BYTETOOLS

Cycling Power Calculator

Estimate the watts needed for a given cycling speed, or the speed a given power buys you, from CdA, rolling resistance, gradient, wind and air density.

Power required

218 W

35.00 km/h at 218 W at the pedals, 211 W actually reaching the road.

218 W
Pedal power
35.00 km/h
Speed
3.02
Watts per kg
1.225 kg/m³
Air density

Where the watts go

ComponentForcePowerShare
Aerodynamic drag18.53 N180 W83%
Rolling resistance3.18 N31 W14%
Gravity (gradient)0.00 N0 W0%
Drivetrain loss—7 W3%

P = (Crr·m·g·cosθ + m·g·sinθ + ½·ρ·CdA·v_air²) × v ÷ η

Air density comes from the International Standard Atmosphere pressure at your altitude divided by R·T, with R = 287.058 J/(kg·K). At sea level and 15 °C that gives 1.225 kg/m³. Aerodynamic drag uses your speed through the air (ground speed plus headwind); the power it costs is that force times your ground speed.

A steady-state model: it assumes constant speed on a constant gradient with a steady wind, and ignores acceleration, bearing losses, tyre scrub in corners and the drafting effect of other riders. Real rides are noisier, so treat these watts as a planning figure rather than a substitute for a power meter. Everything is computed in your browser.

What is the Cycling Power Calculator?

Cycling power is (Crr·m·g·cosθ + m·g·sinθ + ½·ρ·CdA·v²) × v ÷ drivetrain efficiency. It adds the force of rolling resistance, gravity on the gradient and aerodynamic drag, multiplies by speed to get watts at the wheel, then divides by drivetrain efficiency.

  • Solves power from speed and speed from power, using bisection for the inverse
  • Air density from ISA pressure at altitude plus your air temperature
  • CdA presets from upright city bike to time-trial bars, all editable
  • Crr presets from track tarmac to grass, all editable
  • Headwind and tailwind handled through true air speed
  • Force and power split into aerodynamic, rolling, gravity and drivetrain shares
  • Watts per kilogram calculated from rider mass alone

How to use the Cycling Power Calculator

  1. 1

    Choose metric or imperial units and whether to solve for power from a speed, or speed from a power.

  2. 2

    Enter your rider mass and the mass of the bike, bottles and kit.

  3. 3

    Set the gradient, headwind (negative for a tailwind) and drivetrain efficiency.

  4. 4

    Pick a riding position and surface preset, or type your own CdA and Crr values.

  5. 5

    Set the altitude and air temperature, then read the watts, the speed and the force breakdown table.

About the Cycling Power Calculator

The ByteTools Cycling Power Calculator solves the standard road-cycling power equation in both directions. Give it a speed and it tells you the watts required; give it a power number and it inverts the equation by bisection to find the speed you would hold.

Every term is exposed rather than hidden behind a preset: your combined mass, CdA for your riding position, rolling resistance for the surface, gradient, headwind and drivetrain efficiency. Air density is derived properly from altitude and temperature using the International Standard Atmosphere, which is why the same watts buy you noticeably more speed at 2,000 m than at sea level. The results panel splits the total into aerodynamic, rolling, gravity and drivetrain components so you can see where your watts actually go.

All of it runs in your browser with no network access — nothing about your rides is uploaded. This is a steady-state physical model for planning and curiosity, not a substitute for a power meter or for coaching advice.

Frequently asked questions

How many watts do I need to ride at 30 km/h?

For a typical 80 kg rider-and-bike on the flat with a CdA of 0.32 and good tyres, roughly 135–140 W at the pedals. Aerodynamic drag rises with the square of speed and the power to overcome it with the cube, so 40 km/h needs closer to 300 W — not 180.

What is CdA in cycling?

CdA is your drag coefficient multiplied by your frontal area, in square metres, and it is the single biggest lever on flat-road speed. Roughly 0.40 on the hoods, 0.32 in the drops and 0.27 on aero bars for a typical rider — lower is faster, and position usually beats equipment.

How much does altitude affect cycling speed?

A lot on the flat. Air density at 2,000 m is about 78% of sea-level density, so aerodynamic drag drops by nearly a quarter for the same speed. That is why flat time trials and hour records are faster at altitude, even though the thinner air also reduces the power most riders can produce.

What is a realistic drivetrain efficiency?

A clean, well-lubricated chain drive runs at about 97–98%. A dirty or badly cross-chained drivetrain can drop to 93–95%. The default of 97% is a fair assumption for a maintained road bike, and it is editable.

Why does the calculator ask for temperature?

Because air density depends on temperature as well as pressure. Cold air is denser, so the same speed on a 0 °C morning costs meaningfully more watts than on a 30 °C afternoon at the same altitude. The calculator uses the ideal gas law with the specific gas constant for dry air.

How accurate is this compared with a power meter?

It is a steady-state model, so it assumes constant speed on a constant gradient with a steady wind. It ignores accelerations, cornering, bearing drag and drafting. With honest CdA and Crr values it usually lands within about 5–10% on a flat solo effort; in a group or on rolling terrain the gap widens.

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