Speed & Power Calculator
What power holds what speed, on any gradient, surface and riding position. Solve it either direction — built on the same physics engine as the climb calculator.
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The reference figures below still apply — they are part of the page, not the script.
Commonly-cited approximate figures, not a measurement of your bike and body — your actual drag and rolling resistance depend on your position, kit, tyres and tyre pressure.
▶ Gradient & altitude (optional)
Speed
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Pace
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Drag share
Total resistance
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Force breakdown
Estimates from a standard steady-state power model. Wind, drafting, pacing and road-surface variation beyond the selected surface preset are not modelled.
Quick reference · flat road · hoods · sea level · 83.5 kg total
150 W
27.9 km/h
200 W
31.1 km/h
250 W
33.9 km/h
300 W
36.2 km/h
350 W
38.3 km/h
400 W
40.2 km/h
▶ How this is calculated
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Same governing equation as the climb calculator (Stats for Nerds) — this tool only adds the reverse direction and lets CdA/Crr vary instead of holding them fixed.
More tools: Training Zones · FTP Estimator · Climb Calculator
Frequently asked questions
How accurate is this calculator?
It uses the same steady-state power model as the climb calculator — gravity, rolling resistance and aerodynamic drag, adjusted for air density at altitude — solved for either speed or power. Real-world riding adds wind, drafting, pacing and road-surface variation the model doesn't see, so treat the result as a solid planning number rather than a race-day guarantee.
What do the riding position and surface options change?
They set two numbers in the equation: CdA (your frontal drag area — lower in the drops or on aerobars than on the hoods) and Crr (rolling resistance — higher on gravel or cobbles than smooth tarmac). Both are commonly-cited approximate figures, not a measurement of your specific bike and body — your actual numbers depend on your position, kit, tyres and tyre pressure.
Why is there a gradient option if this is a flat-road tool?
The default is 0% because most speed-and-power questions — "what power holds 35 km/h", "how fast at 250 W" — are flat-road questions. The gradient control covers the in-between case, a slight drag or a false flat, without turning this into a second climb calculator. For an actual climb, with a real profile and category, use the Climb Calculator instead.
Why does Speed → Power sometimes show 0 W with a note instead of a number?
On a steep enough downhill, gravity alone is enough to carry you past the speed you asked for without pedalling at all — that freewheeling speed is a genuine physical answer, not a rounding artefact. Rather than print a negative wattage, the calculator says so and gives you the freewheeling speed instead.
A number is a start. A plan gets you there.
This calculator tells you what's possible today. Structured coaching is how next season's numbers become different from this season's.