Cycling Power and Speed Calculator

Written byOleh Soloninupdated 25 September 2026

Solve for

Power, W

Rider weight, kg

Bike weight, kg

Gradient, %

Frontal area CdA, m²

0.4 on the tops, 0.32 on the hoods, 0.25 in an aero position.

Rolling resistance Crr

Result

Result33.35
What was calculatedSpeed, km/h
Watts per kilogram2.67
Share going to air drag, %80.3%
Total mass, kg84
What matters
In still air the drag force grows with the square of speed and the power to overcome it with the cube: twice the speed takes eight times the power for the air.

This result is informational only and is not professional advice.

The power for the air grows with the cube of speed

Links watts to kilometres an hour, accounting for mass, gradient, rolling resistance and aerodynamics.

On the flat almost all your power goes into the air, and not linearly: drag grows with the square of speed, and the power to overcome it with the cube. Going from 30 to 40 km/h costs not a third more watts but 2.1 times as many.

On a climb the picture turns over. The main cost there is lifting mass against gravity, speeds are low and aerodynamics barely matters — which is why time-trial specialists and climbers are built so differently.

How to use

Choose what to solve for: speed from power, or power from speed.
Enter the power in watts or the speed in km/h.
Enter rider and bike weight and the road gradient in percent.
If you like, adjust CdA — the aerodynamics of your position — and the tyres' rolling resistance.

Speed and power on the flat

Every 50 watts buys less speed: from 100 to 150 W is plus 4.5 km/h, from 250 to 300 only 2.6. Meanwhile the share going into the air rises: 80 percent at 200 W, 87 at 400.

Rider 75 kg, bike 9 kg, CdA 0.32, Crr 0.005, a flat road with no wind. Every row comes from the calculator on this page.
Power, W	Speed, km/h	W/kg	Share to air drag
100	25.3	1.33	70.2%
150	29.82	2	76.6%
200	33.35	2.67	80.3%
250	36.3	3.33	82.9%
300	38.86	4	84.7%
400	43.2	5.33	87.3%

A climb changes everything

At the same 250 watts on a 2 percent climb, speed drops from 36.3 to 27.4 km/h and the air's share from 83 to 36 percent. At 6 percent the air takes only 6.2 percent of the power, and weight decides: a 60 kg rider climbs an 8 percent grade at 14.4 km/h, a 90 kg rider at 10.4.

Rider 75 kg, bike 9 kg, CdA 0.32. Every row comes from the calculator on this page.
Gradient	Speed at 250 W, km/h	Share to air drag
0%	36.3	82.9%
2%	27.36	35.5%
4%	20.2	14.3%
6%	15.32	6.2%
8%	12.12	3.1%
10%	9.97	1.7%

What this calculator shows

Speed from power, or power from speed
Power-to-weight in watts per kilogram
What share of your power the air is taking

What to keep in mind

Air density is taken as 1.225 kg/m³ at sea level and drivetrain efficiency as 97 percent.
Wind is not modelled: a headwind adds its speed to the calculation and power climbs very fast.

FAQs

How many watts do I need for 30 km/h?

About 152 W on a road bike in a normal position, on a flat road with no wind. 35 km/h takes about 227 and 40 km/h about 324.

How fast will I ride at 200 watts?

About 33.4 km/h on the flat at 75 kg, a 9 kg bike and CdA 0.32. In an aero position with CdA 0.25 the same 200 W gives 36 km/h.

What is CdA?

The drag coefficient multiplied by frontal area. The calculator uses 0.32 by default; 0.4 for the tops and 0.25 for an aero tuck are example inputs, and your own figure depends on your build and position. At 200 W those give 31.1, 33.4 and 36 km/h.

Which matters more — weight or aerodynamics?

On the flat, aerodynamics: 80 percent of the power goes into the air. From about 4 percent uphill the air takes around a seventh, and weight takes over.

How many watts per kilo does a cyclist produce?

The calculator shows power to weight: 200 W at 75 kg is 2.67 W/kg. Professional racers hold more than five watts per kilo for an hour.

Does it account for wind?

No. A headwind raises your speed through the air, and the drag force grows with its square; the power is that force times your speed along the road. So even a light headwind costs noticeable speed at the same watts.

Worked example

Two hundred watts on the flat

Input: 200 W, rider 75 kg, bike 9 kg, CdA 0.32

Output: About 33.4 km/h, with 80% of the power going to air

Note: Four fifths of the work is pushing air aside. Which is why position and clothing pay more on the flat than shaving a kilogram off the bike.

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Result

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