How much does 1 kg matter on a climb?

On a long climb at threshold, about 1–1.5 % of climbing speed per kilogram for a 70 kg rider — roughly the same as 3–4 W of FTP. The steeper the climb, the closer the relation gets to pure W/kg.

Why does weight matter so little on the flat?

Because at 40 km/h around 90 % of your power is spent on air resistance, which depends on your frontal area and position, not your mass. Two kilograms changes frontal area very little; a lower position changes it a lot.

Should I lose weight to improve W/kg?

Only if you are carrying weight that is not muscle and can lose it while the training stays intact. A lactate test before and after tells you whether the thresholds held; if LT2 in watts fell with the weight, the ratio you gained on paper was not real on the road.

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Weight & Power Calculator

See how body weight changes affect your W/kg and climbing speed

Weight & performance

What a change in body weight does to W/kg, flat speed and climbing.

Current weight (kg)

Target weight (kg)

Current FTP (W)

Assumes same absolute power output. Flat: ~0.4% speed/kg, Climbing: ~1.8% speed/kg

How the weight and power calculator works

Watts per kilogram is the number that decides climbs. On a flat road most of the effort goes into pushing air, which does not care what you weigh; on a steep climb almost all of it goes into lifting you against gravity, and the rider with more watts per kilogram rides away. The calculator shows how a change in body weight moves your W/kg, and what that does to flat speed and to climbing speed separately.

Enter your current weight, your FTP and the weight you are considering. The flat-speed change is small — a lighter rider has slightly less frontal area, and that is about all — while the climbing change follows W/kg almost directly. On a 8 % climb, two kilograms is worth roughly the same as 6–8 W.

Losing weight only helps while the power stays. Below a healthy weight the watts go first, and the ratio gets worse, not better; the calculator assumes the FTP you enter survives the change, which is the assumption to question before you start.

Questions

How much does 1 kg matter on a climb?: On a long climb at threshold, about 1–1.5 % of climbing speed per kilogram for a 70 kg rider — roughly the same as 3–4 W of FTP. The steeper the climb, the closer the relation gets to pure W/kg.
Why does weight matter so little on the flat?: Because at 40 km/h around 90 % of your power is spent on air resistance, which depends on your frontal area and position, not your mass. Two kilograms changes frontal area very little; a lower position changes it a lot.
Should I lose weight to improve W/kg?: Only if you are carrying weight that is not muscle and can lose it while the training stays intact. A lactate test before and after tells you whether the thresholds held; if LT2 in watts fell with the weight, the ratio you gained on paper was not real on the road.

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