What a km-effort is
A flat kilometre and a climbing kilometre cost very different amounts of effort. Km-effort makes trail races comparable anyway by combining distance and elevation gain into a single number. The International Trail Running Association (ITRA) bases its Performance Index on exactly this concept: “Km-effort”.
How the calculator works
The formula is km-effort = distance in km + elevation gain in m / 100. So 100 metres of climbing count the same as 1 extra flat kilometre. A 50 km race with 2,000 m of gain gives 50 + 2,000 / 100 = 70 km-effort.
Your pace per km-effort (time divided by km-effort) gives an estimated finish time for another race: time = pace per km-effort times the target race’s km-effort. Alternatively, enter a time from a race you have already run, and the calculator works out your pace per km-effort automatically.
Why downhill elevation is not included
Descents load the body differently from climbs, but there is no widely accepted, sourced formula for how much downhill elevation should raise or lower the km-effort figure. For that reason, elevation loss here is shown for information only and is not included in the km-effort number or the time estimate. For a reliable figure, use the pure climbing elevation, exactly as ITRA does for its own km-effort calculation.
Limits of the method
The km-effort formula is a rough but workable approximation. It does not account for terrain, altitude, temperature or technical ground with scrambling sections. Two races with the same km-effort can feel very different in practice, for example a short, steep course versus a long, gently climbing one. The time estimate also assumes a steady pace per km-effort, without aid-station stops or fatigue over very long distances.
Frequently asked questions
How many km-effort is a 50 km race with 2,000 m of gain?
50 + 2,000 / 100 gives 70 km-effort.
How do I estimate a new race from one I have already run?
Enter the distance, elevation gain and time of your earlier race as Race A and switch to “From a reference race”. The calculator works out your pace per km-effort and applies it to Race B.
Does downhill elevation count towards km-effort?
No. The ITRA formula uses only the elevation gain on climbs. There is no comparably accepted formula for descents, so they are shown here but not included in the calculation.
Does the km-effort formula work for flat road races too?
It works everywhere, but adds no value on flat courses since the elevation term is zero. For pure road races the <a href="/en/tools/pace-calculator/">pace calculator</a> is the simpler tool.
What if my two races are very different in length?
The formula still calculates a result, but uncertainty grows with the distance gap: fatigue, aid-station stops and altitude matter more over very long races than over short ones.