Power to Speed Calculator

A bicycle reaches a steady speed at the exact point where the power you put through the pedals equals the power the world takes back out. This power to speed calculatorsolves that balance for you: enter the watts you can hold and it finds the velocity at which air resistance, rolling resistance, gravity and drivetrain loss consume precisely that much power.

Where your watts actually go

• Aerodynamic drag — proportional to air density, your CdA and the square of airspeed, so the power it demands rises with the cube of speed. Above roughly 25 km/h on the flat it is the biggest number on the list by a wide margin.
• Rolling resistance — the tyre deforming against the road, proportional to total weight and the coefficient of rolling resistance. It grows only linearly with speed, so it dominates when you are going slowly.
• Gravity — on a gradient, weight multiplied by the slope and by your rate of ascent. Past about 5% it swamps everything else, which is why climbing is a power-to-weight problem and flat riding is an aerodynamic one.
• Drivetrain loss — two to three percent through a clean, well-lubricated chain, and noticeably more through a dirty one.

Using it as a flat road power to speed calculator

Set the gradient to zero and you have a cycling power to speed calculator for a flat road — the case most riders care about, and the one where aerodynamics decides everything. As a rough guide, an 85 kg rider-and-bike total in still air will see something like:

• 150 W → roughly 30–31 km/h on the hoods
• 200 W → roughly 33–35 km/h in the drops
• 250 W → roughly 37–38 km/h in the drops
• 300 W → roughly 40–42 km/h in an aero position

Notice how the returns fall away. Adding 50 watts to 150 buys you around 4 km/h; adding the same 50 watts to 250 buys barely 2.5. That cubic relationship is exactly why every experienced time triallist spends money on position before power.

Climbing: the same tool, a different answer

Enter a gradient and the bike power to speed calculator shifts almost all of your watts into the gravitational term. On a 7% climb an 85 kg total at 250 watts moves at roughly 13–14 km/h, and CdA becomes nearly irrelevant while every kilogram you or the bike carry costs measurable time. This is the calculation to run before you decide whether deeper wheels or lighter wheels are the better purchase for the terrain you actually ride.

Getting a realistic prediction

1. Enter total weight, not body weight. Rider plus bike plus kit, bottles, tools and food. That is often 9–11 kg more than the number on your bathroom scale.
2. Pick the position you will really hold. CdA ranges from about 0.40 sitting up to below 0.25 in a good time trial tuck. It is the single largest lever in the model.
3. Set the surface honestly. Rolling resistance on gravel can be double that of smooth tarmac, and wet roads sit somewhere in between.
4. Account for altitude. Thinner air means less drag, which is why hour record attempts are made at altitude rather than at sea level.

Set those four inputs carefully and the prediction usually lands within a few percent of a real ride. If your actual speed is consistently below the calculator’s figure, the culprit is almost always wind: the model assumes still air, and even a light headwind component moves the answer substantially. To go the other way and work out the wattage a target speed demands, use thespeed to power calculator.