Running calculator
Heat and humidity pace calculator
Adjust your target pace for the conditions using the dew-point chart — how much slower to run, and when to change the session instead. Free, instant, no sign-up.
Heat changes what a session is worth, not just what it costs — the coach adjusts the week around it.
Why heat slows you down
Running generates far more heat than movement, and almost all of it has to leave through sweat evaporating off your skin. When the air is already close to saturated, that evaporation slows, core temperature climbs, and your body diverts blood to the skin to shed heat — blood that is no longer delivering oxygen to your legs. Heart rate rises at the same pace, and eventually pace has to fall. None of that is a lack of fitness or toughness.
Dew point, not humidity
Relative humidity is nearly useless for runners on its own: 90% at 5 °C is a pleasant morning and 60% at 30 °C is brutal. Dew point is the temperature at which the air would saturate, so it measures directly how much evaporative cooling is still available to you. That is why this calculator asks for it, and why the chart is indexed on air temperature plus dew point rather than on either alone.
How the adjustment is calculated
We use the widely published dew-point chart, which sums air temperature and dew point in Fahrenheit and maps that total to a percentage slowdown — nothing below 100, half a percent by 100, and rising steeply past 150. Metric input is converted before the lookup rather than the chart being restated, so the bands stay recognisable to anyone who has seen the original.
Dew-point adjustment chart
| Temp + dew point (°F) | Slow down by | Conditions |
|---|---|---|
| 0+ | 0% | No adjustment |
| 100+ | 0.5% | Minimal |
| 110+ | 1% | Minimal |
| 120+ | 2% | Moderate |
| 130+ | 3% | Moderate |
| 140+ | 4.5% | Hard |
| 150+ | 6% | Hard |
| 160+ | 8% | Severe |
| 170+ | 10% | Severe |
| 180+ | 12% | Dangerous |
Acclimation changes the number
Ten to fourteen days of consistent exposure meaningfully blunts the effect: you sweat earlier, more, and with less salt, and plasma volume rises. A runner acclimated to a hot summer will beat this chart; someone who has trained all winter indoors and met the first warm week will not. Treat the adjustment as an upper bound if you are heat-adapted, and as optimistic if you are not.