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Cycling Power Calculator
This cycling power calculator estimates how many watts you need to ride at a given speed — or, in reverse, how fast a given number of watts will take you on flat ground. Enter your speed, your combined rider and bike weight, the gradient and the headwind, and the calculator splits the power into its three parts: rolling resistance, aerodynamic drag and gravity, so you can see exactly where your effort is going.
Power is the most honest measure in cycling. Speed depends on the wind, the hills and the road, but watts are watts: 200 watts is 200 watts whether you are climbing, cruising or sprinting into a headwind. That is why power meters have become the standard training tool for serious riders. This calculator gives you a good estimate of your power without a meter, and it also shows why small changes — a lighter bike, a lower position, or a tailwind — make such a big difference to your speed.
Gradient is the steepness of the hill: 0 is flat, 5 is a steady climb, 10 is steep. A negative gradient is a descent. Power is split into rolling resistance, air resistance and gravity.
How it works
To keep a bike moving at a steady speed, your legs have to overcome three forces. Rolling resistance is the friction of the tyres on the road: it grows in proportion to your total weight and your speed. Aerodynamic drag is the air pushing back against you and your bike: it grows with the square of your airspeed, and because power is force times speed, the power needed to beat the air grows with the cube of speed. Gravity only matters on hills, where part of your power goes into lifting your weight up the slope.
The three parts in detail. Rolling resistance uses a coefficient of 0.005, a typical value for road tyres on smooth tarmac, and the total mass of rider plus bike. Aerodynamic drag uses an air density of 1.225 kg per cubic metre and a drag area (CdA) of 0.32 square metres, which is a typical value for a rider on the hoods in normal kit. Airspeed is your ground speed plus any headwind, because the wind adds to the air you have to push through. Gravity uses the slope angle from the gradient: a 5% gradient means the road rises 5 metres for every 100 metres travelled. On a descent the gravity term goes negative, which is why you can coast downhill — gravity is doing the work for you.
Why speed is so expensive. Because drag power grows with the cube of speed, small speed gains need big power gains. Going from 25 to 30 km/h on the flat needs about 148 watts instead of 95 — more than 50% more power for 20% more speed. Going from 30 to 40 km/h needs roughly double the power again. That is also why aerodynamics matter so much: dropping your CdA from 0.32 to 0.28 by getting lower on the bike saves more watts at high speed than losing several kilos of weight ever could on the flat.
Reverse mode: speed from watts. If you know your power — from a power meter, a smart trainer, or a fitness test — the calculator works backwards. It tries speeds until it finds the one whose power demand matches your watts on flat ground with no wind. For example, 150 watts on the flat takes an 85 kg rider to about 30 km/h.
Worked example
Here is a worked example, so you can check the maths yourself.
Step 1: A rider weighing 75 kg rides a 10 kg bike, so the total mass is 85 kg. The speed is 25 km/h, the road is flat (0% gradient) and there is no wind.
Step 2: Convert the speed to metres per second. 25 ÷ 3.6 = 6.94 m/s.
Step 3: Rolling resistance power = 0.005 × 85 × 9.81 × 6.94 = 29.0 watts.
Step 4: Aerodynamic drag power = 0.5 × 1.225 × 0.32 × 6.94³ = 65.6 watts.
Step 5: Gravity power = 0 watts, because the road is flat.
Step 6: Add them up. 29.0 + 65.6 + 0 = 94.6 watts.
Try it yourself: enter 25 km/h, 85 kg total weight, 0% gradient and 0 km/h wind, and press Calculate. The tool gives exactly the same result — About 95 watts — with a breakdown of 29.0 W rolling, 65.6 W drag and 0 W gravity.
FAQ
How many watts does the average cyclist produce?
A casual rider on a flat road typically holds 75 to 150 watts. A fit club rider might average 200 watts for an hour, a good amateur racer 250 to 300 watts, and a professional 350 to 400 watts or more. These numbers scale with body weight.
What is a good watts per kilo?
Watts per kilo is your power divided by your body weight, and it decides how fast you climb. Around 2 watts per kilo is a recreational rider, 3 is a fit club rider, 4 is a strong amateur, and 5 or more is professional level. On flat ground, raw watts matter more.
Why does going faster take so much more power?
Because air drag grows with the cube of your speed. Doubling your speed needs about eight times the power to push through the air. That is why the jump from 25 to 30 km/h costs over 50 extra watts, while the jump from 20 to 25 km/h costs far less — and why aerodynamic position matters more than weight on flat roads.
How much power do I need to climb a hill?
On a climb, gravity dominates and drag almost disappears. Power for a climb is roughly your weight times gravity times your vertical speed, so a 5% gradient at 25 km/h needs about 384 watts for an 85 kg rider and bike — four times the power of the same speed on the flat.
Can I use this calculator without a power meter?
Yes, that is the point. It estimates your power from your speed, weight, gradient and wind, which is accurate enough for training, comparing setups and setting targets. If you later get a power meter, you can check the estimate against real data.