Watts per Kg Calculator

Watts per kilogram is how cyclists of different sizes get compared on the same scale. It is one division — power over body mass — and it is the single number that best predicts how you will go uphill. Enter your power, usually your FTP, and your weight:

The arithmetic, and a worked example

W/kg = power (watts) ÷ body weight (kilograms)

A rider weighing 70 kg (154 lb) with an FTP of 250 W is at 250 ÷ 70 = 3.57 W/kg. The same 250 W on a 85 kg (187 lb) rider is 2.94 W/kg — same engine, noticeably different climber. Nothing about their fitness changed; the denominator did.

Reference levels, and where they come from

Level	Threshold power (W/kg)
Untrained	around 2
Moderately trained	around 3.3
Well-trained club cyclist	3.5 to 4.5
Male world class	around 6

These are the landmarks quoted in a study of eleven moderately trained cyclists, whose own group averaged 3.3 ± 0.3 W/kg at threshold. Take them as landmarks rather than grades: they describe men, at FTP, on a bike, and the boundaries between adjacent levels are soft. The calculator’s own bands top out at 5 W/kg, which is below the world-class figure — the top band is a floor, not a ceiling.

Why the ratio decides climbs and not flat roads

Going uphill, most of your power goes into raising your own mass against gravity, and mass appears directly in that equation. On flat ground the dominant resistance is aerodynamic drag, which scales with frontal area and the cube of speed rather than with weight — so a big rider putting out 350 W beats a small rider at 250 W on the flat and loses to them on a 10% grade.

This is not just physics on paper. In a 47 km mass-start mountain bike race, riders’ relative FTP correlated strongly with finishing time (r = −0.74, p < 0.01), while their laboratory VO2 max showed no significant correlation. On terrain with 851 m of climbing, watts per kilogram was the number that mattered.

Two ways to change it

Raise the numerator or lower the denominator. Raising power is the durable route and the one that also improves your flat-road racing; losing mass helps the ratio only until it starts costing you watts, which happens sooner than most riders expect. Junior riders illustrate the trap in reverse: a study of nine male junior road cyclists, aged 16.9 ± 0.8 years, recorded field FTPs of 269 ± 34 W, and their relative numbers move every season simply because they are still growing. Nine riders is a small sample, and the figure is a landmark rather than a norm.

Get the power figure first with the 20-minute test, turn it into training targets with the FTP training zones calculator, and see what FTP is and is not before treating the number as physiology.