Cycling Power Calculator

About the Cycling Power Calculator

Power, measured in watts, is the most honest number in cycling. Speed depends on the wind, the gradient and the road; heart rate drifts with heat and fatigue; but the power you put into the pedals is exactly the work you are doing. Power meters have become common on road bikes and smart trainers, and training plans are now written in watts and watts per kilogram.

This cycling power calculator estimates the power needed to hold a given speed, from the weight of the rider and bike, the gradient, your riding position, the road surface, the wind and the altitude. It shows the total in watts, the power-to-weight ratio, how the effort splits between air resistance, rolling resistance and climbing, and the energy used per hour. It uses the standard physics model that power meters and cycling simulators are built on.

How to Use the Cycling Power Calculator

Enter the rider weight and the bike and kit weight, and choose kg or lb.

Enter the speed and choose km/h or mph.

Enter the gradient as a percentage; use a negative number for a descent.

Choose the riding position and the road surface.

Optionally, enter a headwind (negative for a tailwind), the altitude, or your own measured CdA.

The Formula

Three forces slow a rider down. The power needed is their sum multiplied by the speed, plus a small allowance for the drivetrain:

  gravity  = m × g × sin θ
  rolling  = m × g × cos θ × Crr
  air      = ½ × ρ × CdA × (v + headwind)²

  power    = (gravity + rolling + air) × v ÷ (1 − 3% drivetrain loss)

  m   = rider + bike mass (kg)         g   = 9.80665 m/s²
  θ   = angle of the slope             Crr = rolling resistance coefficient
  ρ   = air density (1.225 kg/m³ at sea level)
  CdA = drag coefficient × frontal area (m²)
  v   = speed (m/s)

Step-by-Step Example

A 75 kg rider on a 9 kg bike rides at 30 km/h on flat ground, hands on the hoods (CdA 0.32), on an average road (Crr 0.005).

  speed    30 ÷ 3.6 = 8.333 m/s
  air      ½ × 1.225 × 0.32 × 8.333² = 13.61 N
  rolling  84 × 9.80665 × 0.005     = 4.12 N
  gravity  0 (flat)

  power at the wheel  (13.61 + 4.12) × 8.333 = 147.8 W
  power at the pedals 147.8 ÷ 0.97           = 152 W

That is 152 watts, or 2.03 W/kg for the rider. Air resistance takes about 113 W (77 percent) and rolling resistance about 34 W (23 percent).

Power Needed at Different Speeds

For the same rider on the flat, on the hoods:

  Speed      Power
  25 km/h     97 W
  30 km/h    152 W
  35 km/h    227 W
  40 km/h    324 W

Going from 30 to 40 km/h — a third faster — needs more than twice the power. This is the defining fact of road cycling: air resistance grows with the square of speed, so the power to overcome it grows with the cube of speed.

Air Resistance and Riding Position

On flat roads above about 20 km/h, air resistance is the largest force. The CdA — drag coefficient multiplied by frontal area — measures how much air you push aside. Typical values are about 0.40 m² sitting upright on the tops, 0.32 on the hoods, 0.30 in the drops and 0.25 on aero bars. At 40 km/h, moving from the hoods to aero bars cuts the power needed from about 324 W to 264 W, a saving of around 60 watts for no extra fitness. Tight clothing, an aero helmet and riding in another rider's slipstream reduce drag further; drafting closely can save a quarter or more of the effort.

Climbing: Where Weight Matters

On a climb, lifting the rider and bike takes over. On a 5 percent gradient at 15 km/h, the same 84 kg needs about 209 W, and about 85 percent of it goes into climbing. Air resistance at that speed is almost irrelevant. This is why lighter riders climb well and why watts per kilogram is the key measure for hills: power divided by weight decides climbing speed, while raw power matters more on the flat.

Rolling Resistance

Rolling resistance comes from tyres deforming against the road. It depends on the tyre, the pressure and the surface: roughly 0.004 for fast tyres on smooth tarmac, 0.005 on an average road, 0.008 on a rough surface and 0.012 or more on gravel. On gravel at 20 km/h, rolling resistance takes more power than the air does. Wider tyres at sensible pressures often roll faster on real roads than narrow, rock-hard ones.

Wind and Altitude

A headwind adds to the airspeed: riding at 30 km/h into a 10 km/h headwind means pushing through air at 40 km/h, and the power rises from about 152 W to 243 W. Altitude works the other way. Air is thinner higher up — about 1.11 kg/m³ at 1,000 metres and 1.01 kg/m³ at 2,000 metres — so the same speed takes less power, about 131 W at 2,000 m in the example. That is why hour records are often attempted on high-altitude tracks.

Understanding Your Result

The headline is the power at the pedals, in watts.

The power to weight line divides it by the rider's weight.

The where the power goes line splits the effort between air, rolling resistance and climbing, and shows the drivetrain loss.

The energy line gives the work done in kilojoules per hour. Because the human body is only about a quarter efficient, kilojoules of work are roughly equal to kilocalories burned — a useful rule of thumb for fuelling long rides.

When Should You Use This Calculator?

Use it to estimate the power a speed or climb will need before a ride or race.

Use it to see how much a better position, lighter bike or faster tyres would save.

Use it to set targets on a smart trainer or plan pacing for a time trial.

Use it to understand why speed is so costly and why hills reward low weight.

Common Mistakes

Forgetting the bike and kit. The mass includes the bike, bottles, shoes and clothing.

Using a flattering CdA. Most riders spend most of their time on the hoods.

Ignoring the wind. A light headwind changes the power more than a few kilograms.

Treating the estimate as a measurement. Only a power meter measures your real watts.

Comparing raw watts on climbs. Use watts per kilogram when the road goes up.