Cycling power and speed calculator
What does your speed cost in watts? On the flat, 20 mph (32 km/h) takes roughly 165 W for a 187 lb rider on the hoods, and only about 130 W in an aero position. Convert in both directions, add gradient and wind, and see on a chart how much of your power goes to gravity, to the air and to the road.
152 W
Where does the power go?
The CdA and Crr values are stated model assumptions (printed on the buttons), averaged for a typical rider and bike. Air density: 1.19 kg/m³, drivetrain efficiency 97.5%.
How is cycling power calculated?
This uses the full cycling power equation, the same model race analysts use: the sum of the three resistances every rider has to overcome.
P = M x g x v x (sin a + Crr x cos a) <- gravity + rolling + 0.5 x rho x CdA x (v + w)^2 x v <- aerodynamic drag (divided by drivetrain efficiency, 0.975) M - rider plus bike mass [kg] g - 9.81 m/s^2 a - road angle Crr - rolling resistance (0.004-0.008) rho - air density 1.19 kg/m^3 CdA - drag area (0.23-0.32 m^2) v - riding speed, w - headwind speed [m/s]
Going from speed to power is arithmetic. Going the other way is not: speed appears cubed, so the equation cannot be rearranged algebraically, and we solve it numerically by bisection instead. Every assumption in the model, the CdA of each position and the Crr of each surface, is printed on the buttons rather than hidden in the code.
Worked example: 187 lb rider, 20 lb bike, flat road, smooth asphalt, no wind, 20 mph on the hoods. Aerodynamic drag takes about 125 W, rolling resistance 35 W and drivetrain losses 4 W, for roughly 165 W total. Put that same rider on a 6% climb at the same power and speed collapses to about 7.5 mph, because nearly all of it now goes into lifting the rider. That is exactly what the chart under the result shows.
Related calculators
Not sure what power you can hold? Establish it with a test in the FTP calculator. Planning a specific climb? Get the time in the climb time calculator. And to make sure those watts turn into speed rather than heat, set your tire pressure properly: badly chosen pressure can cost more than a decent aero upgrade saves.
Frequently asked questions
- How many watts do you need to ride 20 mph?
- On the flat with no wind, hands on the hoods, a 187 lb (85 kg) rider on a 20 lb bike needs roughly 165 W. In an aero position the same speed costs about 130 W, because drag falls with CdA. For comparison, 15 mph takes only about 90 W: aerodynamic power rises with the cube of speed.
- Why does air resistance dominate above 20 mph?
- The power needed to push through the air rises with the cube of speed: going twice as fast takes eight times the aerodynamic watts. At 20 mph on the flat, drag is already around three quarters of your total power, while rolling resistance only rises linearly. That is why position matters more than equipment weight at road speeds.
- How much power does drafting save?
- Riding in a wheel cuts aerodynamic drag by roughly 25-40% depending on spacing and group size, which at 22 mph can mean 60-100 W less. That is why a group rides at speeds no solo rider could hold. This calculator models solo riding, so subtract accordingly when you are sheltered.
- What is CdA in cycling?
- CdA is the drag coefficient multiplied by frontal area for rider and bike together, expressed in square metres. The lower it is, the less power the air takes. Typical values: about 0.32 m² on the hoods, 0.28 m² in the drops and 0.23 m² in an aero position. Professionals on time trial bikes go below 0.20 m².
- How does wind change the power you need?
- Drag depends on your speed through the air, not over the ground. A 11 mph (5 m/s) headwind while riding 20 mph means the air sees 31 mph, which nearly doubles the power required. A tailwind works the other way. That is why the same stretch of road can cost 150 W one day and 250 W the next.