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Blog · 3 October 2026 · 2 min read

What gearing do you need for a hilly 70.3? Start from the steepest half-kilometre

The gear you need is decided by three things: the steepest bit of your course, what you weigh, and the watts you’ll actually push there. Not by what the pros ride, and not by the average gradient of the big climb.

Start from the steepest half-kilometre

Take San Salvador, the hilliest of the 123 IRONMAN 70.3 courses in the library. Its steepest half-kilometre averages 9.9%, from km 2.0.

Our rider holds 200 W normalised over the course, and the predictor has them riding the steep parts at 232 W. At 75 kg on a 9 kg bike, turning the cranks no slower than 70 rpm, how steep a road can they get up?

Which cassette gets you up

On a compact 50/34, the smallest cog that does 9.9% is a 34: 34 × 34 gets them up 10.4%. On a 52/36 it takes a 36.

Why you want a gear to spare

Give yourself a gear to spare, for two reasons. The site reads climbing off a terrain model that averages the ground over a few hundred metres, so a ramp inside that half-kilometre will be steeper than 9.9%. And on race day you may not have 232 W left when you get there. One cog bigger, 34 × 36 on the compact, gets them up 11.1%.

Work out your own

A heavier rider or fewer watts moves all of this, and quickly. The gear page answers it for you: your weight, the watts you’ll hold on the climbs and the slowest cadence you’ll put up with, and it lists every cassette for your chainset and how steep each one gets you. Your course’s climbs, and how steep they are, are on its course page.

How steep each largest cog gets this rider, worked out by the gear page’s own model

How steep each cog gets this rider on a 50/34 compact

25 teeth7.3%

28 teeth8.4%

30 teeth9.1%

32 teeth9.8%

34 teeth10.4%

36 teeth11.1%

San Salvador’s steepest 500 m, 9.9%

A model, not a measurement. What it can show depends on the numbers above (what you typed, and what your files recorded) and on assumptions this page states; anything it cannot stand behind says so rather than drawing a tidier line.

The steepest road each cog climbs at 232 W and 70 rpm

Largest cog	50/34 compact	52/36
25 teeth	7.3%	6.8%
28 teeth	8.4%	7.8%
30 teeth	9.1%	8.5%
32 teeth	9.8%	9.1%
34 teeth	10.4%	9.8%
36 teeth	11.1%	10.4%

For a 75 kg rider on a 9 kg bike with 700 × 28 mm tyres, holding 200 W normalised over the course and 232 W on its steep parts, as the predictor rides it, and turning no slower than 70 rpm, among Shimano’s road cassettes. San Salvador’s steepest 500 m averages 9.9% on the terrain model, from km 2.0.

Work out your own gearing