A 2012 study by El Helou et al., published in PLOS ONE and covering nearly 1.8 million marathon finishes, found pace slows progressively once temperature climbs above each runner's optimal range. The fastest finishers lost about 0.9% per 5°C increase in wet bulb globe temperature, while typical recreational finishers lost closer to 3.2% per 5°C. This calculator applies that 0.9% to 3.2% per 5°C range above a 10°C threshold to your goal pace.
The same study found marathoners run fastest in a fairly narrow band, roughly 4°C to 10°C depending on finishing speed, with slower recreational runners tending to do best nearer the top of that range. Once race-day temperature climbs above it, times slow steadily rather than suddenly, so there is no single hard cutoff, just a widening penalty the hotter it gets.
Among the Abbott World Marathon Majors in Marathon Ballot's climate data, Sydney's 2023 edition ran hottest overall: a spring heatwave pushed the high to 32°C, sending 26 runners to hospital and treating 40 more for heat exhaustion. Berlin's 2025 edition was its own hottest on record, with a 27.6°C peak and a 22°C start. Both are preloaded as quick-picks in this calculator.
Yes. The El Helou et al. figures are based on wet bulb globe temperature, which folds in humidity, sun and wind alongside air temperature, not air temperature alone. This tool asks only for temperature, so treat the low end of the output range as a dry, breezy day and the high end as a humid or sunny one at the same temperature.
No. The underlying study found faster, likely more heat-adapted marathoners lose the least pace per degree, while typical recreational finishers lose the most. That is why this calculator always shows a range rather than one number. Heat acclimatization training narrows the gap but does not erase it.