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Cycling Power from Speed Calculator

See exactly how many watts a speed costs — and where every watt goes.

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 vair 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.

Worked example

Take 30 km/h, 80 kg rider plus bike, a 2 % grade, no wind, sea level, 15 °C. Speed is 8.33 m/s and air density is 1.225 kg/m³. Aero drag costs 113 W, rolling 33 W and the climb 131 W, about 277 W at the wheel. Dividing by 0.97 efficiency adds a 9 W drivetrain loss, so the result is 285 W, which is 3.57 W/kg and 1,028 kJ/h.

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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.

Sources

About this calculator

Holding a cycling speed costs power across four physical channels: aerodynamic drag (which dominates at speed and scales roughly with the cube of airspeed), rolling resistance from tire deformation against the road, the gravitational cost of climbing on any gradient, and drivetrain friction losses through the chain and derailleurs. Total power output is simply the sum of these four, and because aero drag depends on the rider's CdA (drag coefficient times frontal area), air density and wind, the same speed can cost very different watts depending on position, equipment and conditions.

Cyclists without a power meter use this breakdown to estimate what a given speed should cost in watts, to sanity-check a power meter's readings, or to see how much of their effort at a given speed is being spent fighting wind versus gravity versus rolling resistance — the split matters for deciding whether an aero upgrade, a lighter bike, or better tires would help more for their typical riding.

Speed, rider and bike weight, gradient, wind, altitude and temperature (the last two set air density) are used to estimate the watts you are pushing, split into aero drag, rolling resistance, gravity and drivetrain loss.

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