Cycling Power Calculator
The watts needed to hold a speed, split between air resistance, rolling resistance and the gradient.
Results
What this tool does
Three forces stand between a cyclist and the horizon: the air, the tyres and the hill. On the flat at any real speed the air dominates completely, which is why a tuck or a skinsuit buys more than a lighter bike ever will. Point the road upwards and the balance flips within a few percent of gradient, and from there it is all weight. CdA of around 0.32 m² is a rider on the hoods; on the drops it is nearer 0.28.
Formula
P = (Crr m g v + 0.5 rho CdA v³ + m g slope v) / yield
Variables
| Symbol | Meaning | Unit |
|---|---|---|
kg | Total weight | kg |
sp | Speed | km/h |
cd | Drag area CdA | m² |
cr | Rolling resistance coefficient | — |
gr | Gradient | % |
ro | Air density | kg/m³ |
ef | Drivetrain efficiency | % |
PW | Power | W |
PA | Power against the air | W |
PR | Power against rolling resistance | W |
PG | Power against the gradient | W |
WK | Power per kilogram | W/kg |
Worked example
- Total weight80 kg
- Speed30 km/h
- Drag area CdA0.32 m²
- Rolling resistance coefficient0.005
- Gradient0 %
- Air density1.225 kg/m³
- Drivetrain efficiency97 %
- Power150.6 W
- Power against the air113.43 W
- Power against rolling resistance32.69 W
- Power against the gradient0.00 W
- Power per kilogram1.883 W/kg
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
Frequently asked questions
Why is going faster so disproportionately hard?
Air resistance takes power proportional to the cube of speed. Going from 30 to 35 km/h is only 17% faster but needs nearly 60% more power against the air, and above about 25 km/h on the flat the air is already most of what you are fighting. Climbing is the opposite: gravity is proportional to speed, so on a steep hill your weight matters far more than your position on the bike.