How this cycling power calculator works
Cycling power is the sum of three resistive forces, each multiplied by your speed through the air (which is your ground speed plus any headwind):
- Rolling resistance: total weight (rider + bike) x 9.81 x rolling resistance coefficient (Crr) x speed. Crr depends on tyre type and road surface, from about 0.004 on smooth asphalt to 0.012 on gravel.
- Aerodynamic drag: 0.5 x air density (1.225 kg/m³ at sea level) x drag area (CdA) x effective wind speed squared x rider speed. This is the dominant force at speeds above about 25 km/h, since the power needed to overcome drag grows with the cube of speed (the drag force itself grows with the square of speed).
- Gravity: total weight x 9.81 x gradient (as a fraction) x speed. Zero on flat ground, and it quickly dominates on climbs.
The three components are added together and divided by a typical drivetrain efficiency of 96 percent, since some power is lost to friction in the chain and derailleurs before it reaches the road. This model matches the physics used by most cycling power estimation tools and is a reasonable approximation for a road bike in normal conditions, though real-world power also varies with drafting, tyre pressure, bike stiffness and rider fatigue.
Power to weight ratio (W/kg)
Watts per kilogram of rider body weight is the standard way to compare cycling performance across riders of different sizes, especially on climbs where gravity dominates. As a general guide, under 1.5 W/kg is untrained, 1.5 to 2.5 W/kg is recreational, 2.5 to 3.5 W/kg is a trained club rider, 3.5 to 4.5 W/kg is competitive, and above 4.5 W/kg sustained is elite or professional level.
Worked example
Target speed: 30 km/h, rider 75 kg, bike 10 kg, flat road (0% gradient), no wind, normal asphalt (Crr 0.005), hoods position (CdA 0.32)
Speed in m/s: 30 / 3.6 = 8.33 m/s. Total weight: 85 kg.
Rolling resistance: 85 x 9.81 x 0.005 x 8.33 = 35 W
Aerodynamic drag: 0.5 x 1.225 x 0.32 x 8.33 x 8.33 x 8.33 = 113 W
Gravity: 0 W (flat road)
Subtotal: 35 + 113 + 0 = 148 W. After dividing by drivetrain efficiency (0.96): 148 / 0.96 = 154 W.
W/kg: 154 / 75 = 2.05 W/kg (recreational level)
Energy rate: 154 x 3.6 = 554 kJ/hour of mechanical work. Because a cyclist's body is only around 20 to 25 percent efficient, the dietary energy burned is close to the mechanical work measured in kJ, so this works out to roughly 554 kcal/hour.
Over a 40 km ride at a steady 30 km/h, ride time is 40 / 30 = 1 hour 20 minutes.
Frequently asked questions
How many watts do I need to cycle at 30 km/h? As shown in the worked example above, a 75 kg rider on a 10 kg bike on flat ground with no wind needs around 150 to 155 watts to hold 30 km/h. Wind, gradient and riding position all change this significantly.
How is cycling power calculated? By adding rolling resistance, aerodynamic drag and the gravity component (on a gradient), then adjusting for drivetrain loss, as set out in the method above.
How long will a ride take at a given power output? Switch to the Power to Speed and Time tab, enter your power output and ride distance, and the calculator solves for the speed that power would produce under your conditions, then divides your distance by that speed.
Related calculators
Sources: Standard cycling power model (rolling resistance, aerodynamic drag, gravity) as widely used in cycling physics references and power estimation tools, including work derived from Martin et al., "Validation of a Mathematical Model for Road Cycling Power" (Journal of Applied Biomechanics, 1998).
This calculator provides estimates only, based on a simplified road bike physics model at sea level. It does not account for drafting, tyre pressure, bike stiffness, altitude, temperature or rider fatigue, all of which affect real-world power and speed.
