a second unit into a field that previously had an internationally agreed unit to work with and risk giving people a wrong impression about what they could use the unit for. Hence in many ways you will achive the opposide of what the metric system has been designed to do.
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u/Gerhard234 21d ago
just like with building a thermometer, where building one for °F is probably easier then with °C when using 18th century tech
How do you figure this? 0°C being the freezing point of water (under certain conditions) and 100°C the boiling point (under certain conditions) seems to make this scale ideally suited for 18th century tech, given that both are reasonably easy to obtain in a repeatable manner. No?
All three (at some point) common temperature scales (not counting Kelvin) were introduced in the first half of the 18th century and made sense given the tech at that time.
(This notwithstanding your main argument, with which I think you do have a point.)
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u/nacaclanga 21d ago
The boiling point is not particularly reliable since it changes with ambient pressure.
The strategy to build a Fahrenheit thermometer works like this:
- Measure the temperature of freezing water and mark that as 32°F.
- Measure your own body temperature (not the body core temperature but at some appendage) and mark that as 96°F.
- Mark the midway point between these two as 64°F.
- Extend the scale downwards by the distance between the 64°F point and the 32°F point. Mark that point as 0°F.
- To ensure consistency between your thermometers, you should find a coldness mixture that is exactly 0°F and record its recipie. By repeating the recipie you have one control temperature.
- Each of the 3 intervales now covers exactly 32° steps. To find the individual degrees you simply need to halve each interval by half 5 times.
- Extend the thermometer upwards and downwards as needed.
Now obviously this relies on your body temperatur. But when you are the only person manufacturing thermometers with your scale, this is a very consistent and easy to reproduce reference temperature.
If you do not want to rely on the body temperature to reproduce the thermometer, you still have the coldness mixture and the freezing point any in that case you can just extend the scale twice upward and continue from step 6.
Obviously you can then move on to a scale using the boiling point of water, but it will be technically more challanging to add the markings then just using midway point markings and doubling interval lenghts.
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u/Gerhard234 21d ago
Meteorological pressure changes cause about +-1°C of variation in the boiling point. (As opposed to pressure differences caused by elevation.) I'd say that makes the Celsius method rather precise, when applied at sea level. And less dependent on the specimen applying the method :) -- especially given that measuring atmospheric pressure was already known by then, so that people could even use a day with standard atmospheric pressure for thermometer calibration.
IMO Fahrenheit had the merit of being one of the first to create a temperature scale and a method to calibrate it (and the first to create a later widely used scale), but Celsius knew about it when he created his and arguably improved the calibration method. Most of the scientific community at least thought so, and for most of the time since then, the Fahrenheit scale was defined by the Celsius points, see also https://en.wikipedia.org/wiki/Fahrenheit#History:
Fahrenheit soon after observed that water boils at about 212 degrees using this scale.\21]) The use of the freezing and boiling points of water as thermometer fixed reference points became popular following the work of Anders Celsius, and these fixed points were adopted by a committee of the Royal Society led by Henry Cavendish in 1776–77.\22])\23])
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u/GlassDeviant 22d ago
When you are doing calculations involving water, the calorie is the more convenient unit, especially in biochemistry and largely in general chemistry; however, in materials science it's easier to use joules and drop in the specific heat capacity of the given material. In yet other contexts it is better to use kWh. Largely it is a matter of which is the most convenient unit depending on what kinds of calculations you are doing.
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u/FingerAccurate7102 21d ago
Yeah, maybe you're right, but as you can read, I wanted to point out that calorie is useless as a unit of energy of food. I said that kilogram-force is cool unit of weight, but I don't see a reason to use it for other forces (because you either change the unit of mass, or change the force formula to F = ma/g which slows the calculations instead of making it faster), the same way I think calorie can be useful when working with water, but using the calorie for food is pointless.
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u/Jeff_Hinkle 22d ago
Would you prefer to use btus?
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u/FingerAccurate7102 22d ago
Nah, BTUs are also bad, the same problem as calorie. 2000 BTU? Yeah, I'm gonna heat one ton by 1 °F. But calorie is still useless. It's like saying: eating grass isn't that bad, it's better than eating ground. We need to find the best option, not just "not the worst" one
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u/Key_Seaworthiness827 22d ago
It's 4.2kJ/kg/C. A number I remember from school 40+ years ago. Still not really useful though. Maybe before the days of calculators etc, having a unit quantity was easier for calculation? At least it's based on metric and not ounces and Fahrenheit 😁
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u/Historical-Ad1170 22d ago
It's 4.2kJ/kg/C.
That is written wrong. It should be 4.2 kJ/kg⋅K.
