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Graduvex

How much power do you need to ride at a given speed?

Climbing at 12 km/h, holding 32 km/h on the flat: every goal has a price in watts, set by gradient, weight, position and surface. This tool applies the full physics model — the same one racing simulators use — to price your goal before you attempt it.

Your weight fully dressed, shoes and helmet included.

Bike, full bottles, bag, tools. A road bike weighs 7 to 10 kg.

The segment’s average gradient: GPS unit, or elevation ÷ distance × 100. Negative downhill.

The speed you want to hold on that segment.

Sets your aerodynamic drag (CdA). Position matters mostly above 25 km/h.

Sets the rolling resistance (Crr).

Required power

206W

Watts per kilo of rider
2.75W/kg

Hard effort: trained-rider pace, sustainable from minutes to an hour depending on fitness.

The method

Three forces hold a bike back: gravity (total weight times gradient), rolling resistance (tyres deforming on the road) and air. The first two grow with speed; aerodynamic drag grows with the CUBE of speed — which is why gaining 2 km/h on the flat costs so much.

Uphill, gravity dwarfs everything: total weight becomes the dominant factor and position barely matters. On the flat it flips: aerodynamics accounts for up to 90% of the spend.

The computed power is what leaves your legs: drivetrain losses (about 2.5%) are already included.

In the formula: m is total mass (kg), θ the slope angle, Crr the rolling coefficient, ρ air density, CdA the effective frontal area (m²), V speed (m/s), η drivetrain efficiency (0.975).

P = [m·g·(sin θ + Crr·cos θ)·V + ½·ρ·CdA·V³] / η

Good to know

  • Wind is not modelled: a 15 km/h headwind costs as if you were riding 15 km/h faster in the aero term. In real wind, the result is optimistic.
  • The CdA presets are averages: your build, clothing and equipment shift the real value by ±15%.
  • On a steep descent gravity supplies more than the resistances: the tool then shows 0 W (freewheeling), not negative power.
  • Altitude helps on the flat (thinner air) and taxes the body: the calculation uses sea-level air density.
  • Benchmark the result: a leisure cyclist holds 1.5–2.5 W/kg for hours; a trained amateur racer, 3–4 W/kg for an hour.

FAQ

How many watts to climb at 10 km/h?

It depends on weight and gradient: at 8%, an 84 kg rider-plus-bike needs about 190 W at 10 km/h — 2.5 W/kg, a steady tourer’s pace. The same climb at 15 km/h needs nearly 290 W. Put your own numbers in the fields above: that is exactly what the tool is for.

Losing 2 kg of body weight vs 2 kg off the bike: same thing?

For climbing physics, yes: only total mass matters, and 2 kg less buys roughly 2.5% more speed at equal power on a steep slope. The difference is financial: losing 2 kg of body weight is free, taking it off the bike often costs over €1,000.

Why do 2 extra km/h on the flat cost so much?

Because aerodynamic drag grows with the cube of speed: going from 30 to 32 km/h raises the required power by about 20%, not 7%. It is also why drafting saves 25–35% of the energy.

What do my watts per kilo mean?

W/kg is the climbing currency of cycling. One-hour ballparks: 2 W/kg occasional rider, 3 W/kg regular rider, 4 W/kg amateur racer, 5.5 W/kg and up, elite level. Over a few minutes, everyone holds noticeably more.

Does this work for an e-bike?

The physics is identical: the result is the TOTAL power required. On an e-bike, the motor supplies part of it (250 W nominal in Europe) and your legs the rest — so the tool also tells you what the motor must deliver, and why batteries melt on climbs.

Where do the CdA and Crr values come from?

From ranges published in cycling literature (wind-tunnel and power-meter measurements): CdA from 0.25 m² in a time-trial tuck to 0.42 m² upright; Crr from 0.004 for a good road tyre on smooth asphalt to 0.015 on trails. These are typical values, not measurements of YOUR equipment.