What the inputs mean
- Power: any power in watts from a power meter or smart trainer. Most riders use FTP (functional threshold power, usually described as the power you can hold for about an hour), but a best 5 minute or 1 minute power works the same way. The label only changes the headline.
- Body weight: in pounds or kilograms (use the US/Metric switch). W/kg is by convention per kilogram of rider.
- Target W/kg: optional. The tool shows the power you would need at your weight, or the weight you would need at your power.
- Bike weight: optional. On a climb you lift the bike too, so W/kg of rider plus bike tells you more there.
The formula
W/kg = power (W) ÷ weight (kg)
weight (kg) = weight (lb) × 0.45359237
power for a target = target W/kg × weight (kg)
weight for a target = power (W) ÷ target W/kgWorked example
A rider with an FTP of 250 W who weighs 165 lb. 165 × 0.45359237 = 74.84 kg, and 250 ÷ 74.84 = 3.34 W/kg. That is also 250 ÷ 165 = 1.52 W per pound. Each extra 10 W adds 10 ÷ 74.84 = 0.13 W/kg.
To reach 4.0 W/kg at the same weight, the power would have to be 4.0 × 74.84 = 299 W, 49 W more. At the same 250 W, the weight would have to be 250 ÷ 4.0 = 62.5 kg (137.8 lb). With a 20 lb bike included, 250 W over 185 lb (83.91 kg) is 2.98 W/kg.
Why W/kg matters
On a steep climb most of your power goes into lifting weight, so two riders with the same W/kg climb at about the same speed. On flat roads air drag matters more, and absolute watts count for more than W/kg. The cycling power calculator shows how the split changes with slope and speed. For cars and motorcycles, the power to weight ratio calculator works in hp and pounds.
Why there is no category chart here
Many sites print tables that turn a W/kg number into a rider level. We have not found one source that such tables agree on, so we leave them out. Compare your number with your own past numbers for the same duration, measured on the same power meter.
Questions
How do you calculate watts per kg?
Divide your power in watts by your weight in kilograms. 250 W at 165 lb (74.84 kg) is 250 ÷ 74.84 = 3.34 W/kg.
How do I convert pounds to kilograms for W/kg?
Multiply pounds by 0.45359237. 150 lb is 68.04 kg, so 300 W at 150 lb is 4.41 W/kg.
How many watts do I need for 4 W/kg?
Four times your weight in kilograms. At 165 lb (74.84 kg) that is 299 W; at 70 kg it is 280 W.
Should I include my bike in W/kg?
W/kg is normally quoted per kilogram of rider. For climbing speed the bike counts too: 250 W is 3.34 W/kg for a 165 lb rider but 2.98 W/kg with a 20 lb bike added.