Tools › Heat and humidity pace calculator for runners
RunningEnter a US ZIP code, or the temperature and humidity from your weather app, to see roughly how much the heat may slow your running pace. Runs in your browser; nothing is saved. The optional forecast lookup sends a map point for your ZIP code, or your location, straight to the US National Weather Service, along with the internet (IP) address every website visit carries.
Enter a US ZIP code or use your location, and we’ll fill in the temperature and dew point below from the National Weather Service hourly forecast. Racing? Pick your race day and start time first.
Your browser sends the map point for your ZIP code, or your location, straight to the US National Weather Service (weather.gov), along with the internet (IP) address every website visit carries. It isn’t sent to us, and we don’t store it.
Type minutes and seconds, like 9:00, or a finish time, like 4:00:00.
Step 1, the heat level. We estimate WBGT from the temperature and humidity with the approximation the Australian Bureau of Meteorology uses: WBGT = 0.567 × air temperature + 0.393 × vapor pressure + 3.94 (in °C; vapor pressure is the amount of moisture in the air, measured in hectopascals, hPa, and we work it out from the humidity or dew point). It assumes a fairly sunny day with a light wind, so under heavy cloud, in a strong wind or before sunrise the real figure is usually lower. A 2022 analysis of 1,258 endurance races used the same approximation and found it tracked the WBGT values reported by race organizers closely (Mantzios et al., 2022). The heat levels are the World Athletics heat-stress bands as listed in that analysis: up to 10 °C (50 °F) cool, 10–18 °C (50–64 °F) neutral, 18–23 °C (64–73 °F) moderate, 23–28 °C (73–82 °F) high, above 28 °C (82 °F) extreme.
Step 2, the slowdown. In results from seven big-city marathons, finish times slowed steadily as WBGT rose from about 5 to 25 °C, for men and women, and slower runners lost more time than the fastest (Ely et al., 2007; Montain et al., 2007). Across 1,258 races, the best performances came at a WBGT of about 7.5–15 °C, and endurance performance fell about 0.3–0.4 % for every °C outside that zone; in the marathon data that analysis reviews, the drop was about 0.3 % per °C for elite women and about 0.6 % per °C for well-trained, non-elite runners (Mantzios et al., 2022). We start counting at 10 °C WBGT and use 0.3–0.6 % per °C above it. Faster runners can plan near the low end of the range; most recreational runners near the high end.
The same pattern shows up beyond the elite field: higher temperatures slowed amateur runners more than elites at the Great North Run half marathon (Hodgson et al., 2022), and air temperature had the strongest link to finish times at the Stockholm Marathon, with slower runners affected more (Vihma, 2010). In a pacing study, slower marathoners ran slower from the start in warm weather rather than fading more at the end (Ely et al., 2008), so start at the adjusted pace. For training runs, the same percentage is a useful guide, and running by effort or heart rate is the simpler way to handle a hot day.
The forecast lookup (US only). For a ZIP code, we look up the center point of that ZIP code area from the US Census Bureau’s ZIP code area list, which this site keeps a copy of. Your browser then asks the National Weather Service for its hourly forecast at that point, and we fill in the air temperature and dew point for the hour you pick. “Use my location” does the same with your device’s position, rounded to about 100 meters first. The hourly forecast runs about 6 days ahead and is updated through the day, so look again the evening before and on race morning. The map point for your ZIP code, or your location, goes straight from your browser to weather.gov, along with the internet (IP) address every website visit carries; it isn’t sent to us, and we don’t store it. Outside the US, or if you prefer, type the numbers in yourself.
Above 28 °C (82 °F) WBGT we give guidance in place of a pace. Heat illness can affect healthy athletes, even on days that aren’t very hot, and spotting it early and cooling fast reduce harm; confusion or odd behavior is a warning sign of heatstroke (American College of Sports Medicine, 2007). In a study of marathons, races started above about 21 °C (70 °F) WBGT with runners who weren’t used to the heat often ended in cancellation or with many runners needing medical care (Roberts, 2010). Getting used to the heat (heat acclimatization: about 1–2 weeks of repeated training in warm conditions) is the most important way to reduce the strain, along with starting well hydrated (Racinais et al., 2015). To plan your drinking, measure your sweat rate on a warm day.
General information for healthy adult runners, not medical advice. If you have a heart condition or another medical condition, or are pregnant, talk to a doctor before running hard in hot weather.