Why heat slows you down
Running at race pace produces a lot of heat. To get rid of it, your body sends more blood to the skin and sweats — and sweat only cools you when it evaporates. On a hot day your heart has to supply both your working muscles and your skin, so heart rate rises at any given pace. When the air is humid, sweat evaporates poorly, and your core temperature climbs faster still.
Research on marathon results has consistently found that performance is best in cool conditions, roughly 10–15 °C (50–60 °F) for most runners, and declines as it gets warmer — with slower runners, who are out on the course longer, often affected the most.
The temperature + dew point rule of thumb
A widely shared coaching rule of thumb combines temperature and dew point, both in °F, into a single number and suggests a pace adjustment for each range. It is a practical guide rather than a precise physiological model, but it captures the combined effect of heat and humidity well enough to plan a sensible race.
| Temp + dew point (°F) | Pace adjustment | At 5:00/km | At 8:00/mile |
|---|---|---|---|
| 100 or less | No adjustment | 5:00 | 8:00 |
| 101–110 | 0.0–0.5 % | 5:00–5:01 | 8:00–8:02 |
| 111–120 | 0.5–1.0 % | 5:01–5:03 | 8:02–8:05 |
| 121–130 | 1.0–2.0 % | 5:03–5:06 | 8:05–8:10 |
| 131–140 | 2.0–3.0 % | 5:06–5:09 | 8:10–8:14 |
| 141–150 | 3.0–4.5 % | 5:09–5:14 | 8:14–8:22 |
| 151–160 | 4.5–6.0 % | 5:14–5:18 | 8:22–8:29 |
| 161–170 | 6.0–8.0 % | 5:18–5:24 | 8:29–8:38 |
| 171–180 | 8.0–10.0 % | 5:24–5:30 | 8:38–8:48 |
| Above 180 | Hard running not recommended | — | — |
Example: 80 °F (27 °C) with a 65 °F (18 °C) dew point gives 145, which suggests slowing by roughly 3.8 %. A 5:00/km goal pace becomes about 5:11/km.
Dew point or humidity?
Relative humidity tells you how close the air is to saturation at its current temperature, so it changes throughout the day even when the moisture in the air does not. Dew point is the temperature at which the air would become saturated, and it directly measures how much water vapour is present. Above a dew point of about 15–16 °C (60 °F), most runners notice the air feeling sticky. If you only know humidity, the calculator converts it to dew point using the Magnus formula.
Racing smart in the heat
Start slower
Begin at the adjusted pace or slower. You can always speed up in the second half if conditions ease.
Run by effort
Heart rate and breathing tell you more than pace on hot days. Expect heart rate to drift upward over time.
Cool externally
Pour water over your head and neck, use sponges and seek shade. Wear light, loose clothing and a cap.
Acclimatise
10–14 days of training in the heat improves sweating and lowers heart rate at the same pace.
Know the warning signs. Dizziness, confusion, stopping sweating, nausea or a pounding headache in the heat can signal heat illness. Stop, get into the shade, cool down and seek medical help. No finish time is worth it.
Work out how much to drink with the sweat rate calculator, and turn your adjusted pace into splits with the marathon pace calculator.
- Ely MR, Cheuvront SN, Roberts WO, Montain SJ (2007). Impact of weather on marathon-running performance. Med Sci Sports Exerc 39(3):487–493.
- Alduchov OA, Eskridge RE (1996). Improved Magnus form approximation of saturation vapor pressure. J Appl Meteorol 35(4):601–609.
Last reviewed: September 2026 · Health disclaimer