Cyclist training indoors on a turbo trainer
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Free indoor cycling tool

Indoor Trainer
Resistance Calculator.

Estimate the power in watts your turbo trainer produces at a given speed, plus watts per kg and the equivalent flat-road outdoor speed.

Your setup
Results
Estimated power
258 W
Easy · spin Tempo Hard · race
Watts per kg 3.0 W/kg
Wheel speed 20.0 mph
Same speed in km/h 32.2 km/h
Equivalent flat road 21.2 mph
outdoor speed that power would give

Power is estimated from a fluid-trainer curve adjusted by type. Real watts vary with tyre pressure, roller tension and temperature; use a smart trainer or power meter for exact figures.

The maths

How the calculator works

A trainer turns wheel speed into resistance along a fixed power curve. Faster wheel speed means sharply more watts.

P
Power curve = 5.24 x speed + 0.0192 x speed cubed (mph), the published fluid-trainer curve. Wheel speed is the main driver of resistance.
T
Trainer type = a factor on that curve. Wind trainers read higher, magnetic and rollers a little lower, because each unit brakes the wheel differently.
/
Watts per kg = power divided by rider plus bike weight in kg. The fairest way to compare effort between riders of different sizes.
v
Flat-road speed = the outdoor speed that same power would produce on flat ground, using a standard rolling and air-drag model.
^3
Cubic effect = because the speed term is cubed, a few mph faster on the wheel can add a hundred watts or more of resistance.

On a trainer, resistance comes from the unit rather than from lifting your body, so weight barely changes the watts at a given wheel speed. It matters mostly for watts per kilogram and the outdoor comparison.

Want a printable turbo session card?
A one-page speed-to-watts reference for your trainer is coming soon to our shop.
FAQ

Indoor trainer resistance FAQ

Each trainer has a power curve that links wheel speed to resistance. Fluid trainers follow a well-known curve of roughly 5.24 x speed plus 0.0192 x speed cubed in mph, so power rises steeply as you go faster. This tool applies that curve and adjusts it for magnetic, wind and roller trainers.

Trainer resistance grows with the cube of speed, so small increases in wheel speed cause large jumps in power. Going from 18 to 22 mph on a fluid unit can add 100 watts or more. That cubic relationship is what makes a turbo feel punishing when you push the pace.

They are a solid estimate, not a calibrated power meter. Real output varies with tyre pressure, roller tension, temperature and how hard you clamp the wheel. Use the figure to gauge effort and compare sessions, and rely on a smart trainer or power meter when you need exact numbers.

It depends on your size and fitness. Many recreational riders hold 150 to 200 watts for a steady session, trained cyclists sit around 200 to 280, and racers push well beyond that. Watts per kilogram is the fairer yardstick because it accounts for body weight.

Producing a given wattage indoors takes the same effort as producing it outdoors, but the resulting speed differs because there is no wind, gradient or coasting. The tool shows the flat-road speed that same power would give you outside, so you can picture the effort in real terms.

Barely. On a trainer the resistance comes from the unit, not from lifting your body, so a heavy and a light rider at the same wheel speed produce almost identical watts. Weight matters here mainly for working out watts per kilogram and the equivalent outdoor speed.