Grade adjusted pace

GAP converts the pace you actually ran on a slope into the pace the same effort would have produced on flat ground, so a mountain session and a track session can be compared on one scale. It is an effort-comparison unit, not a race pace.

A runner holding 6:00 min/km on the flat covers the same effort at 7:03 on a 3% grade, 7:49 at 5%, 9:57 at 10% and 12:22 at 15%. Roughly, every 1% of gradient slows your pace by 6-7%, about 22-24 seconds per kilometre for a 6:00 min/km runner, and the cost accelerates as the slope steepens.

Less than people expect. The gain peaks around a 13% descent at about 11% below flat pace: a 6:00 min/km runner gets to roughly 5:20 at best. Steeper descents give the gain back through braking and footing, and beyond 25% most runners are slower than on the flat. Calculators that run the raw energy curve backwards produce impossible numbers here, such as double speed at -20%.

The uphill side comes from Minetti et al. (2002), who measured the energy cost of running across gradients; Strava GAP rests on the same physical basis. Their data holds to about 45%, which is why we cap the tool at ±30%. The downhill correction is observational rather than physiological: runners cannot convert the full saving into speed.

Uphill it comes very close, because both use the same energy cost curve. Downhill it can differ: Strava uses a closed curve calibrated on its own data, and while we limit the gain in a similar way, the numbers are not identical. Neither accounts for technical ground, mud or fatigue.

For the uphill side yes, with the caveat that a 1% treadmill incline only compensates for outdoor air resistance, so comparisons effectively start there. For the downhill side no: most treadmills cannot go negative, and those that can do not reproduce the braking load of a real descent.

It is enough for a single slope, not for a whole course: on a trail race the gradient changes every hundred metres and technical ground adds its own cost. For a full course use the trail effort tool, which works from distance and total ascent; this page answers what one climb or descent is worth.

For most runners the threshold sits around a 15-20% gradient: above that, fast hiking gives almost the same speed as running for far less energy. This is why experienced trail and ultra runners hike steep ramps on purpose: the speed loss is small and the energy saving is large.

8-12% is common for short power repeats and 4-6% for longer climbs at threshold pace. Above 15% your running form changes and the session turns into strength work. The same applies on a treadmill, except you cannot reproduce the descents.

Put the course distance and total ascent into the trail effort tool to get the flat-kilometre equivalent, then multiply that by your flat target pace. Use this page for what a single climb costs and the effort tool for the whole course.

A 1996 study by Jones and Doust showed that a 1% incline on a treadmill roughly matches the energy cost of air resistance outdoors. That makes 1% the closest setting to flat outdoor running; 0% is slightly easier at the same speed.