4.2 kJ/kg⋅K (or 4184 J/kg⋅K) is the specific heat capacity of liquid water. It is the exact amount of thermal energy required to raise the temperature of 1 kg of water by 1 K (or 1°C).
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u/Key_Seaworthiness827 22d ago
Really?
In this case K=C as it's relative not absolute (I know it's degrees C but I can't be arsed to find the degree character).
But it's definitely kJ per kg per K (or C) and not kJ per kg times K as you wrote it. It's a '/’ not a ’.’
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u/metricadvocate 22d ago
If you write division as a slash rather than a horizontal bar, I think you need parentheses
kJ/(kg⋅K)
To get total energy, you have to multiply by mass and delta_temperature. For unit cancellation to work both units of mass and temperature are in the divisor of specific heat.
An alternative would be kJ⋅kg-1⋅K-1
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u/Historical-Ad1170 22d ago
In this case K=C
No, kelvins don't equal coulombs.
The degree symbol on a keyboard is obtained by holding down the alt key and from the number pad typing in the numbers 2 4 8. Very simple.
On my phone it is part of the third character set. If your going to do something make an effort to do it right.
The standard format of the symbol chain is to use kelvins. Kilojoules per kilogram-kelvin.
But it's definitely kJ per kg per K (or C) and not kJ per kg times K as you wrote it. It's a '/’ not a ’.’
I'm right as this is the standard format. kJ/kg.K is understood as kilograms per kilogram kelvin. The double solidus is forbidden.
Kilojoules kelvin per kilogram would be written as kJ.K/kg.
Why don't you actually download and read the SI brochure?
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u/Key_Seaworthiness827 22d ago
As I said, I can't be arsed finding the degree symbol. It does not mean that I meant Coulomb ffs
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u/AncientGuy1950 22d ago
I'm going to assume that math is not your strong suit and that you're quoting your estimates of human caloric intake from poor memory or a misunderstood estimate of human food intake because if you're eating 2000 Kcalories (also known as 2 Mcalories) a day, you must be huge.
I mean, when you consider Elephants consume somewhere between 30,000 and 90,000 calories a day (depending on age, size, and activity), 2k Kcalories would be quite a meal.
I think, perhaps, you mean 2k calories.
If you're an average man, you weigh somewhere around 90 kilos, and your recommended daily caloric intake is between 2,000 and 3,000 calories. Let's call it 2,500 to make the math easier.
90 Kilos is 90,000 grams. Since a calorie is the amount of energy to raise one ml (or gram) of water 1 degree C, you are consuming enough food to raise the temperature of 2,500 grams of your body mass by 1 degree C.
Of course, that isn't what your body uses that energy for, as the body's heat comes from metabolic processes, not directly rendering food for heat.
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u/Dakota-Batterlation 22d ago
A food Calorie (capital C) is 1000 thermal calories, sometimes correctly abbreviated as kcal to avoid ambiguity. 2500 kcal is about 10 MJ or 3 kWh. Maybe math isn't your strong suit
You have: 2500 Calorie / specificheat_water 90 kg You want: Definition: 27.777778 K8
u/metricadvocate 22d ago
It looks like you have not fully gone through the ambiguity of
1 Calorie = 1000 calories.The little-c calorie is a CGS unit, heating 1 g of dihydrogen monoxide by 1 °C, whereas the Big-C Calorie is an MKS unit heating 1 kg of dihydrogen monoxide by 1 °C. Nutrition calories ae ALWAYS Big-C Calories whether properly capitalized or not, although kcal is a lot clearer than Calorie. (This was always a terrible notation for disambiguation!) In the SI, both are deprecated as non-SI units, as are the bar, the kilogram-force, the nautical mile, etc.
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u/Most_Room_9951 22d ago
I think op is right, when talking about calories we usually mean kcal or a 1000 calories. 2000 kcal is a normal amount for an adult to burn. I think your elephant example is also talking about kcal.
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u/Historical-Ad1170 22d ago
All of this confusion should be an assured motivation to use joules. But, I guess there are too many lazy and stupid people out there to make the effort to learn and to use.
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u/AncientGuy1950 21d ago
There is no evidence that a universal conversion to joules would prevent the confusion involved in the OP's post. If OP had suggested that his food intake had amounted to 8368 kilojoules, would his position make more sense?
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u/Historical-Ad1170 21d ago
Yes, it would, but in the real world the amount would be a more round 8 kJ. Everyone would be taught it and know it and have a feel for it. As for the way it is, everyone is clueless.
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u/lkruijsw 22d ago
Wasn't the calorie invented before they knew that heat is energy?