How to calculate cycling power
The power you put into the pedals goes into three resistances: air drag, rolling resistance and – uphill – gravity. On top of that about 2.4 % is lost in the chain and bearings.
- Air drag
- ½ × air density × CdA × (speed + headwind)². It grows with the square of speed, and the power needed to overcome it with the cube.
- Rolling resistance
- Rolling resistance coefficient × total weight × 9.81. Smooth asphalt is around 0.004, gravel around 0.010.
- Gravity
- Total weight × 9.81 × gradient (more precisely: the sine of the slope angle). On climbs this is by far the largest share.
- Power
- (Sum of resistances) × speed ÷ 0.976 (drivetrain efficiency).
What costs the most watts
On the flat it is almost all air: at 25 km/h about 70 % of your power already goes into air drag, from 30 km/h more than three quarters. A lower riding position therefore saves more than any lighter wheelset.
On climbs the picture flips: at 8 % and 15 km/h most of your power goes into lifting rider and bike – every kilo counts, and your power-to-weight ratio in W/kg decides how fast you reach the top.
Wind acts like extra speed: a headwind costs disproportionately much, a tailwind saves accordingly.
Tables: watts by speed and gradient
| Speed | Upright (MTB/trekking) CdA 0.45 | Hoods CdA 0.32 | Drops CdA 0.28 | Aero (forearms) CdA 0.24 |
|---|---|---|---|---|
| 20 km/h | 70 W | 56 W | 52 W | 48 W |
| 25 km/h | 120 W | 94 W | 86 W | 78 W |
| 30 km/h | 192 W | 147 W | 133 W | 119 W |
| 35 km/h | 290 W | 218 W | 196 W | 174 W |
| 40 km/h | 418 W | 311 W | 278 W | 245 W |
| 45 km/h | 582 W | 429 W | 382 W | 335 W |
| Gradient | 150 W | 200 W | 250 W | 300 W |
|---|---|---|---|---|
| 0 % | 30.3 km/h | 33.9 km/h | 36.9 km/h | 39.5 km/h |
| 2 % | 20.0 km/h | 24.2 km/h | 27.8 km/h | 30.8 km/h |
| 4 % | 13.4 km/h | 17.1 km/h | 20.5 km/h | 23.5 km/h |
| 6 % | 9.7 km/h | 12.7 km/h | 15.5 km/h | 18.2 km/h |
| 8 % | 7.5 km/h | 9.9 km/h | 12.3 km/h | 14.5 km/h |
| 10 % | 6.1 km/h | 8.1 km/h | 10.1 km/h | 12.0 km/h |
| 12 % | 5.2 km/h | 6.9 km/h | 8.6 km/h | 10.2 km/h |
| W/kg | Power | Speed | Time | VAM (m/h) |
|---|---|---|---|---|
| 2.0 | 150 W | 7.5 km/h | 1:20 h | 600 m |
| 2.5 | 188 W | 9.3 km/h | 1:04 h | 750 m |
| 3.0 | 225 W | 11.1 km/h | 54:02 min | 890 m |
| 3.5 | 263 W | 12.8 km/h | 46:43 min | 1,030 m |
| 4.0 | 300 W | 14.5 km/h | 41:15 min | 1,160 m |
| 5.0 | 375 W | 17.8 km/h | 33:42 min | 1,420 m |
VAM (vertical metres per hour) is approximate: 10 km at 8 % is about 800 m of climbing.
Frequently asked questions
How many watts do you need to ride at 30 km/h?
On the flat, without wind, with a 75 kg rider and an 8.5 kg bike: about 147 W on the hoods, 133 W in the drops and 119 W in an aero position. Sitting upright like on a trekking bike it is about 192 W. 30 km/h is 18.6 mph.
How fast can I ride with 200 watts?
With 200 W you ride about 33.9 km/h (21.0 mph) on the flat (hoods, 83.5 kg total, no wind), 35.3 km/h in the drops. On a 6 % gradient it drops to 12.7 km/h.
How many watts does a headwind cost?
A lot, because air drag grows with the square of the air speed. Riding at 30 km/h takes 147 W without wind and 345 W into a 20 km/h headwind. With a 20 km/h tailwind 47 W is enough.
How much does losing one kilo help?
A lot on climbs, little on the flat. At 8 % and 15 km/h every kilo costs about 3.6 W, on the flat at 30 km/h only about 0.4 W. There a more aerodynamic position helps far more.
What does watts per kilogram (W/kg) mean?
Power-to-weight is your power divided by your body weight. On climbs it almost alone determines your speed, because the force of gravity is proportional to weight. As a rough guide for one-hour power: around 2 W/kg for recreational riders, 3 to 4 W/kg for well-trained amateurs, while pros are well above 5 W/kg.
How many watts do you need to climb?
On an 8 % gradient a 75 kg rider on an 8.5 kg bike needs about 202 W for 10 km/h and about 310 W for 15 km/h. The table above shows how fast you climb at 150 to 300 W on different gradients.
How accurate is the power calculator?
It assumes steady riding, typical values for drag area (CdA) and rolling resistance, and a fixed drivetrain efficiency of 97.6 %. Depending on body size, clothing, tyres and road surface, real values can differ by 10 to 20 %. It is well suited for comparisons – faster, heavier, steeper – but it does not replace a power meter.