Cycling Power Output Calculator
This calculator works out how much power you are putting through the pedals, in watts, based on your speed and riding conditions, or flips the sum around to estimate the speed a given power output would produce. In Speed to Power mode you enter your cycling speed, rider weight, bike weight, road gradient, wind conditions and road surface, plus your Functional Threshold Power (FTP) if you know it. In Power to Speed mode you enter a target wattage, rider weight and gradient instead. The results show your power output or estimated speed, your watts per kilogram (W/kg) ratio, and your energy burn rate in kJ and kcal per hour, along with a power category from untrained through recreational, trained and competitive to elite. Add your FTP and it also flags which training zone your effort sits in, from active recovery up to anaerobic. The maths accounts for aerodynamic drag, rolling resistance from your tyres and road surface, and the extra effort needed on climbs, with a standard four percent drivetrain loss built in. Use it to check whether a target average speed is realistic for your fitness, to compare your output against typical rider benchmarks, or to see how much a headwind or gravel will cost you in watts. These are indicative physics based estimates only, as your actual power depends on riding position, drafting, fatigue and bike setup.
Estimates based on a standard road bike physics model (CdA 0.32, sea-level air density). Real-world power varies with rider position, drafting, fatigue and equipment.
How the calculation works
Cycling power is the sum of three resistances, each multiplied by speed:
- Aerodynamic drag: 0.5 x air density (1.225 kg/m) x drag area (CdA 0.32 for road bike on hoods) x effective wind speed squared x rider speed. This dominates at higher speeds.
- Rolling resistance: total weight x gravity (9.81) x rolling resistance coefficient (Crr) x speed. Crr varies from 0.004 on smooth asphalt to 0.012 on gravel.
- Gravity: total weight x 9.81 x gradient fraction x speed. Zero on flat ground, dominant on climbs.
The three components are added and divided by 0.96 to account for a typical 4 percent drivetrain loss. Energy burn rate is simply watts x 3.6 to convert to kJ per hour (since 1 W = 1 J/s and there are 3,600 seconds in an hour), then divided by 4.184 to convert kJ to kcal.
Power categories by W/kg
Watts per kilogram (W/kg) is the standard way to compare riders of different sizes, especially for climbing. Typical ranges for sustained efforts:
| W/kg | Category |
|---|---|
| Below 1.5 | Untrained |
| 1.5 to 2.5 | Recreational |
| 2.5 to 3.5 | Trained |
| 3.5 to 4.5 | Competitive |
| Above 4.5 | Elite |
FTP training zones
When you enter your Functional Threshold Power, the calculator shows which training zone your current output sits in. FTP-based zones let you target specific physiological adaptations, from easy aerobic base building (Zone 2) through threshold work (Zone 4) to short, hard VO2 max intervals (Zone 5).
Worked example
75 kg rider, 10 kg bike, 28 km/h on flat road, no wind, normal asphalt (Crr 0.005)
Speed in m/s: 28 / 3.6 = 7.78 m/s. Total weight: 85 kg.
Rolling resistance: 85 x 9.81 x 0.005 x 7.78 = 32 W
Aerodynamic drag: 0.5 x 1.225 x 0.32 x 7.78 x 7.78 x 7.78 = 92 W (effective wind = rider speed only)
Gravity: zero (flat road)
Total before drivetrain loss: 124 W. After 4% loss: 124 / 0.96 = 130 W.
W/kg: 130 / 75 = 1.7 (recreational category)
Energy: 130 x 3.6 = 468 kJ/hour (112 kcal/hour)
Related calculators
- Calories Burned Calculator: energy expenditure for a range of activities.
- VO2 Max Calculator: estimate your aerobic capacity from fitness test data.
- Training Pace Calculator: running pace zones by target race time.
- Physics Power Calculator: general work, force and power relationships.
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