By Zencalcy Editorial TeamLast reviewed About us
Estimate the steady-state power required to hold a given speed, using the standard road-load equation that balances rolling resistance, the pull of gravity on a slope, and aerodynamic drag. Enter your speed, total mass, and grade, and adjust the aerodynamic and rolling terms if you have better numbers.
Estimate the watts needed to hold a speed on the road.
P = (Crr × m × g + m × g × grade + 0.5 × rho × CdA × v²) × v
Defaults are Crr 0.005, air density rho 1.225 kg/m³, CdA 0.32 m², and g 9.81 m/s². This is a steady-state estimate on smooth tarmac with no wind, and it ignores drivetrain losses of roughly two to three percent.
| Symbol | Meaning |
|---|---|
| v | Road speed in metres per second, converted from km/h. |
| m | Total mass in kilograms, rider plus bike plus kit. |
| grade | Road gradient as a fraction, so 5% becomes 0.05. |
| Crr | Coefficient of rolling resistance, about 0.005 on good tarmac. |
| rho | Air density in kilograms per cubic metre, about 1.225 at sea level. |
| CdA | Drag area in square metres, roughly 0.32 for a road position. |
Holding 32 km/h (8.89 m/s) on level ground at 80 kg with the default values needs about 173 W: around 35 W for rolling resistance, nothing for gravity, and about 138 W to push the air.
The power needed to hold a speed on a bicycle is the sum of three resistance terms, each multiplied by velocity. Gravity on a slope costs mass times gravity times the sine of the incline. Rolling resistance costs the rolling coefficient times mass times gravity times the cosine of the incline. Air drag costs half the air density times the drag area times velocity squared.
Multiplying the total force by speed gives power. Because the aerodynamic force grows with the square of speed, the power to overcome it grows with the cube, so doubling speed needs roughly eight times the aero power. That is why the flat road load rises so steeply and why a small increase in top speed is so expensive.
The drag area, written CdA, combines the frontal area of rider and bike with a shape factor, and it is the single biggest lever on flat speed. Lowering your torso, tucking your elbows, or using aero equipment reduces CdA and cuts the cubic drag term directly.
The rolling term carries the coefficient Crr, which rises with soft or rough tyres and falls with firm, supple ones, and the gravity term depends only on your total mass and the gradient you face. Climbing is dominated by the gravity term, which is linear in mass, so losing weight pays off most on hills while cutting drag pays off most on the flat.
Continue with the Cycling Power Zones Calculator, Cycling Time Calculator, or the FTP Calculator — or browse more cycling tools and all sports calculators.
Aerodynamic drag grows with the square of speed, so its share of the total rises quickly. Above about 30 km/h it usually dominates the power you need.
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Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026