Col de l'Iseran

Col de l'Iseran Cycling Climb

The Col de l'Iseran sits in a Category 1 classification, and the numbers justify it. From the start at 1,814 m elevation, the road climbs 1,552 m across 15.4 km to the summit, giving an average gradient of 6.15%. That average, however, papers over a climb that is anything but uniform — the 22.13% maximum gradient is a sharp reminder that gradient variance here is significant.

How the Gradient Unfolds

The opening kilometres from the base set a persistent, grinding tone. The road through the valley narrows as you gain height, and the gradient rarely lets you coast. The middle section sustains the 6% average without much respite, demanding consistent output rather than explosive efforts.

The crux arrives in the upper third. The gradient spikes — up to 22.13% at its steepest — on ramps that punish anyone who has spent too much energy on the long approach. These sections are short but severe enough to crack a poorly paced ride entirely.

Above the steepest pitches, the road opens onto exposed terrain. The final approach to the summit is more moderate but is fought at altitude, where thinner air compounds any accumulated fatigue.

Pacing Strategy

With 15.4 km and 1,552 m of total ascent, this is a long effort. Treat the first half as investment, not attack. A common mistake is chasing the steeper early ramps above a sustainable FTP-based power target — you will pay for it on the upper slopes.

Key pacing principles:

• Establish your ceiling early. Target a power output you can sustain for 60–90 minutes depending on your fitness. Use the FTP estimator to set a realistic ceiling before you arrive.
• Protect reserves for the upper ramps. The 22.13% sections arrive when fatigue is already accumulating. If your power spikes sharply on those pitches, you are spending future watts.
• Altitude matters. Starting at 1,814 m and climbing to the summit means you are operating in progressively thinner air throughout. Riders not acclimatised should reduce their target power by 3–5% for every 1,000 m above their usual training elevation.

Gearing Notes

For a 6.15% average with a max of 22.13%, compact gearing is not optional — it is sensible. A 34-tooth inner chainring paired with a 32- or 34-tooth cassette will protect your legs on the steepest pitches without sacrificing too much on the easier grades.

Anyone attempting the climb on a standard 53/39 setup should know that the 22.13% maximum will force a very high cadence sacrifice unless the cassette runs to at least 30 teeth. Grinding out steep alpine ramps at low cadence accelerates muscular fatigue and compromises the descent.

What to Expect Overall

The Col de l'Iseran rewards methodical riding. The 1,552 m of ascent means you will be on the bike for a long time regardless of fitness level. Discipline in the first half, conservative power through the steepest gradients, and adequate gearing are the three levers that separate a strong finish from a survival crawl.

For comparison with other major alpine passes, see the Alpe d'Huez climb profile.