How to use
- Type the energy you have.
- Choose its unit: J, kJ, cal, kcal or eV.
- Choose which calorie the page should use. Thermochemical is the default and is the one behind the kcal/mol in biochemistry.
- The other four appear underneath, each with a copy button. Negative values are fine — a reaction can release energy.
💡 Exponents can be entered with e — for example, 1.5×10⁻⁵ is entered as 1.5e-5.
Formula & notes
The joule is the unit; everything else on this page is a fixed multiple of it. The trap is not the arithmetic — it is that the word calorie names two different units, both of them exact definitions, 0.067 % apart. A result you cannot reproduce because nobody wrote down which one was meant is not a result.
The factors, and where they come from
| Unit | In joules | Exact? | Source |
|---|---|---|---|
| cal (thermochemical) | 4.184 | definition | NIST SP 811 B.8 · JIS Z 8000-5 Annex B 5-6.B.c |
| cal (International Table) | 4.1868 | definition | NIST SP 811 B.8 note 9 · JIS Annex B 5-6.B.b |
| eV | 1.602 176 634 × 10⁻¹⁹ | exact since 2019 | CODATA 2022, NIST CUU |
Everything on the page is these, applied in whichever direction is needed.
- Calories to joules
- E/J = E/cal × 4.184 (thermochemical)
- Electronvolts to joules
- E/J = E/eV × 1.602 176 634 × 10⁻¹⁹
How much 0.067 % is
A ΔG of −10.0 kcal/mol is −41.84 kJ/mol on the thermochemical calorie and −41.868 kJ/mol on the IT one. Nobody will ever measure the difference in one binding constant. Across a table of forty of them, converted by two different people, it is a systematic offset that will not average out — which is why the tool prints which calorie it used rather than assuming you agree with it.
Practical notes
- The thermochemical calorie is 4.184 J exactly and the International Table calorie is 4.1868 J exactly. Both come from NIST SP 811 Appendix B.8 and JIS Z 8000-5:2014 Annex B; the IT value was set by the Fifth International Conference on the Properties of Steam, London, 1956.
- Biochemistry means the thermochemical one. Bond energies, ΔG in kcal/mol, thermodynamic tables — that lineage is 4.184. The IT calorie comes from steam tables and engineering. This page defaults to thermochemical and names it under every result.
- 1 eV = 1.602 176 634 × 10⁻¹⁹ J, and since the 2019 revision of the SI that is exact rather than measured, because the elementary charge is now a defined constant. ⚠️ JIS Z 8000-10:2015 10-28.b still carries the 2010 CODATA value with an uncertainty. The standard predates the redefinition; it is old rather than wrong.
- ⚠️ An electronvolt is an energy per particle and a kcal/mol is an energy per mole. This page does not cross between them — that step needs the Avogadro constant, not a unit factor. For the record the bridge is 1 eV per particle = 96.485 kJ/mol, which is the Faraday constant in disguise.
- JIS Z 8000-5 introduces the annex these units sit in with 「これらの単位は用いないほうがよい」 — better not to use them. It is passed on here rather than argued with: the joule is the unit, and the calorie is carried because the literature is full of it.
FAQs
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