How long will it take me?
Based on your UTMB® Index, each segment's duration comes from actual times logged by runners in your bracket on previous editions. When the race has no history, a model trained on 45 other races steps in. Riegel's law is a last resort only, if no data exists.
Calculation runs three levels deep, and the closest to reality wins. Across 240 races, results are plentiful enough that each segment gets the time runners in your band actually spent on it, edition after edition — races are matched by location and by distance to within 4%, never by name, which organizers change from one year to the next. Elsewhere, a model trained on 45 races predicts the shape of yours from all the others: distance, elevation gain, elevation loss, altitude, start time, how much of the race is already behind you, and how many nights you've started.
Riegel's law is the safety net. It raises the ratio of the distances to a power slightly above 1 to express the slowdown, with an exponent around 1.06 on road. Applied as-is to trail it gets it wrong — ten flat kilometers and ten with 800 m of elevation gain are not the same effort — so its exponent is adjusted for elevation, then recalibrated race by race. That's what answers when no result exists, and only then: everywhere else, you no longer model a pace, you copy the one that was run. Checked against an edition the model had never seen, the predicted percentiles land within 2.5% of the real ones. Aid station stops are modeled separately from movement: 1,667 stops measured at 103 stations across 26 races.
On a real race: Mont-Blanc — UTMB®
On Mont-Blanc — UTMB®, the gap between levels is considerable: an Index of 500 is projected well beyond 37h10, an Index of 800 well inside it. The table below shows the real progression — and why a gut-feel estimate almost always errs in the same direction, optimism.
| UTMB® Index | Estimated time | Carbs | Fluid | Sodium |
|---|---|---|---|---|
| 500 | 43h01 | 2,581 g | 30.1 L | 21,508 mg |
| 600 | 37h10 | 2,230 g | 26 L | 18,583 mg |
| 800 | 24h36 | 1,476 g | 17.2 L | 12,300 mg |
Temps repris des passages réels des coureurs de chaque tranche de cote sur les éditions précédentes, arrêts aux ravitaillements compris. Besoins calculés à 60 g/h de glucides, 700 mL/h de liquide et 500 mg/h de sodium, par temps tempéré.
Use this projection for two things. First, check that the cut-off is reachable at your level: no organiser gives you that answer directly. Second, size your nutrition, since everything is counted per hour. A three-hour error in the projection is close to 200 grams of carbohydrate.
RAVIT applies this calculation to 2,778 races across 49 countries, with 6,547 aid stations whose contents are taken from the official rulebooks.