What is CdA and how do I find mine?

CdA is the product of drag coefficient (Cd) and frontal area (A), measured in square metres. Typical values range from 0.20–0.24 m² for a time-trial aero position, 0.30–0.35 m² for a road-race drops position, and 0.38–0.45 m² for an upright commuter position. Wind-tunnel testing or on-road field protocols (such as the Chung method) can provide individualised estimates; otherwise, published averages for similar positions serve as reasonable starting points.

Why does the aero component increase so rapidly with speed?

Aerodynamic drag scales with the square of velocity, and power is the product of force and velocity, so aero power scales with velocity cubed. Doubling speed from 20 km/h to 40 km/h increases aero drag by a factor of four and aero power by a factor of eight, which is why high-speed efforts demand disproportionately more watts than slower riding.

How accurate is the fixed air density of 1.226 kg/m³?

Air density at sea level and 15°C is approximately 1.226 kg/m³. At 1,500 metres elevation, density drops to roughly 1.056 kg/m³ (about 14% lower), reducing aero drag and power by a similar margin. The calculator does not adjust for altitude, temperature, or humidity, so results at elevation or in hot conditions will overestimate aero power slightly.

What rolling resistance coefficient should I use?

Crr values depend on tyre type, pressure, and surface. Smooth asphalt with high-pressure clinchers typically yields 0.004–0.006, tubeless or latex tubes may reach 0.003–0.004, and rough or wet pavement or lower tyre pressures increase Crr toward 0.007–0.010. Off-road or gravel surfaces can exceed 0.012. The default of 0.005 represents a mid-range road setup.

Can I use this calculator for downhill speeds with negative gradients?

Yes. The gradient input accepts values from −30% to +30%, and the gravity term becomes negative on descents, representing the propulsive force from gravity. At high downhill speeds, aero drag typically dominates the power balance, and riders may need to pedal minimally or brake to control speed rather than produce net positive power.

Cycling

Cycling Power Calculator (Watts from Speed)

Estimate cycling power from speed, mass, drag area, rolling resistance, and gradient.

Last updated: 22 April 2026

What this tool does

This calculator estimates the mechanical power required to cycle at a given speed by applying the Martin et al. (1998) cycling power model, which partitions total watts into aerodynamic drag, rolling resistance, and gravitational climbing forces. It accepts rider and bike mass, speed, drag area (CdA), rolling resistance coefficient (Crr), and gradient as inputs and returns an estimated power output in watts, alongside a breakdown of the three resistance components. The model assumes sea-level air density and standard gravity; results are most accurate for steady-state riding on road surfaces within the typical range of the published coefficients.

Inputs

Result

—

Formula Used

P = \frac{1}{2} ρ C_{d} A v^{3} + C_{r r} m g v + m g v sin θ

v

Speed in km/h

m_{r}

Rider mass in kg

m_{b}

Bike mass in kg

C_{d} A

Drag area

C_{r r}

Coefficient of rolling resistance

θ

Road angle θ = arctan(gradient_pct / 100); enter the gradient as a percent and the calculator converts to the angle internally

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How Cycling Power Calculator (Watts from Speed) works

This calculator estimates the mechanical power output required to sustain a given cycling speed under specified conditions. It accounts for three distinct forces acting on a rider: aerodynamic drag, rolling resistance, and gravitational load (climbing or descending). By entering speed, rider and bike masses, drag coefficient × frontal area (CdA), rolling resistance coefficient (Crr), and gradient percentage, the tool returns an estimated wattage and breaks it into component contributions from aero, rolling, and gravity forces.

The formula

The calculator implements the Martin et al. (1998) standard cycling power model:

Power = 0.5 × ρ × CdA × v³ + Crr × m × g × v + m × g × sin(θ) × v

where v is velocity in metres per second (converted from km/h by dividing by 3.6), ρ is air density held constant at 1.226 kg/m³ (sea level, 15°C), m is the sum of rider and bike masses in kilograms, g is gravitational acceleration (9.8067 m/s²), CdA is the drag area in square metres, Crr is the dimensionless rolling resistance coefficient, and θ is the road angle computed as arctan(gradient_pct / 100). The aero term scales with the cube of velocity because drag force is proportional to v² and force multiplied by velocity yields power. The rolling and gravity terms are linear in velocity.

