Enter your watts and get the speed they will hold, or enter a target speed and get the watts it needs. It works in km/h or mph.
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Assumes a typical road setup (hoods position, standard wheels, race tires, a well-lubricated drivetrain) at sea level. For control over aerodynamics, tires and elevation, use the full watts calculator.
The table below answers the question for a typical rider so you can sanity-check the calculator or read off a quick figure. It assumes a 75 kg rider on an 8 kg road bike, hands on the hoods, race tires, a clean drivetrain, sea level and still air.
| Power | Flat road | 4% climb | 8% climb |
|---|---|---|---|
| 100 W | 26.2 km/h (16.3 mph) | 9.4 km/h (5.9 mph) | 5.1 km/h (3.1 mph) |
| 150 W | 30.5 km/h (19 mph) | 13.6 km/h (8.4 mph) | 7.5 km/h (4.7 mph) |
| 200 W | 34 km/h (21.1 mph) | 17.3 km/h (10.8 mph) | 10 km/h (6.2 mph) |
| 250 W | 36.8 km/h (22.9 mph) | 20.6 km/h (12.8 mph) | 12.3 km/h (7.7 mph) |
| 300 W | 39.3 km/h (24.4 mph) | 23.6 km/h (14.7 mph) | 14.6 km/h (9.1 mph) |
| 350 W | 41.5 km/h (25.8 mph) | 26.3 km/h (16.4 mph) | 16.8 km/h (10.4 mph) |
Read down the flat column: each extra 50 W buys less speed than the one before, because you are fighting air. On the 8% column the steps stay almost the same size, because on a steep hill you are fighting gravity, which responds to power in a straight line.
Speed is most useful when it becomes time. Here is how long common distances take on the flat at a steady power, using the same setup:
| Power | 10 km | 40 km | 100 km |
|---|---|---|---|
| 150 W | 19:39 | 1:18:35 | 3:16:28 |
| 200 W | 17:40 | 1:10:41 | 2:56:44 |
| 250 W | 16:18 | 1:05:12 | 2:43:00 |
| 300 W | 15:16 | 1:01:04 | 2:32:41 |
A rider who holds 200 W for a whole 100 km on the flat would finish in about 2:56:44 if they never stopped. Real events add stops, hills and wind, so treat it as the floor.
Push hard on a flat road and most of your effort goes into moving air. The force of that air grows with the square of speed, and because power is force times speed, the watts needed grow with the cube. From 200 W to 400 W in this setup, speed rises only from 34 to 43.5 km/h. Going the other way, holding 40 km/h takes about 315 W. That cubic relationship is also why small aerodynamic gains are worth so much to fast riders.
This page keeps inputs short by fixing a standard road setup. If you want to know what a headwind, a lower riding position or a high-altitude ride does to the same power, open the cycling watts calculator. It runs the same physics with every assumption exposed and splits your power into gravity, drag, rolling resistance and drivetrain loss. To measure your own position, the CdA calculator works out drag area from a ride, and the rolling resistance calculator shows what your tires cost. Not sure what power you can hold? Estimate it with the FTP calculator.
The 4% and 8% columns above show how quickly gradient overtakes drag. For a specific climb, the hill climb calculator takes your power, weight and the climb's length and returns a time and a rate of ascent, which is the number climbers actually compare.
