Cycling Power Calculator

How it is calculated

The speed is converted to v = km/h / 3.6 (m/s). Three forces resist the rider: gravity Fg = m·g·sin θ, rolling resistance Fr = Crr·m·g·cos θ and air drag Fa = ½·ρ·CdA·v². The gradient is a percentage, with θ = atan(grade / 100). Power at the wheel is (Fg + Fr + Fa) × v and is divided by 0.97 for drivetrain losses to get the rider's power in watts.

The tool uses g = 9.81 m/s² and air density ρ = 1.225 kg/m³ (sea level, 15 °C). Mass is rider plus bike in kg. If CdA or Crr are empty or zero, defaults of 0.32 m² and 0.005 are used, and grade defaults to 0. There is no wind. On a steep descent the result can come out negative, meaning no pedalling is needed.

Common mistakes

• Entering rider weight only: the weight field should be rider plus bike (and bottles, bag), because the engine uses it as total mass.
• Typing the grade as a decimal fraction (0.08 instead of 8): it is a percentage, so 0.08 means a nearly flat road.
• Using climbing results for flat riding, or the other way round: on a climb gravity dominates, but at 30 km/h on the flat most of the power (about three quarters with the default settings) goes into air drag.
• Comparing the answer with a power meter reading in the wind: the estimate assumes still air and sea-level density, so headwind, altitude and drafting change the real figure.