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Heat-adjusted pace calculator

Heat slows every runner — the question is how much to adjust. This calculator turns your goal and the conditions into a pace range based on published research you can check below. It can also pull the live forecast for your race.

Your race

Distance
Goal
Pace unit
Race conditions

Dew point drives how well sweat evaporates — most weather apps list it hourly.

Unknown sky assumes 75% of full-sun solar load. That can materially change estimated WBGT near the no-pace boundary.

The research — and our modeling choices

The model's anchor coefficients trace to the studies below. The choices that connect them — solar fractions, range caps, and cohort widening — are Wyndo modeling decisions documented separately from the published findings:

  • Ely MR, Cheuvront SN, Roberts WO, Montain SJ (2007). Med Sci Sports Exerc 39(3):487-493

    Marathon slowdown grows with WBGT (studied range ~5-25 °C) and is cohort-dependent: fastest runners ~1% per +5 °C WBGT, three-hour runners roughly 1.5% per +5 °C — our upper bound.

  • El Helou N, Tafflet M, Berthelot G, et al. (2012). PLoS ONE 7(5):e37407 (1.8M marathon results)

    Performance-vs-temperature is an inverted U: optimum roughly 4-10 °C, cooler for faster cohorts, with accelerating slowdown away from it — our cohort optima and curve shape.

  • Mantzios K, Ioannou LG, Panagiotaki Z, et al. (2022). Med Sci Sports Exerc 54(1):153-161

    Large multi-race analysis found materially smaller heat slowdowns than Ely (roughly half to two-thirds), with observed maxima near ~8% — our lower bound and reality check.

  • Vihma T (2010). Int J Biometeorol 54(3):297-306 (Stockholm marathon)

    Independent confirmation that slower finishers lose more time per degree of heat than faster ones — supports the cohort ordering.

  • Riegel PS (1981). American Scientist 69(3):285-290

    Endurance-time scaling t2 = t1·(d2/d1)^1.06 — used only to place a non-marathon goal time into the marathon-based cohort bands.

  • Yaglou CP, Minard D (1957). AMA Arch Ind Health 16(4):302-316

    Defines WBGT = 0.7·T_wet-bulb + 0.2·T_globe + 0.1·T_air — the index every anchor study and race flag system uses.

  • Stull R (2011). J Appl Meteorol Climatol 50(11):2267-2269

    Wet-bulb temperature from air temperature and relative humidity (valid roughly -20…50 °C, RH 5-99%) — used by our simplified WBGT estimate.

  • Armstrong LE, Casa DJ, Millard-Stafford M, et al. (ACSM position stand) (2007). Med Sci Sports Exerc 39(3):556-572

    Event-alert flag categories for road racing; black flag above ~28 °C WBGT — where this calculator stops giving pace numbers and defers to the race’s flag guidance.

How it works — and what it doesn’t know

Your goal is placed into a cohort by marathon-equivalent time (slower cohorts slow more per degree). Conditions become a heat-load estimate (WBGT). Above your cohort’s optimum, the slowdown range grows with heat: the lower bound follows the smaller published effect (Mantzios 2022), the upper bound the larger one (Ely 2007, accelerating per El Helou 2012’s curve).

Not modeled in v1: wind (so treat the heat estimate as approximate in breezy conditions), heat acclimatization, course shade or surface, and cold-weather slowing. The WBGT estimate is a documented simplification; a full physical model (Liljegren) is the planned upgrade.

This is planning guidance from published field studies — not medical advice, and never a green light to race in dangerous heat. Above ~28 °C WBGT the calculator stops giving pace numbers and defers to your race’s event-alert flags.