Colle Blegier

Overview

Colle Blegier is a hors catégorie climb, the hardest classification cycling has to offer, and its numbers explain why. Over 34.9 km the road rises from a start elevation of 489 m to a summit of 2373 m, delivering 1884 m of net elevation gain. Because the profile includes small dips and false flats, the total ascent adds up to 1959 m — a fraction more than the net figure. The average gradient of 5.4% sounds moderate on paper, but at this length that average is the whole story: it is not a short, sharp wall but a sustained, hour-plus effort that grinds down riders who start too hard.

How the Gradient Unfolds

The defining tension of Colle Blegier is the gap between its 5.4% average and its 17.96% maximum. A climb that averages 5.4% but touches nearly 18% is not evenly graded — it hides its difficulty. Long stretches sit below the average, lulling you into a rhythm, and then ramps far steeper than 5.4% arrive to break it. With 1884 m of vertical to gain across 34.9 km, expect the road to alternate between shallower gradients where you can breathe and pitches that force you well out of the saddle. The final push toward 2373 m is where thin air compounds the gradient, so effort that felt sustainable lower down starts to bite.

Pacing Notes

On a climb this long, pacing is everything. Anchor your effort to your FTP and resist the urge to chase the steep ramps — surging on an 17.96% pitch early can cost you dearly across the remaining kilometres. Aim to ride the 5.4% sections at a controlled, repeatable power and treat the steep spikes as brief over-efforts you settle back down from quickly. If you don't know your threshold, run the numbers through the FTP estimator before you tackle the full 34.9 km. Fuel and drink early and often; with 1884 m to climb, the difference between a strong summit and a survival crawl is usually made in the first two-thirds.

Gearing

Do not underestimate the gearing you'll want here. A compact chainring paired with a wide-range cassette is the sensible baseline. A 34×32 will get many fit riders up the sustained 5.4% sections, but the 17.96% maximum is where that ratio runs out of road — you'll be mashing at low cadence exactly when your legs are most fatigued. For most riders, and especially anyone carrying luggage or riding after a long day, a 34×34 is the smarter choice, giving you a spinnable gear to protect your legs on the steep ramps and across the full 34.9 km.

Summary

Colle Blegier is a true hors catégorie test: long, sustained, and steep in the wrong places. Respect the length, pace off your threshold, and gear low.

Length: 34.9 km
Average gradient: 5.4%
Max gradient: 17.96%
Start: 489 m
Summit: 2373 m
Net gain: 1884 m
Total ascent: 1959 m
Category: HC