Understanding your power result
The table shows model estimates for a 75 kg rider on a 9 kg bike on a flat road in still air, using this calculator's assumptions (Crr 0.005, CdA 0.32 m², air density 1.225 kg/m³, drivetrain efficiency 97.5%). Note how the aerodynamic share climbs with speed.
| Speed | Estimated power | Share spent on air resistance |
|---|---|---|
| 20 km/h | ≈ 58 W | ≈ 59% |
| 25 km/h | ≈ 97 W | ≈ 70% |
| 30 km/h | ≈ 152 W | ≈ 77% |
| 35 km/h | ≈ 226 W | ≈ 82% |
| 40 km/h | ≈ 323 W | ≈ 85% |
- All constants are typical assumptions, not measurements of your setup. CdA alone commonly ranges from roughly 0.20 m² (aggressive time-trial position) to 0.40 m² or more (upright riding), and Crr varies severalfold across tires and surfaces.
- Still air is assumed. A headwind raises the required power substantially because drag depends on airspeed, not ground speed.
- Air density falls with altitude and rises in cold weather, changing the aerodynamic term.
- On descents the model can produce very low or zero power — gravity supplies the propulsion; the calculator floors displayed power at 0 W.
- A calibrated power meter measures actual output directly and is the reference standard; model estimates are for education and rough planning.
What does a cycling power calculator do?
Cycling power is the rate at which a rider does mechanical work, measured in watts. On the road that work goes to three physical loads: lifting the combined mass of rider and bike against gravity on a gradient, overcoming rolling resistance between tires and road, and pushing air aside. A well-established physics model, validated against measured power in studies such as Martin et al. (1998), sums these forces and multiplies by speed to give the required power.
Because the true drag and rolling parameters of a specific rider and bike are rarely known, this calculator uses typical published values as fixed assumptions: a rolling-resistance coefficient (Crr) of 0.005, representative of road tires on asphalt; an effective frontal area (CdA) of 0.32 m², representative of a road rider on the hoods; air density of 1.225 kg/m³, the standard sea-level value at 15 °C; and a drivetrain efficiency of 97.5%, in line with published chain-drive measurements. Still air is assumed — no head- or tailwind.
Real-world values vary around these assumptions: position (drops versus upright), clothing, equipment, tire choice, road surface, altitude and wind can each change the required power by a meaningful margin. A direct-force power meter remains the reference standard for measuring an individual's actual output.
How to use this cycling power calculator
- Enter your riding speed in km/h.
- Enter your body weight and your bike's weight — their sum determines the gravity and rolling-resistance loads.
- Enter the road gradient as a percentage: 0 for flat, positive for climbing (e.g. 5 for a 5% climb), negative for descending.
- Read the estimated power in watts, the watts-per-kilogram figure, and how much of the power goes to fighting air resistance.
The physics behind cycling power
The model sums three resistive forces and multiplies by ground speed, then divides by drivetrain efficiency to account for friction losses in the chain and bearings. Gravity force is the combined mass times g times the sine of the road angle; rolling resistance is mass times g times the cosine of the angle times Crr; aerodynamic drag grows with the square of speed, so the power to overcome it grows with the cube of speed.
Worked example: a 75 kg rider on a 9 kg bike at 30 km/h on flat ground. Rolling resistance contributes about 4.1 N and aerodynamic drag about 13.6 N; total power is roughly 152 W, of which about 77% is spent against the air. The same rider at 40 km/h needs about 323 W — more than double for a one-third speed increase, because aerodynamic power scales with speed cubed.
Watts per kilogram divides power by rider weight. It is the standard normalization used in cycling because climbing speed on steep gradients is governed largely by power relative to total weight, whereas flat-road speed is governed largely by absolute power relative to aerodynamic drag (Allen, Coggan & McGregor, Training and Racing with a Power Meter).
Common mistakes
- Comparing the model estimate directly with a power-meter reading and attributing the gap to fitness — differences in CdA, Crr and wind explain most of it.
- Entering gradient as a decimal (0.05) instead of a percentage (5).
- Ignoring wind: the model assumes still air, and even a modest headwind materially raises the real power requirement.
- Assuming power scales linearly with speed — the aerodynamic term scales with speed cubed, so small speed gains at high speed are expensive.
- Using rider weight only for the climbing load; the model correctly uses rider plus bike mass for gravity and rolling resistance.
常见问题
How many watts does it take to ride at 30 km/h?
Using this calculator's standard assumptions (75 kg rider, 9 kg bike, flat road, still air, Crr 0.005, CdA 0.32 m²), riding at 30 km/h requires roughly 150 watts, with about three quarters of that spent overcoming air resistance. Actual requirements vary with position, equipment, wind and road surface.
What is CdA in cycling?
CdA is the drag coefficient multiplied by frontal area, expressed in square metres — the single number that summarizes how aerodynamic a rider and bike are. This calculator assumes a typical road-riding value of 0.32 m². Published values range from roughly 0.20 m² for an aggressive time-trial position to 0.40 m² or more for upright riding, which is why position changes speed so much at the same power.
What is a good watts-per-kilogram value?
Watts per kilogram is power divided by rider weight, and it matters most on climbs, where speed is governed largely by power relative to total weight. Sustainable W/kg varies enormously with training background, duration of effort and age, so a single 'good' number does not exist; the power-based training literature (Allen, Coggan & McGregor) profiles riders across wide ranges. Comparing your own value over time is more informative than comparing with others.
Why does air resistance dominate at higher speeds?
Aerodynamic drag force grows with the square of airspeed, and the power to overcome it grows with the cube. Rolling resistance, by contrast, is nearly constant with speed. At 20 km/h the model attributes about 59% of power to air resistance; by 40 km/h that share reaches about 85%, which is why aerodynamics dominates racing at speed.
How accurate is a cycling power model compared with a power meter?
Validation research such as Martin et al. (1998) found the physics model agrees closely with measured power when the actual CdA, rolling resistance, wind and slope are measured for the specific rider and course. This calculator uses typical fixed values instead of measured ones, so treat its output as an educational estimate; a calibrated direct-force power meter is the reference standard for an individual.
How does gradient change the power needed?
On a climb, a gravity term proportional to total mass and the sine of the road angle is added. For an 84 kg rider-plus-bike system, each 1% of gradient adds roughly 8.2 newtons of resistive force — at 15 km/h that is about 35 additional watts per 1% of slope. This is why climbing speed tracks watts per kilogram so closely.
参考文献
- Martin JC, Milliken DL, Cobb JE, McFadden KL, Coggan AR. Validation of a mathematical model for road cycling power. Journal of Applied Biomechanics 1998; 14(3): 276–291.
- Wilson DG. Bicycling Science, 3rd edition. MIT Press, 2004 — rolling resistance, aerodynamic drag and drivetrain efficiency of bicycles.
- Allen H, Coggan AR, McGregor S. Training and Racing with a Power Meter, 3rd edition. VeloPress, 2019.
- International Civil Aviation Organization (ICAO). Standard Atmosphere — sea-level air density 1.225 kg/m³ at 15 °C.
- American College of Sports Medicine. ACSM's Guidelines for Exercise Testing and Prescription, 11th edition. Wolters Kluwer, 2021.