How is the required power calculated?

The tool sums three resistances: rolling resistance, gravity on a slope, and aerodynamic drag. Each force is multiplied by your speed to get watts, then divided by drivetrain efficiency. Aerodynamic drag rises with the cube of speed, which is why high speeds need so much power.

Why does aerodynamic drag dominate at speed?

Drag force grows with the square of speed and power with the cube, so doubling your speed on the flat needs roughly eight times the power. On flat fast rides, position and clothing matter far more than weight.

What assumptions does it use?

It assumes a rolling resistance coefficient of 0.005 (good road tyres), a drag area CdA of 0.32 m² (hoods position), sea-level air density of 1.225 kg/m³, and 97% drivetrain efficiency. Real wind, surface, and position will shift the result.

Why does climbing need so much power even at low speed?

On a slope, gravity adds a force proportional to your total weight and the gradient. At low climbing speeds drag is tiny, so power is dominated by lifting your mass — which is why weight matters most uphill.

Can I use it for headwind or tailwind?

Not directly. The model assumes still air. A headwind effectively raises your air speed, increasing drag, while a tailwind reduces it. For windy estimates, adjust your target speed to approximate the air speed you experience.

Cycling Speed vs Power Calculator

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Cycling speed vs power calculator

Ever wondered how many watts it takes to hold 35 km/h, or why a small climb feels so much harder than the flat? This calculator answers both using the standard cycling power model. Enter your weight, bike weight, the gradient, and a target speed, and it returns the power required plus a clear breakdown of where that power goes.

How it works

Three resistive forces oppose your motion. The calculator computes each, multiplies by road speed to get power, and divides by drivetrain efficiency:

F_roll = Crr · m · g · cos(θ)        rolling resistance
F_grav = m · g · sin(θ)              gravity on the slope
F_aero = 0.5 · ρ · CdA · v²          aerodynamic drag
P = (F_roll + F_grav + F_aero) · v / 0.97

Here m is total mass, θ is the slope angle from the gradient, v is speed in metres per second, ρ is air density, Crr is rolling resistance, and CdA is your drag area. The defaults model good road tyres and a hoods riding position.

Example and tips

Holding 32 km/h on the flat for a 75 kg rider on an 8 kg bike needs roughly 180–200 W, most of it fighting air. Add a 5% gradient at the same speed and gravity suddenly dominates. To go faster on the flat, lower your CdA (aero position, tighter kit); to climb faster, lose system weight or raise FTP.

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