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this post was submitted on 01 Sep 2026
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My exercise bike gives me a calorie count and apparently they are a bit better because the bike can actually measure how many watts of power you are generating which is the dream scenario for any calorie counting device. However even then they are only 80-85% accurate in counting calories because apparently we humans are all differently efficient in converting calories into watts. Despite this I still use that calorie number, but not to measure how many muffins I can now eat post workout, but just to compare my different workouts to each other. Recently I have been doing 1k kcal per day. Who knows maybe I'm efficient and it's actually 800kcal. But it gives me a goal to move towards each day.
Even 80-85% sounds like too much.
Compare the legs of a slim person with the legs of a severely overweight person. It's like adding shake weights onto the legs that do the pedalling.
And overweight people have different weight distributions, some have the weight on the legs some in the belly. Depending on where the weight is, that's a massive difference.
Watts input and watts output is massively different with every machine (compare e.g. an electric engine with 90+% efficiency to a combustion engine with maybe 20-30% efficiency), and humans are no different.
We're not that different though. It doesn't vary that much between people. It's pretty universally 20-25% of energy gets converted into mechanical energy, with the other 75-80% lost as heat.
What does differ is how mechanically efficient the bike is. A commercial gym stationary bike might only be 80% efficient at converting the energy put into the pedal down into the flywheel, and that is what gets read by the power meter. A high end race bike is above 95%.
Due to the roughly 25% efficiency, the math of kilojoules to calories burned is almost exactly 1:1.
Outside of a clinical lab where you get hooked up to a machine that reads the actual CO2 from your breath as you exercise, a power meter is the most accurate you're going to get though.
My Apple Watch is within 2% of the reading on the exercise bike and is noted as the most accurate in the study. I'd wager your exercise bike reading is pretty close to your actual calorie burn.