Kilojoules vs kilocalories — why WHOOP’s energy looks 4× too high
If one device says you burned four times what another says, the likeliest cause is not effort or accuracy — it is that one of them is counting in kilojoules.
The 4.184× factor
Energy has two common units in fitness data. Most of the ecosystem — Apple, Garmin, Fitbit — reports kilocalories, the “calories” everyone means. WHOOP reports kilojoules, the SI unit, and one kilocalorie is about 4.184 kilojoules. So a day WHOOP records as roughly eight thousand kilojoules is under two thousand kilocalories — the same energy, a different unit — and an import that treats the kilojoule figure as calories reports you burning four times what you did.
Why this is not a disagreement
Nothing here is a measurement error or a device being wrong. It is a units mismatch, the same category as Apple’s fractional SpO₂ or FIT’s cadence encoding: two correct numbers in two systems that only conflict when combined without conversion. Divide the kilojoule figure by 4.184 and it lands where the kilocalorie devices are.
Reading energy across devices
Before comparing any two devices’ energy, check the unit each reports, convert to one, and only then look at the gap. What remains after conversion is the genuine modelling difference — active versus total energy, different effort models — which is a smaller and more interesting disagreement than a 4× units artefact standing in front of it.