All factors here reduce to joules, the SI unit of energy, sourced from NIST’s own published conversion tables — and deliberately using the same calorie definition this site’s calorie and macronutrient calculators already build on, so a figure converted here never quietly disagrees with one computed there.
How it works
Exact and near-exact factors
1 kWh = 3,600,000 J exact · 1 kcal = 4184 J exact
1 BTU ≈ 1054.350 J
The kilowatt-hour is exact because it follows directly from the SI definitions of the watt and the hour. The thermochemical calorie (4.184 J) is exact by definition and is the “food Calorie” convention. NIST’s own table marks the BTU value as not exact, and this page keeps that distinction rather than presenting all three with equal confidence.
Why the calorie has more than one definition
The “International Table” calorie (4.1868 J) and the thermochemical calorie (4.184 J) differ by about 0.06% — invisible on a nutrition label, but they are not the same unit. This converter uses the thermochemical definition throughout, matching the daily-calorie-needs and macronutrient-split calculators elsewhere on this site.
How to use this calculator
- Type an energy value into any field.
- The rest update — kilojoules, kilocalories, kilowatt-hours and BTU up front; joules, small calories and watt-hours behind “More units”.
Frequently asked questions
Why do nutrition labels say “Calories” with a capital C?
Because a food Calorie is actually a kilocalorie — 1,000 of the small calories chemists use. The capital C is a convention meant to flag this, though it is often dropped in casual use, which is a common source of confusion.
Is a kilowatt-hour an exact unit?
Yes, exactly 3,600,000 joules — it follows directly from the SI definitions of a watt (one joule per second) and an hour (3600 seconds), with no rounding anywhere in the chain.
Why isn’t the BTU marked exact like the others?
Because NIST’s own published table does not mark it exact — the thermochemical BTU used here is a defined but rounded figure, and this page states that plainly rather than implying a precision the number does not have.
Which calorie does this site’s calorie calculator use?
The thermochemical calorie, the same one used here — so a figure you convert on this page and one computed by the daily calorie needs or macronutrient split calculators will always agree.
