Km-effort: definition, formula, example

Km-effort is the trail runner’s way of folding climbing into distance: every 100 metres of ascent counts as one extra kilometre. A 25 km race with 1,000 m of climbing becomes 35 km-effort, and you can compare it with a 35 km road run, or with another trail of the same total, more honestly than with distance alone.

It is a convention, not a law of physiology. It grew up in French trail running (kilomètre-effort), where coaches and race organisers needed a quick way to say how hard a course was, and ITRA uses the same sum, distance plus D+ divided by 100, to classify races. Its appeal is that you can do it in your head at the start line.

How km-effort is calculated

Descents are ignored: they add neither credit nor penalty. Some coaches weight climbing differently on very steep or very runnable courses, and some add part of the descent, but 100 m per kilometre is the common version and the one used here.

Once you have km-effort, you can turn a past race into a prediction. The trail time estimator applies Peter Riegel’s endurance formula to km-effort instead of distance:

A worked example

• Reference: a 25 km trail with 1,000 m D+, so 25 + 1,000 / 100 = 35 km-effort, finished in 3:00:00. That is 10,800 s / 35 = 5:09 per km-effort.
• Target: 42 km with 2,000 m D+, so 42 + 20 = 62 km-effort.
• Low end: 3:00:00 × (62 / 35)^1.06 = 5 h 30.
• High end: 3:00:00 × (62 / 35)^1.10 = 5 h 38.
• So 5 h 30 to 5 h 38, about 5:20 per km-effort and an average of 7.5 km/h over the real 42 km.

The 1.06 exponent comes from Riegel’s work on road and track records. The 1.10 upper bound is our assumption for the extra slowdown of long days on trails, not a published value, which is why the result is a window rather than a time to the minute.

How to use it in training

Count your week in km-effort as well as kilometres. Twelve kilometres with 600 m of climbing is 18 km-effort: a week of 55 km with 2,000 m of climbing is 75 km-effort, as much as 75 km on the flat, and a plan that only counts kilometres will overload you without saying so.

Pick long runs by km-effort. If the goal race is 62 km-effort, a long run of 30 to 35 km-effort with a realistic share of climbing tells you far more than a flat 30 km. Keep the D+ per kilometre close to the race: around 30 m/km on a rolling course, 60 m/km or more for an alpine one.

Use pace per km-effort as a rough gauge of fitness across courses. If your pace per km-effort on similar terrain keeps falling over a block, that is a better sign than any single fast loop. The trail training plans are built around this kind of progression, and for climbs themselves, grade-adjusted pace says what each one cost in energy.

Limits

One rule for all slopes is a simplification. On a gentle, runnable climb, 100 m of D+ cost less than a kilometre; on a 25% wall where you walk with poles, they can cost more. Terrain matters even more: rocks, roots, mud, snow and scree slow you down far beyond what the elevation suggests, and two races with the same km-effort can finish far apart.

Ignoring descents is the biggest blind spot. On a course with long, steep descents, your quads set the pace in the second half, not your lungs, and km-effort does not see it coming. Night, altitude above about 2,000 m, heat and the time spent at aid stations are also outside the formula. A reference from a road race assumes runnable trails and carries your road speed onto the climbs: on technical ground, expect to finish past the window.

Finally, km-effort says nothing about a UTMB or ITRA performance index, which use their own models. Treat it as a common currency for planning, not as a score.