Free calculator
Trail race time estimator with km-effort
Enter the race distance and elevation, then a race you have already run. You get a finish-time window, the pace per km-effort and the average speed, with the formula under the result.
The race you are preparing
D− is optional: leave it empty on a loop. It adds the course profile to the result.
h:mm:ss or mm:ss
Estimated finish time
5h30to5h38
Low end with a Riegel exponent of 1.06, high end with 1.10.
Km-effort
62.0
Pace per km-effort
5:19–5:27
Speed
7.5–7.6 km/h
Average pace per km
7:51–8:02 /km
Where the time goes
- Distance (42.0 km): 3h44–3h49
- Climbing (2000 m D+): 1h46–1h49
How it is calculated
Km-effort turns climbing into distance: km-effort = km + D+ / 100, so every 100 m of ascent counts as one extra kilometre. Your reference race gives a time over its own km-effort. The estimate scales it with Riegel's formula applied to km-effort: T = T_ref × (E / E_ref)^k. The exponent k is set to 1.06, Riegel's road value, for the low end, and to 1.10 for the high end, to allow for the extra slowdown of long days on trails. Both are assumptions, not measurements, which is why you get a window rather than a time to the minute.
km-effort = km + D+ / 100
T = Tref × (E / Eref)1.06 … 1.10
Example: a 25 km race with 1,000 m D+ is 35 km-effort. Finished in 3:00:00, that is 5:09 per km-effort. For a 42 km race with 2,000 m D+ (62 km-effort): 3:00:00 × (62 / 35)^1.06 = 5h30, and 3:00:00 × (62 / 35)^1.10 = 5h38. So 5h30 to 5h38, about 5:20 per km-effort and 7.5 km/h.
Variants: 100 m of climbing per kilometre is the usual French convention. Some coaches weight climbing differently on very steep or very runnable courses, and some add part of the descent. ITRA uses the same sum, distance + D+ / 100, to classify races. This calculator is not affiliated with ITRA and does not compute an ITRA or UTMB index.
Sources: km-effort is a French trail running convention, also used by ITRA to classify races. The exponent comes from Peter Riegel, "Athletic Records and Human Endurance", American Scientist, vol. 69, 1981. The 1.10 upper bound is our assumption for long trail races, not a published value.
What the formula does not know
- Terrain. Rocks, roots, mud, snow and scree slow you more than the elevation says. Two races with the same km-effort can finish far apart.
- Night. A head torch narrows what you see, and technical descents go slower in the dark.
- Altitude. Above about 2,000 m, the same effort gives a slower pace, more so if you live low.
- Heat. A valley at 2 pm in July costs time, and the formula does not read the forecast.
- Aid stations. A trail reference already includes the stops you made; a longer race means more of them, and longer. A road reference includes none.
Send these targets to your watch
Write your sessions in OpenAthlete with targets as a percentage of your VMA or max heart rate. They reach your Garmin, Suunto or COROS with the paces filled in.
Questions
A distance that counts the climbing: kilometres plus one per 100 m of ascent. A 30 km race with 1,500 m D+ is 45 km-effort. It lets you compare a mountain 30 km with a flat 45 km, and it is how French trail runners talk about how hard a race is.
Because the slowdown over long trail races varies a lot from one runner to the next. The window goes from a Riegel exponent of 1.06, the classic road value, to 1.10, for a runner who fades more on long days. If your long races usually go to plan, aim for the low end.
A trail race, if you have one: it already contains your climbing, your descending and your stops. From a road race, the estimate assumes runnable trails and is often optimistic on technical courses. The closer the reference is to the target in km-effort and terrain, the better.
It gives an order of magnitude. Past roughly 100 km-effort, sleep, food, night sections and time at aid stations weigh more than any formula. Take the high end of the window as a starting point, then check it against the cut-offs and the results of runners you know.
No. ITRA uses km-effort to classify races, but its performance index compares your results with other runners' in its database. This page only scales one of your own results. It is not affiliated with ITRA or UTMB.
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