Cycling Power from Speed Calculator

How it is calculated

The tool converts speed to m/s (v = km/h ÷ 3.6) and adds three forces at the wheel: gravity m·g·sin(θ)·v, rolling Crr·m·g·cos(θ)·v and aero ½·ρ·CdA·vair·|vair|·v. Here m is rider plus bike in kg, θ = atan(grade ÷ 100), and v_air is ground speed plus headwind (a tailwind is a negative wind value, and a strong one can make the aero term negative). Air density ρ is computed from altitude (m) and temperature (°C). The wheel total is divided by drivetrain efficiency to give pedal power. Empty CdA, Crr and efficiency fall back to 0.32, 0.005 and 97 %; speed must be 0–150 km/h and weight up to 400 kg.

Common mistakes

• Entering rider weight alone: the field is rider plus bike plus kit, so leaving out 8-10 kg understates the gravity and rolling terms.
• Getting the wind sign backwards: a positive value is a headwind, a tailwind must be entered as a negative number.
• Typing a grade as a fraction (0.02) instead of percent (2). The tool reads it as a nearly flat road.
• Treating the result as a pedal-only figure on a long descent: a steep negative grade can give a negative watt value, which just means gravity supplies more than the ride needs.
• Using one CdA for every position: the default 0.32 suits an upright hoods position, while a drops or time-trial position is lower and changes the aero share a lot.