Where this method is most accurate

The model is most reliable on steady-grade roads at constant speed with known rider position and equipment. CdA values are typically measured in wind tunnels or estimated from published field data; small errors in CdA produce proportional shifts in the aero component at high speeds. Rolling resistance depends on tyre pressure, surface texture, and tube type; the default of 0.005 represents a smooth road with clincher tyres at moderate pressure. Air density is fixed at sea level; at altitude, lower density reduces aerodynamic drag by roughly 3% per 1,000 metres of elevation. The gradient term uses the trigonometric sine to maintain accuracy across steep climbs and descents from −30% to +30%.

What this tool does not do

This calculator does not model accelerations, changes in wind speed or direction, drafting effects, changes in rider position mid-ride, or mechanical drivetrain losses (typically 2–5%). It assumes constant speed and does not account for variations in air density due to temperature, humidity, or altitude beyond the fixed sea-level value. The tool returns mechanical power at the crank; it does not estimate metabolic cost, caloric expenditure, heart rate, or training stress. For time-trial pacing, race strategy, or interval prescription, additional context and physiological data are required.

Disclaimer

This tool is for educational and informational purposes only. It is not a substitute for professional coaching, medical advice, or personalised training guidance. The results are estimates based on standardised physical models and do not account for individual physiology, fitness level, technique, or health status. Always consult a qualified coach or healthcare provider before making changes to training intensity, volume, or equipment selection.

Questions

What is CdA and how do I find mine?: CdA is the product of drag coefficient (Cd) and frontal area (A), measured in square metres. Typical values range from 0.20–0.24 m² for a time-trial aero position, 0.30–0.35 m² for a road-race drops position, and 0.38–0.45 m² for an upright commuter position. Wind-tunnel testing or on-road field protocols (such as the Chung method) can provide individualised estimates; otherwise, published averages for similar positions serve as reasonable starting points.
Why does the aero component increase so rapidly with speed?: Aerodynamic drag scales with the square of velocity, and power is the product of force and velocity, so aero power scales with velocity cubed. Doubling speed from 20 km/h to 40 km/h increases aero drag by a factor of four and aero power by a factor of eight, which is why high-speed efforts demand disproportionately more watts than slower riding.
How accurate is the fixed air density of 1.226 kg/m³?: Air density at sea level and 15°C is approximately 1.226 kg/m³. At 1,500 metres elevation, density drops to roughly 1.056 kg/m³ (about 14% lower), reducing aero drag and power by a similar margin. The calculator does not adjust for altitude, temperature, or humidity, so results at elevation or in hot conditions will overestimate aero power slightly.
What rolling resistance coefficient should I use?: Crr values depend on tyre type, pressure, and surface. Smooth asphalt with high-pressure clinchers typically yields 0.004–0.006, tubeless or latex tubes may reach 0.003–0.004, and rough or wet pavement or lower tyre pressures increase Crr toward 0.007–0.010. Off-road or gravel surfaces can exceed 0.012. The default of 0.005 represents a mid-range road setup.
Can I use this calculator for downhill speeds with negative gradients?: Yes. The gradient input accepts values from −30% to +30%, and the gravity term becomes negative on descents, representing the propulsive force from gravity. At high downhill speeds, aero drag typically dominates the power balance, and riders may need to pedal minimally or brake to control speed rather than produce net positive power.

Sources & Methodology

Implements the Martin et al. (1998) cycling power model: Power = 0.5 × ρ × CdA × v³ + Crr × m × g × v + m × g × sin(θ) × v, where v is speed in m/s (converted from km/h ÷ 3.6), ρ = 1.226 kg/m³ (sea level air density), m = rider + bike mass, g = 9.8067 m/s², and θ = arctan(gradient_pct / 100).

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