Col Fenêtre (Perloz)

Overview

Col Fenêtre from Perloz is an hors catégorie climb that asks a serious question of every rider who points a wheel uphill. Over 12.8 km it averages 8.3%, dragging you from a start elevation of 284 m to a 1347 m summit. That is 1063 m of net gain, with 1038 m of measured total ascent — meaning the road climbs almost relentlessly, with very little coasting to break the effort. Add a max gradient of 19.54% and you have a climb that rewards restraint early and punishes anyone who spends too freely on the lower slopes.

How the Gradient Unfolds

An 8.3% average across 12.8 km is already demanding, but averages hide the story. With total ascent of 1038 m against a net gain of 1063 m, the climb barely gives anything back — there are almost no false descents to recover on. Instead, the elevation piles up steadily.

The defining feature is the spread between the 8.3% mean and the 19.54% peak. That gap tells you the road is not evenly pitched: expect stretches sitting near or above 10% that make the 8.3% figure feel optimistic, punctuated by ramps steep enough to force you out of the saddle. From 284 m at the base to 1347 m at the top, the mountain rarely relents, and the steepest pitches tend to arrive when your legs are already loaded.

Pacing Notes

On a climb this long and steep, pacing is everything. The goal is to hold a sustainable effort just below your threshold rather than chasing the steep ramps. Anchor your ride to a percentage of your FTP and refuse to spike over it on the shallower sections — you will need that headroom when the gradient bites toward 19.54%.

If you do not know your threshold number, estimate it first with the FTP estimator, then target roughly tempo-to-threshold on the sustained middle. With 1063 m of climbing to absorb, a rider who paces the first third conservatively will pass those who went out hard. Treat the steep pitches as brief surges over your target, then settle straight back down.

Gearing

Do not under-gear this one. An average of 8.3% over 12.8 km, with ramps to 19.54%, demands a wide range at the back. A compact 34-tooth inner chainring is the sensible baseline. Pair it with at least a 34×32, and if you weigh more, ride at a steady effort, or simply want to spin the steep ramps rather than grind them, go to 34×34. That extra sprocket keeps your cadence up when the road tilts toward 20%, protecting your legs across the full 1063 m of gain. The difference between surviving and suffering here is often one or two teeth of cassette.

Summary

Col Fenêtre from Perloz is a true HC test: long, steep and unforgiving, with a summit at 1347 m earned the hard way. Pace it below threshold, gear it generously, and respect the ramps.

Length: 12.8 km
Average gradient: 8.3%
Max gradient: 19.54%
Start: 284 m
Summit: 1347 m
Net gain: 1063 m
Total ascent: 1038 m
Category: HC