Estimate cycling speed from power (watts) using your weight, bike weight, gradient, surface type and wind. Physics-based with realistic rolling resistance and CdA presets.
Built and maintained by Paul Clark, Redmoon Software · Checked against what this tool computes on
| Force component | Watts | % of total |
|---|
Total power = rolling resistance + aerodynamic drag + gravity + drivetrain loss. Specifically: P = Crr·m·g·v + ½·ρ·CdA·(v + wind)²·v + m·g·sin(atan(grade))·v, divided by 0.95 drivetrain efficiency.
Constants: air density ρ = 1.225 kg/m³, g = 9.8067 m/s². Speed solved by binary search.
The tool inverts the standard cycling power equation. Power at the pedals is (Crr·m·g·v + ½·ρ·CdA·(v+wind)²·v + m·g·sin(atan(grade/100))·v) ÷ 0.95, with air density ρ = 1.225 kg/m³, gravity g = 9.8067 m/s² and a 0.95 drivetrain efficiency. That cannot be rearranged for v algebraically, so it runs sixty rounds of binary search between −20 and 30 m/s to find the speed your wattage actually produces.
Surface / bike sets the aerodynamic and rolling constants together: Road bike CdA 0.32 and Crr 0.005, TT bike 0.24 / 0.004, Gravel 0.36 / 0.012, MTB 0.42 / 0.020, Commuter 0.45 / 0.008. On the flat CdA dominates — at the default 75 kg rider, 8 kg bike and 200 W the breakdown shows aerodynamic drag taking about 80% of your power. Tilt the Gradient field to 5% and gravity takes 85% instead.
Fixed CdA presets are the weak point. Your real frontal area swings enormously between hoods, drops and a tuck, and 0.32 describes a moderately aggressive road position rather than any specific rider, with no field to enter a measured value. Air density is pinned at sea level, so mountain speeds read low. Wind is a pure head- or tailwind with no crosswind or yaw angle, and drafting is not modelled at all.
About 33 km/h for a 75 kg rider on an 8 kg road bike in still air, which is the default setup. Aerodynamic drag takes roughly 80% of that power, so the same wattage on the TT bike preset buys nearly 37 km/h.
Because gravity scales with total mass and speed. At 83 kg combined the climbing term absorbs about 85% of 200 W, dropping speed to roughly 14 km/h — under half the flat figure — while aerodynamic drag falls below 7% of the total.
The Road bike option uses a single CdA of 0.32, roughly a moderately aggressive hoods position. Real riders span about 0.28 in the drops to 0.40 sitting upright, and there is no custom field, so pick whichever surface preset is closest to your posture.
No. The model treats you as riding alone in still or steady air. Sitting in a bunch can cut aerodynamic drag by 30% or more, so real group speeds at a given wattage will always run higher than this estimate suggests.
On the flat, almost none — mass only enters through rolling resistance, a small share of total power. On a climb it is close to proportional, because the gravity term is multiplied directly by rider weight plus bike weight combined.
Cycling power divides between overcoming aerodynamic drag, rolling resistance, gravity on a climb and drivetrain losses. Which of those dominates depends almost entirely on speed and gradient, and the split shifts dramatically between them.
Above roughly 25 km/h on the flat, aerodynamic drag is most of the total, and it scales with the cube of speed — so going 10% faster needs about a third more power. That is why small aerodynamic changes matter at speed and almost nothing at low speed.
On a climb the picture inverts: gravity dominates, drag becomes minor because speed is low, and total system weight — rider plus bike plus everything on it — becomes the number that matters. This is why climbing performance is discussed in watts per kilogram and flat performance is not.
The inputs the model cannot know well are the aerodynamic drag coefficient and frontal area, which vary hugely with position on the bike. Sitting up versus getting low can change effective drag by 30% or more, which is larger than most equipment differences and free.
Wind is the other unknown, and headwind adds to the speed the air sees rather than to ground speed, so a modest headwind costs disproportionately.
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