I.Effort distance in one sentence
An outing rarely reduces to its distance. Ten kilometres along a canal towpath and ten kilometres in the mountains have almost nothing in common — yet on paper they're the same « ten kilometres ». Effort distance, often called km-effort, exists to fix that misunderstanding. It fuses distance and climb into a single figure: the real difficulty of the route, expressed as if everything were flat.
The recipe fits on one line, and every trail runner knows it: take the distance in kilometres and add the positive elevation gain in metres divided by a hundred. An outing of ten kilometres with six hundred metres of positive elevation gain is therefore worth sixteen kilometres-effort. The underlying idea: a hundred metres of climb « cost » roughly as much as one kilometre on the flat.
It's one of the thirteen measures of a GPX track, but of a slightly different nature: it records nothing new on the ground, it sums up two numbers you already have. Hence its use — a single number to rank outings that have nothing in common.
II.Why a hundred metres of climb equal a kilometre
Why add metres of climb to kilometres of distance, two quantities that have nothing to do with each other? Because what we actually care about is neither one nor the other: it's the effort. And climbing is expensive. Gaining a hundred metres of elevation on an ordinary gradient takes roughly the same time and energy as covering one kilometre on the flat at the same easy rhythm. From that observation came the coefficient: one for one.
Take it for what it is — an empirical rule, not a law of physics. The « hundred metres for one kilometre » ratio fits running and hiking well on reasonable gradients. At the extremes it breaks down: on a thirty-percent wall you can only walk, a hundred metres of climb cost far more than a kilometre of flat, and km-effort then underestimates the difficulty. It's a robust benchmark for most outings, not a verdict.
Effort distance doesn't claim to be exact. It claims to be comparable — and that's exactly what's missing when you look at two routes of different shapes.
III.Comparing two routes that have nothing in common
Here's the use case that justifies effort distance all on its own. Picture yourself preparing a race whose profile looks like nothing you've ever run — say twenty-five kilometres for fifteen hundred metres of climb, where your usual outings are long and flat. Distance tells you nothing useful: you've already run far more than twenty-five kilometres, but never with this relief. Is it within reach, and in what time?
Effort distance answers by translating both routes into the same unit. Your upcoming race? Twenty-five kilometres plus fifteen hundred divided by a hundred, that is forty kilometres-effort. Your biggest outing to date — forty-five kilometres with seven hundred metres of climb — weighs in at fifty-two kilometres-effort. Surprise: you've already done harder, even though on paper your race looked like another planet.
All that's left is turning this equivalence into a time estimate. Take the track of your big outing and isolate the portion that matches forty kilometres-effort — the first kilometres, up to where the effort counter reaches forty. Read the time you took. There's your anchor: a duration your legs actually sustained, for exactly the same load as the race you're targeting. Not a theoretical projection — a lived one.
IV.What effort distance can't see
A single figure can't say everything, and effort distance has blind spots worth keeping in mind before you trust it blindly.
First, the descent. The calculation only counts the climb; negative elevation doesn't enter it. Yet a long technical descent wrecks the quads as much as a climb burns the lungs. Two routes with the same effort distance, one finishing on a gentle slope, the other in a thousand-metre plunge, don't run the same. Then, the terrain: mud, rocks, roots, snow — none of it shows up in two numbers. And altitude, heat and wind weigh in too, leaving no trace in the formula.
Effort distance ranks outings by difficulty reliably; it replaces neither how they felt nor the other measures. To separate two climbs with similar effort distance, look instead at climbing speed and gradient: they tell the intensity, where effort distance tells the volume.
V.Effort distance for a portion, not just a whole outing
Most tools only show an effort distance for the whole activity. But it's often a portion you care about: the big climb in the middle, the first kilometres before the aid station, or — as in the example above — the stretch of your old outing that matches the load of your next race.
In GPXchunk, effort distance recomputes for the portion you select, just like the other twelve measures. Drag two handles on the elevation profile, or type two kilometre bounds, and the figure updates for that segment alone. That's what turns effort distance from a mere difficulty label into a genuine preparation tool: you cut your past tracks to the size of the goal, and you read your own times like a road map.