How is Index 42 calculated?

Two pillars: route and climate

Index 42 rates marathon, half-marathon, 10 km and 5 km difficulty on a 0–100 scale. It is a comparative score, not percentage time loss.

How T is calculated — route difficulty

100 m of ascent equals 1 additional effort-km; steep and late climbs cost more. Gentle descents provide limited credit and each sharp turn costs 5 m. Cost is normalized to race distance: T = 100 × clamp(E / Emax)^1.12. Components add to T.

How C is calculated — climate difficulty

5–12°C is the optimal band. Cold load grows below 5°C; warmth and humidity load grow above 12°C, with an additional heat multiplier above 27°C. Low dew point alone is not a cold penalty. Wind and rain contribute separately.

How race distance is included

Emax is 15 effort-km for a marathon, 7.5 for a half-marathon, 3.555 for 10 km and 1.777 for 5 km. C = 100 × clamp((R / 150) × √(D / 42.195)). Climate exposure factors: marathon 1.000; half-marathon 0.707; 10 km 0.487; 5 km 0.344. These are comparative model assumptions, not physiological measurements. Weather uses each event’s own local start and a 5, 3, 2 or 1-hour window respectively.

How T and C combine into one Index

Index = 0.8T + 0.8C − 0.006TC

Terrain or climate alone can contribute at most 80 points each; together they can reach 100. This version adds no separate altitude points.

Where the weather data comes from

Catalog climate uses mean weather by local race hour over the latest three full calendar years available before the edition, within a ±7-day seasonal window. Incomplete years mean no climate result. Forecast is separate and available from seven days before the race.

And how the Index works for your own logged runs

My Run uses the same model for all four distances, with actual race weather when available. With a known start time and finish time, weather is weighted over the duration of the run. Older caches are recalculated; weather alone never changes T.