About this tool
The Cycling Power & Speed Estimator uses the steady-state physics model (Martin et al. 1998 / Kreuzotter) to relate speed, grade and power. It computes the power at the wheel as P = v × (Crr·m·g·cosθ + m·g·sinθ + ½·ρ·CdA·(v+wind)²), where θ = atan(grade), then divides by a drivetrain efficiency of 0.976 to get the power you put through the pedals. Everything is computed locally in your browser.
The three resistances are reported separately so you can see where the watts go: rolling resistance (tyre on road), gravity (climbing or descending), and aerodynamic drag, which dominates at speed. Defaults are g = 9.8067 m/s², Crr = 0.005 for a road tyre on asphalt, and air density ρ = 1.225 kg/m³, adjusted for altitude and temperature via ρ = 1.225 × 273/(273+T) × e^(−altitude/8000). CdA is chosen by riding position — tops ≈ 0.40, hoods ≈ 0.32, drops ≈ 0.30, aero bars ≈ 0.27 m² — or entered directly.
In power-from-speed mode you enter a target speed and read the required watts; in speed-from-power mode the tool solves the cubic relationship numerically by bisection to find the speed a given power produces. Mass is rider plus bike. Results are estimates for still, steady conditions and ignore acceleration, drafting and road surface variation.