How to use the cycling power to weight calculator
- Choose Watts per kilogram to find the ratio, or Watts for a target to find the power a ratio needs.
- Enter your power in watts (or the target ratio) and your body weight in kilograms or pounds.
- Open More options to add the weight of the bike and gear, and for a climb enter the grade and length with a rolling resistance value. Air drag is not included.
- Read the ratio, the ratio with the bike included, the total mass and the climb speed and time if you entered a climb.
Formula
Watts per kilogram = power in watts / body weight in kg
Watts needed = target ratio x body weight in kg
Climb speed = power / (total mass x g x (sin(angle) + Crr x cos(angle)))
Climb time = length / speed
The angle comes from the grade, g is 9.80665, and Crr is the rolling resistance coefficient. The climb estimate leaves out air drag, wind and drivetrain loss, so a real climb is slower.
250 watts at 70 kg
250 / 70 = 3.57 watts per kilogram. With a 9 kg bike and gear, the total mass is 79 kg and the ratio becomes 3.16. For a target of 3.5 W/kg at 70 kg you need 245 W. In pounds, 165 lb is 74.84 kg, so 250 W is 3.34 W/kg, or 2.98 with a 20 lb bike.
A steady 8 percent climb of 5 km at 250 W with 79 kg and a rolling resistance of 0.005 comes to 13.7 km/h, about 21 min 53 s, with 399 m of vertical gain. With 300 W, 83 kg total, a 6 percent climb of 10 km and Crr 0.004, the speed is 20.8 km/h and the time 28 min 53 s.
Why weight matters on climbs
On a steep climb most of your effort goes into lifting your mass against gravity, so power divided by weight describes how fast you can climb better than power alone. On flat ground, air drag matters far more and size helps instead. That is why two riders with the same watts per kilogram can ride very different flat speeds. The ratio is a way to compare power across body sizes, and it does not describe overall fitness or health.
For pedal speed and gearing, see the Cycling Speed from Cadence Calculator. To set training zones from your heart rate, use the Heart Rate Zone Calculator.
Reading the climb estimate
The estimate assumes a steady grade, a steady power and no wind. It also ignores drivetrain loss, which is a few percent, and air drag, which grows with the square of speed. At slow climbing speeds drag is a small part of the effort, but on shallow grades it is not, so the estimate is best for steep climbs and optimistic otherwise. Use it to compare efforts, not to predict a finish time. For a bike that fits you, the Bike Frame Size Calculator helps, and for cars the Power to Weight Ratio Calculator does the same idea in horsepower.
Assumptions and limits
- The power is the average or sustained watts you enter, measured at the pedals.
- The climb is steady, with no wind and no air drag, and the rolling resistance is the number you enter (0.005 is an example).
- Pounds are converted at 0.45359237 kilograms each.
- Everything is calculated in your browser, and no data is stored.
Frequently asked questions
How do I calculate watts per kilogram?
Divide your power in watts by your weight in kilograms. 250 watts at 70 kg is 3.57 W/kg. If you weigh in pounds, the tool converts to kilograms first.
Should I include the weight of the bike?
The usual ratio uses body weight only. The tool also shows the ratio with the bike and gear included, which matters when you compare climbing speeds.
Why is the climb time only an estimate?
It assumes a steady grade and steady power and ignores air drag, wind and drivetrain loss. Real climbs are slower, especially on shallow grades.
Does a higher ratio mean I am fitter?
No. It describes power relative to weight, and says nothing about health or overall fitness. It is a number for comparing efforts.