Climb calculator
Your FTP and weight, turned into watts per kilo — and into honest times on the famous cols and the big climbing sportives. Physics, not wishful thinking: weight against gravity, rolling resistance, and air that gets thinner (and trims your power) as you go up.
The famous cols
| Climb | Time | km/h | VAM |
|---|
VAM = vertical metres climbed per hour — climbing's own speedometer. Lengths and gradients are the classic published figures for each side named. Long climbs self-limit: past the one-hour mark the tool quietly caps your chosen effort at what's actually holdable for that duration, so a "95% all-out" Stelvio becomes ~88% — because that's what your legs would do too.
The big climbing sportives
| Event | Time on the climbs | Rough day, total |
|---|
Event climbs are ridden at 75% of FTP no matter what you pick above — nobody climbs Alpe d'Huez fresh after 160 km. The total adds the kilometres between the climbs at ±30 km/h and 20 minutes of stops. Treat it as a ballpark: give or take half an hour, more if the weather turns.
Where it bends: the model assumes a steady average gradient, no wind, no wheel to sit on, and a road-bike position (CdA 0.34 m², Crr 0.0045, 2.5% drivetrain loss). Thinner air up high trims your power too — we use the acclimatised altitude curve (Bassett et al. 1999), so if you sleep at sea level, add roughly 3–4% on the high cols. And an FTP from a fresh 20-minute test is not the FTP you have on day four of a cycling holiday.
Under the hood
Going uphill, almost all of your power goes into lifting you and the bike against gravity. The tool balances the power you can produce against the three things taking it away — gravity, the tyres, and the air — and finds the speed where those two are equal. Then it divides the climb by that speed.
Two adjustments sit on top of that, and both matter more than you would guess.
The air gets thinner as you climb — which cuts drag slightly, but cuts your power a lot more. We use the average height of the climb, not the summit, because you spend the ride getting there.
You cannot hold your hour-power for two hours. The longer the climb, the lower the fraction of your threshold power you can sustain, so the tool works out the time and the sustainable power together until they agree.
The numbers that go in
| Number | What it is | Where it comes from |
|---|---|---|
| 9.8067 | gravity | The standard value. A definition. |
| 1.225 kg/m³ | air density at sea level | The standard atmosphere, at 15 °C. |
| 0.975 | drivetrain efficiency | Martin and colleagues (1998) measured 97.7% for a road chain and bearings. |
| the altitude curve | how much power you lose with height | Bassett and colleagues (1999). They publish two curves — one for riders who have spent weeks at altitude and one for riders who have been there a few days. We use the second, because that is the rider going up a col on holiday. |
| CdA 0.34 | how much air you push, climbing | Our assumption. A plausible figure for an ordinary rider on the hoods, but we have not found a published measurement for this position that we could point you to. |
| Crr 0.0045 | rolling resistance | Our assumption, in the range usually quoted for a good clincher on asphalt. |
| the fade with duration | the fraction of threshold power you hold | Our assumption — see below. |
| the climbs | length, gradient, summit height | Published profiles for the named side of each climb, checked against the organisers' own figures where they exist. |
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