How the time is worked out
The calculator uses the standard road-cycling power model (Martin et al., 1998): your power, minus about 2.5% lost in the drivetrain, has to lift you and the bike, roll the tyres and push through the air. It finds the steady speed where those balance on the climb’s average grade, then divides the distance by that speed.
Real climbs are not steady. Hairpins, steeper ramps and wind all add time, and most riders cannot hold the same power for 20 minutes as for 60. For a power you can actually sustain, start from your FTP: about 105% of FTP for a 20-minute climb, close to 100% for one that takes about an hour, and less for anything longer. The FTP calculator can help.
The VAM cross-check
Under the result you’ll see a second estimate from a well-known rule of thumb, usually credited to the Italian coach Michele Ferrari: W/kg ≈ VAM ÷ (200 + 10 × grade %). Turned around, it predicts VAM from your W/kg. Because it leaves out bike weight and air drag, it usually lands within about 10% of the physics model on 5–11% grades and drifts further on shallower or steeper climbs, where drag or weight matter more than it allows for.
On Wikipedia’s scale, a VAM of 1,650–1,800 m/h is top-10 form in the Tour de France, and 1,100–1,300 m/h is the pace of the riders at the back on mountain stages. For amateur benchmarks, see the VAM pacing guide.
Frequently asked questions
How long will it take me to ride up a climb?
It depends mostly on your power-to-weight. For example, a 75 kg rider on a 9 kg bike holding 250 W (3.3 W/kg) takes about 35 minutes to climb 8 km at 7%, which is about 560 m of climbing at a VAM of roughly 960 m/h. Enter your own climb, power and weight in the calculator.
What is VAM in cycling?
VAM (velocità ascensionale media) is how many vertical metres you climb per hour: elevation gain in metres ÷ climbing time in seconds × 3,600. Climbing 600 m in 36 minutes is a VAM of 1,000 m/h. Because it ignores distance and speed, it lets you compare efforts on different climbs.
How do you estimate W/kg from VAM?
A common rule of thumb is W/kg ≈ VAM ÷ (200 + 10 × grade %). On an 8% climb, a VAM of 1,500 m/h works out to about 5.4 W/kg. It ignores bike weight, rolling resistance and air drag, so it is a rough guide; the calculator shows it next to a full physics estimate.
How many watts do I need to climb in a target time?
Switch the calculator to “I have a target time”. It works out the speed you need and the power to hold it. For the same 8 km at 7%, a 75 kg rider on a 9 kg bike needs about 216 W to finish in 40 minutes.
How much faster would I climb if I lost weight?
On a steep climb almost all your power goes into lifting you and the bike, so time scales roughly with total weight. Losing 2 kg from 84 kg of rider and bike makes you about 2% faster at the same power, roughly 45 seconds on a 35-minute climb. Losing weight only helps if your power holds.
Sources
- Martin et al. (1998), Validation of a mathematical model for road cycling power, Journal of Applied Biomechanics
- Wikipedia: VAM (bicycling), with the VAM to W/kg relationship
- Best Bike Split: typical CdA by riding position
Rolling resistance is fixed at 0.004 (road tyre on asphalt). Air density uses your altitude at 20 °C.