Watts to Speed Calculator
How fast does 200 watts actually go? This calculator solves the published power-balance model both ways — watts to speed, or the watts a target speed demands — with every assumption printed on the page.
- Martin et al. power-balance model
- Solves both directions
- Every default shown
Rider and road
0 is flat, 5 is a steady climb, −3 a gentle descent.
Air density is fixed at 1.225 kg/m³ (sea level, 15 °C) and drivetrain efficiency at 97% — both printed in the method below.
Your results
Estimated speed
—
| Air drag | — |
|---|---|
| Rolling resistance | — |
| Gravity (gradient) | — |
| At the pedals | — |
Still air, steady effort. A headwind or an optimistic position preset moves the real number.
The model and its defaults
This is the power-balance model published by Martin and co-authors and validated against measured road rides: the power at the wheel is spent three ways — pushing air aside (the cubic drag term), flexing tyres (rolling resistance), and lifting mass on a gradient (gravity). Divide by drivetrain efficiency, 0.97, to get the power your legs must produce. The equation is cubic in speed, so the calculator solves it numerically to within a fraction of a watt. Air density ρ is 1.225 kg/m³ and g is 9.81 m/s² throughout. The methodology page sets out the model and its validation in full.
A 70 kg rider on an 8 kg bike holding 200 W on flat, smooth asphalt, on the hoods: drag takes 183 W, rolling resistance 11 W, gravity nothing — an estimated 34.0 km/h (21.1 mph). Turn it around: holding 30 km/h in the same setup needs about 143 W. Put the same 200 W on a 5% climb and speed falls to about 15.5 km/h — gravity takes the difference.
Position and surface presets
Both presets are labelled estimates, drawn from published wind-tunnel and rolling-resistance ranges. Your own CdA depends on size, flexibility and kit — treat the presets as the middle of each range, not a measurement.
| Preset | Value | Notes |
|---|---|---|
| Hoods | CdA 0.32 | Upright endurance position, hands on the hoods. |
| Drops | CdA 0.28 | Lower and narrower; the classic fast road position. |
| Aero bars | CdA 0.25 | Clip-on extensions on a road bike. |
| Full time-trial | CdA 0.22 | Dedicated TT bike, skinsuit and aero helmet. |
| Smooth asphalt | Crr 0.004 | Good tarmac, quality road tyres at sensible pressures. |
| Rough road | Crr 0.005 | Worn or coarse-chip surfaces. |
| Gravel | Crr 0.008 | Loose or rutted unpaved roads. |
Power is only half the story — the speed it buys still has to be paced over distance and time, which is the homepage solver’s job, and produced by a gear you can actually turn, which is the gear speed calculator’s.
Limitations
- Still air is assumed. Wind is the largest real-world error and always costs more on the way out than it repays on the way back.
- CdA and Crr presets are population estimates; a wind tunnel or field test is the only way to know your own.
- The model is a steady-state average — accelerations, corners and stops are outside it.
- Air density falls with altitude and heat, making high or hot rides slightly faster than shown.
Frequently asked questions
How accurate is the watts to speed estimate?
On a calm, flat road with the right position preset, riders usually land within about 1–2 km/h of the estimate. Wind is the biggest error source — even a light headwind costs more than any other single factor — followed by an optimistic CdA preset.
Why does riding position change the result so much?
Above about 25 km/h, aerodynamic drag is most of the resistance, and it grows with the cube of speed. Moving from the hoods (CdA 0.32) to a time-trial position (0.22) cuts the drag term by nearly a third — at 200 W that is worth roughly 4 km/h.
What is CdA?
CdA is drag area: the drag coefficient multiplied by frontal area, in square metres. It bundles your size, position and clothing into the one number the drag formula needs. The presets on this page are published estimate ranges for each position, not measurements of you.
Does weight matter on a flat road?
Barely. On the flat, weight only feeds the rolling-resistance term — about 11 W of a 200 W effort in the worked example. On a climb it dominates: at a 5% gradient the same 200 W holds only about 15.5 km/h, because most of the power is lifting mass against gravity.
Why is there no wind setting?
Because a single wind number misleads: wind varies by direction, shelter and gusts, and out-and-back rides never cancel out. The model assumes still air so the estimate stays honest — treat a headwind day as a slower day than the number shown.
Keep working the numbers
Power explains speed; the rest of the site uses it. The bike speed calculator converts the speeds here into pace and ride times; the gear speed calculator finds the cadence that produces them; the triathlon pace calculator turns a sustainable wattage into a bike split and a finish time; the stationary bike calculator covers indoor power on an erg; and the bike pace chart prints the speed-to-pace table.
Bike Speed Calculator
Any two of speed, pace, distance and time — get the other two.
Open the calculator →Gear Speed Calculator
Speed from chainring, cog, wheel size and cadence.
Open the calculator →Triathlon Pace Calculator
Swim, bike and run splits for Ironman, 70.3, Olympic and Sprint.
Open the calculator →Bike Pace Chart
Every speed from 10–50 km/h with mph and pace — printable.
Open the calculator →- Martin et al. — “Modeling road cycling performance” (PubMed) — the power-balance model and its validation
- Analytic Cycling — forces on a rider — an independent implementation of the same physics
- Bicycle performance — Wikipedia — power, drag and typical CdA ranges in context