Speed to Power Calculator

Every cyclist has a speed in mind — the average that gets you round the loop in an hour, the pace that holds the wheel in front, the number on the time trial start sheet. Thisspeed to power calculator tells you the wattage that speed costs. It takes the standard cycling power model and solves it in the direction riders actually plan in: speed in, watts out.

The calculation behind the number

Total power is the sum of four terms, all evaluated at the speed you name. Aerodynamic drag is half the air density multiplied by your CdA and by the cube of airspeed. Rolling resistance is the coefficient of rolling resistance multiplied by total weight and by speed. Climbing power is total weight multiplied by gravity, by the gradient and by speed. A drivetrain efficiency factor of about 97–98% is then applied, because the power you produce at the pedals is slightly more than the power that reaches the road.

Because the aerodynamic term is cubic, the wattage curve steepens sharply as speed rises. Going from 30 to 35 km/h on the flat is a 17% increase in speed but roughly a 55% increase in power. Going from 40 to 45 km/h costs more watts again for the same 5 km/h. Seeing that curve laid out is the fastest cure for unrealistic pacing plans.

Typical wattage at typical speeds

For an 85 kg rider-and-bike total on flat tarmac in still air, riding on the hoods, expect roughly:

• 25 km/h → about 95–110 W
• 30 km/h → about 150–170 W
• 35 km/h → about 230–255 W
• 40 km/h → about 335–370 W

Drop into the drops or onto aero bars and every one of those figures falls, because the term that is growing fastest is the one CdA controls. Run the same numbers on a 5% climb and the ranking of what matters inverts: weight, not position, sets the price.

Pacing a time trial or a climb

This is where a speed to power calculator earns its place. Take the distance and your target time, work out the average speed that requires, and enter it with the correct gradient and position. The calculator returns the power you must average. Compare it honestly against your FTP:

1. Below 95% of FTP — sustainable for an hour or more. A realistic target.
2. 95–105% of FTP — a genuine 20–40 minute effort. Achievable, but it will hurt and leaves no margin for a headwind.
3. Above 105% of FTP — not sustainable over a long effort. Either the target time needs to move, or the position and equipment do.

Drafting changes the arithmetic

Sitting in another rider’s slipstream cuts the aerodynamic term by roughly a quarter to a third, and deep inside a bunch it can reach 40% or more. Set the drafting option and the calculator applies that reduction, which is why holding 40 km/h in a group can be an endurance-pace effort while the rider on the front is at threshold. It also explains why a turn on the front feels so disproportionately expensive.

Getting an accurate figure

The same discipline applies as with any power model: use total weight including bike, kit and bottles; pick the position you will hold for the entire effort rather than the one you manage for five minutes; match the surface to the road; and remember that the model assumes still air, so budget extra for any headwind on the day. To run the problem the other way and see what speed a given wattage buys, use thepower to speed calculator.