Doi Inthanon

Overview

Doi Inthanon is an hors catégorie (HC) climb, the hardest classification a road ascent can carry — above categories 1 through 4. Over 15.2 km it averages a punishing 8.54%, hauling you from a start elevation of 1258 m up to a 2560 m summit. That is a net elevation gain of 1302 m, with 1319 m of total ascent once the handful of brief flatter moments are accounted for. The gap between net gain and total ascent is only 17 m, which tells you almost everything: this road climbs, and it keeps climbing. There is very little respite baked into the profile.

How the Gradient Unfolds

An 8.54% average is already steep, but averages hide the teeth. The maximum gradient here hits 19.84% — nearly a wall — and on a climb this long, a pitch like that arrives when your legs are least equipped to absorb it. Because the difference between the 1319 m of total ascent and the 1302 m of net gain is a mere 17 m, you cannot count on downhill or false-flat recovery. The climb effectively goes up the entire way. Expect the gradient to oscillate around and above that 8.54% mean, with the steepest ramps approaching the 19.84% max forcing sharp, out-of-the-saddle efforts. Over 15.2 km, those repeated spikes accumulate into deep fatigue.

Pacing Notes

On a 15.2 km HC climb, discipline early is everything. Anchor your effort to your FTP: riding the first two-thirds at or just under threshold leaves headroom for the steepest ramps toward the 19.84% max. If you do not know your current threshold, run the numbers first with the FTP estimator so your target wattage reflects reality rather than optimism. With an 8.54% average sustained for over 15 km, going into the red before the halfway point is a common and costly mistake — the 1302 m of vertical does not forgive a blown match. Meter your effort, keep cadence smooth, and treat each near-20% ramp as a controlled surge rather than an all-out sprint.

Gearing

Gearing matters more here than on almost any other climb. With sustained 8.54% slopes and a 19.84% maximum, most riders will want a compact crank paired with a wide-range cassette. A 34×32 is the sensible baseline for the average gradient, giving you a low enough ratio to spin the long middle section without grinding. But given that 19.84% max, seriously consider a 34×34 — that extra sprocket is the difference between spinning up the wall and lurching over it at a stall. If your cassette allows it, err toward the easier 34×34; on a 1302 m climb, spare gears preserve your legs for the pitches that count.

Summary

Doi Inthanon is a genuine HC monster: 15.2 km of near-constant climbing at 8.54%, punctuated by ramps to 19.84%, delivering you from 1258 m to a lofty 2560 m. Pace it against your threshold, gear low, and respect the near-total absence of recovery — the 17 m gap between total ascent and net gain says it all.

Length: 15.2 km
Average gradient: 8.54%
Max gradient: 19.84%
Start: 1258 m
Summit: 2560 m
Net gain: 1302 m
Total ascent: 1319 m
Category: HC