Cycling Power to Speed Calculator
Indoor speed on a trainer is virtual and doesn't transfer to the road — power does. Enter your sustained power in watts and this estimates your speed on flat ground using the physics of cycling.
Why indoor speed doesn't convert
On a stationary trainer the displayed speed is a simulation — it has no fixed relationship to real-world speed, because outdoors your speed depends on aerodynamics, road surface, gradient, wind and air density, none of which the trainer reproduces. The quantity that genuinely carries over is power. So instead of converting one speed to another, this tool takes the power you can hold and estimates how fast that would move you outdoors.
How it's calculated
It uses the established cycling power model (Martin et al., 1998): the power needed to ride is the sum of rolling resistance, aerodynamic drag and the work against gravity, divided by drivetrain efficiency. Given your power, the calculator solves that equation for speed. Defaults assume no wind and sea level; a typical road-bike CdA is 0.30–0.40, and Crr around 0.004–0.006 for good tyres. Mass is rider plus bike.
Frequently asked questions
What CdA should I use?
Roughly 0.40 sitting upright, 0.32 on the hoods, 0.28 in the drops, and 0.22–0.25 in an aero position. Lower CdA means more speed for the same power, especially at higher speeds.
Why is my outdoor speed lower than the trainer showed?
Most trainers flatter you. Air resistance rises with the cube of speed, so on flat road a large share of your power is spent pushing air — something a trainer in a fixed resistance mode doesn't fully capture.