How the calorie to BTU conversion works
Most confusion comes from two facts: (1) people say “calorie” but often mean kilocalorie (kcal, equal to food Calorie), and (2) there are slightly different BTU definitions. Here we use the widely accepted BTU (IT) = 1055.056 J, and the thermochemical calorie = 4.184 J, which is consistent with engineering sources like EngineeringToolbox.
Formulas
cal → J: J = cal × 4.184
kcal → J: J = kcal × 4184
BTU → J: J = BTU × 1055.056
Energy conversion: convert source → joule → target.
Power conversion: convert all to watt, using:
- 1 kcal/h = 4184 J / 3600 s = 1.163 W
- 1 BTU/h = 1055.056 J / 3600 s ≈ 0.293071 W
Why use this tool over generic unit converters
This page is tuned specifically to heat/energy contexts. It lets you work in the three most common domains at once: lab/physics (J, kJ), food/thermochemistry (cal, kcal), and HVAC/engineering (BTU, BTU/h, kcal/h, W). You don’t have to guess constants: they are embedded.
FAQ
1. What’s the difference between cal and kcal?
1 kcal = 1000 cal. Food labels use kcal but often write “Calorie” with a capital C.
2. Why are there different BTUs?
Yes, there are slightly different BTU definitions (IT, mean, thermochemical) differing within about 0.1%. We use the IT BTU, common in HVAC.
3. Can I use this for boiler sizing?
Yes, in the power tab you can convert kcal/h (common in European boilers) to BTU/h (common in US specs) or to kW.