How a desk bike's calorie burn is measured
The calorie number a desk bike shows you (if it shows one at all) is a guess. The bike has no way to know how hard you're pedalling. We measure the real watts at the pedal on every bike, every resistance level, then turn those watts into a live calorie count in the SitZip app. This page walks every step, with the studies behind each number.
Quick summary
- A desk bike can't sense how hard you're pedalling. Any calorie number it shows is a guess. Most don't show one at all.
- We measure the real watts at the pedal with professional power meters.
- Watts at the pedal ÷ body efficiency = real energy spent. Multiply by 0.86 to get calories per hour.
- The SitZip app does all of this live as you ride.
Why the bike can't tell you your real burn
Most desk bikes have no screen. The ones that do can't tell which resistance level you're on. So any calorie, speed, or distance figure they show is a guess.
To get a real number you have to start at the source: the force going through the pedals.
How it works, in brief
Three steps get you from pedalling to a trustworthy calorie count:
- Measure the watts at the pedal, at every resistance level, with professional power meters.
- Watts to energy spent. Your body uses more energy than ends up at the pedal. Divide by gross efficiency.
- Energy to calories. A simple unit conversion finishes the job.
The SitZip app runs this whole chain live as you ride. The rest of this page walks through each step.
1. Measuring the watts at the pedal
The only way to know a bike's real output is to measure it. We fit professional power pedals to every bike, then ride each resistance level at a steady cadence and log what the pedals report. The result: a full map of watts for each setting. No manufacturer claims, no guesses.
Here's how the SitZip app works out your calorie burn. Every number traces back to one measurement: the real watts at the pedal. The steps below show how we capture it.
- 1
Fit Assioma PRO RS power pedals. Strain gauges inside the pedal axle measure the actual torque your legs apply, accurate to roughly ±1% — the gold standard for cycling power.
- 2
Ride every resistance level at fixed cadences. We hold a steady 30 and 60 rpm on each level and log the watts the pedals report — dozens of readings per bike.
- 3
Map each level to real watts. We store the numbers for every resistance setting in the SitZip app. Tell the app which level you’re on and it converts your cadence into accurate wattage, calorie burn, and distance — no guessing.
Our Assioma PRO RS pedals and the live power they read at the pedal.
2. Watts to the energy your body actually spends
The watts at the pedal are only the part that actually moves the cranks. Your body burns more than that, because most of the energy leaves as heat rather than movement. Divide the pedal watts by your body's gross efficiency (the share of energy that becomes pedal movement) to get the real energy you spend.
Why 18 %? Efficiency depends on how hard you're working. Trained cyclists at full effort hit 20–25 %. Desk biking is much lighter (30–50 W), where resting metabolism takes a bigger share of the burn. The measured numbers at desk-bike power:
- 16.1 % ± 2.0 in healthy 18–40 y adults at 50 W (Wezenbeek & Van Hooren, J Appl Physiol 2017)
- 21.4 % ± 1.1 in inactive subjects at 120 W (Fares & Bouillanne, Physiol Rep 2017)
We use 18 % as one safe number across the whole desk-bike range. It's slightly on the cautious side vs the 16 % measured at 50 W. We use gross efficiency, which keeps your baseline metabolism in the total. That's the same convention Fitbit, Strava, Garmin and Apple Health use, so our calorie numbers line up with what your other trackers say.
The rule is simple: pedal watts ÷ 0.18 = real energy your body spends.
3. Energy to calories
This step is pure unit math. One watt sustained for an hour is one watt-hour of energy. One watt-hour is 0.86 kilocalories. Multiply your metabolic watts by 0.86 and you have calories burned per hour.
Here's the whole chain as a worked example:
What the pedals actually measure
Total energy your body uses — pedalling plus heat
Energy your body uses per hour
How the app gives you live calories
You only need to tell the app two things while you ride: your resistance level and your cadence. Together they decide your watts.
The SitZip app already knows the watts for every resistance level on your bike (from step 1). So the flow is:
- You set your resistance level in the app.
- A sensor reads your cadence in real time.
- The app looks up the watts, runs steps 2 and 3, and shows live calories, distance, and effort. Plus heart-rate support and the option to race friends.
Instead of a guess (or no number at all), you get real numbers from how your bike was actually measured.
Ready to track your own rides? Get the SitZip tracker, or see which desk bike fits you.