The three calorie definitions, with a day's intake worked out in each
The calorie has three values in use. The thermochemical calorie is exactly 4.184 J, the International Table calorie exactly 4.1868 J, and the 15 degree calorie about 4.1858 J. The gap is 0.0669 percent, so a 2,000 kcal day is 8,368.0 kJ under the first definition and 8,373.6 kJ under the second, a spread of 5.6 kJ.
The three calorie definitions
The joule column is quoted from NIST SP 811 Appendix B.8. The thermochemical and International Table calories are exact by definition; the 15 degree calorie rests on a measurement of the heat capacity of water between 14.5 and 15.5 degrees Celsius, so sources differ in its last digit and it is marked approximate. The last two columns are calculated: the kilojoule total for 2,000 kcal, which is 8,368,000 J under the thermochemical definition, and the gap from that definition.
| Definition | Joules per calorie | Status | Where you meet it | Kilojoules in 2,000 kcal | Gap versus thermochemical |
|---|---|---|---|---|---|
| Thermochemical calorie (cal_th) | 4.184 | exact by definition | chemistry, nutrition and the food Calorie | 8,368.0 | +0.0000% |
| International Table calorie (cal_IT) | 4.1868 | exact by definition | steam tables and older mechanical and HVAC engineering | 8,373.6 | +0.0669% |
| 15 degree calorie (cal_15) | about 4.1858 | measured, sources differ | classical calorimetry, now largely historical | 8,371.6 | +0.0430% |
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Why there is more than one calorie
The calorie began as a description of an experiment rather than a definition of a unit: the heat that raises one gram of water by one degree Celsius. Water does not oblige. Its heat capacity varies slightly with temperature, so the answer depends on which degree you pick, and that single fact produced every calorie in this table.
Fixing the experiment at 14.5 to 15.5 degrees gives the 15 degree calorie, around 4.1858 J. Steam-table work needed a value that agreed internationally and settled on exactly 4.1868 J. Chemists cut the dependence on water entirely by defining the thermochemical calorie as exactly 4.184 J, a pure conversion factor with no experiment behind it.
That last one won. It is the calorie behind nutrition figures, the calorie in physical chemistry, and the one this site converts with. Where you still meet the International Table calorie is older mechanical, refrigeration and HVAC literature, where a BTU or a calorie figure copied from a mid-century table carries the 4.1868 definition with it.
Why this matters when the numbers are large
At 0.0669 percent the difference is invisible on a single food item and easy to dismiss. Multiply it out and it stops being invisible: 2,000 kcal is 8,368.0 kJ thermochemical and 8,373.6 kJ International Table, a difference of 5.6 kJ, which is roughly the energy in a small biscuit, arriving purely from which definition a database used.
Over a year of tracked intake, or across a food database with tens of thousands of entries, that drift compounds into a systematic bias rather than noise. It is the kind of error that never trips a sanity check, because every individual number looks reasonable.
The rule is the same one that applies to every unit on this site: pick a definition, state it, and do not mix. A nutrition tool that imports one dataset in International Table calories and another in thermochemical calories will produce totals that no single source agrees with.
The capital C and the kilocalorie
The food Calorie, written with a capital C, is a kilocalorie: 1,000 calories. A 500 Calorie meal is 500 kcal, or 500,000 calories in the physics sense. The capital is the only thing distinguishing them in older American writing, which is why so much nutrition text is off by a factor of a thousand when quoted carelessly.
Modern labelling avoids the trap by writing kcal, and in the European Union by printing kilojoules alongside. That kilojoule figure is not a conversion of the kcal figure: EU labels calculate both independently from the Annex XIV factors, which is a separate gotcha covered in its own reference table on this site.
So a label that reads 2,000 kcal states 8,368,000 joules of energy under the thermochemical definition, and any kilojoule figure printed next to it will be close to but usually not equal to 8,368.0.
Frequently asked questions
Is a calorie 4.184 or 4.1868 joules?
Both, depending on which calorie. The thermochemical calorie is exactly 4.184 J and is the one used in nutrition and chemistry. The International Table calorie is exactly 4.1868 J and survives in steam tables and older engineering work. They differ by 0.0669 percent.
How many kilojoules is a 2,000 calorie diet?
8,368.0 kJ using the thermochemical calorie, which is what nutrition figures are built on. Using the International Table calorie you would get 8,373.6 kJ instead, 5.6 kJ higher, from the same intake. The commonly quoted round figure of 8,400 kJ is a rounding of the first of those.
What is the 15 degree calorie and should I use it?
It is the heat needed to raise one gram of water from 14.5 to 15.5 degrees Celsius, tabulated by NIST at about 4.1858 J. You should not use it for conversion. It is a measured quantity rather than a defined one, published values differ in the last digit, and every modern context has replaced it with the thermochemical calorie.
Is a food Calorie the same as a calorie?
No. The food Calorie with a capital C is a kilocalorie, 1,000 calories. A 250 Calorie snack is 250 kcal. The distinction only ever mattered in writing that used the capital letter to carry the factor of a thousand, which is why labels now print kcal instead.
Which calorie does this site convert with?
The thermochemical calorie, exactly 4.184 J, so 1 kcal is exactly 4.184 kJ. That choice is stated on the methodology page and applied consistently across every energy conversion here, including the BTU and kilowatt-hour pages.
Can I republish this table?
Yes, under Creative Commons Attribution 4.0 International (CC BY 4.0). Credit UnitGlide, https://unitglide.net and link back. The underlying joule values are from NIST, a United States government publication; the compilation and the calculated columns are covered by the licence.
Sources
- NIST Special Publication 811, Appendix B.8, for the joule value of the thermochemical calorie, the International Table calorie and the 15 degree calorie.
- NIST Special Publication 811 (full guide), for the status of the calorie as a non-SI unit and the rules for stating which calorie is meant.
- Regulation (EU) No 1169/2011, Annex XIV (EUR-Lex), for the separate legal factors an EU nutrition label is calculated with, which are not this conversion.