How to use
- 1Enter your goal pace for cool weather at sea level (min:sec per km or mile).
- 2Type the temperature (°C/°F) and relative humidity, or the dew point directly.
- 3Add the altitude (m or ft) and, if you like, choose a race distance.
- 4Read the adjusted pace, total slowdown percentage and estimated finish.
The temperature + dew point rule
A popular rule of thumb among runners adds the temperature and dew point in Fahrenheit. Below 100 there is no effect; 101–110 suggests 0–0.5% slower, 111–120 0.5–1%, 121–130 1–2%, 131–140 2–3%, 141–150 3–4.5%, 151–160 4.5–6%, 161–170 6–8% and 171–180 8–10%; above 180 hard running is not recommended. The tool interpolates within these bands and derives dew point from humidity with the Magnus formula. Example: 81 °F (27 °C) at 70% humidity gives a dew point near 70 °F, a sum of about 150, and turns a 5:00 /km goal into roughly 5:14 /km (8:03 to 8:25 per mile).
How altitude slows you
At altitude the oxygen pressure in the air drops, so sustainable endurance pace falls. Using an approximation based on research on runners at moderate altitude, the tool adds about 0.7% for every 300 m (1,000 ft) above 300 m: roughly 1.6% at 1,000 m and about 3% in Denver (1,600 m / 5,280 ft). A few weeks of acclimatization reduce, but do not remove, the effect. Short sprints are barely affected.
Racing in the heat
Summer races in the US South, Gulf states or Southeast Asia often see dew points above 68 °F (20 °C). Slow down from the first mile, drink at every aid station and pour water over your head and neck. Stop if you notice signs of heat illness such as dizziness, nausea or chills. These are approximate values and not medical advice.
Frequently asked questions
How much slower should I run in the heat?
It depends on the temperature + dew point sum in °F. At 77 °F with a 59 °F dew point expect about 2–3%; at 86 °F with a 72 °F dew point about 5–6%.
What is dew point and why does it matter?
It is the temperature at which air becomes saturated. Unlike relative humidity it directly reflects how well sweat can evaporate; above 65–70 °F it feels oppressive.
How much does altitude affect running pace?
Roughly 2.5–3% slower at 5,000 ft (1,500 m) and about 4% at 6,500 ft (2,000 m). Acclimatization reduces the gap.
Is it safe to run a marathon in high humidity?
It can be, if you relax your goal. Once the temperature + dew point sum exceeds 180 °F, racing hard carries serious health risks.
Need a website for your club?
Fast, SEO-friendly websites and Google Ads management for sports clubs, academies and gyms — talk to Webin Agency.