How it works
index = temperature°F + dew point°F → pace slowdown band
The method adds two Fahrenheit readings — air temperature and dew point — and reads the total against a banded chart. A sum of 100 or lower needs no adjustment at all. From there, each band eases the pace a little more: the default inputs on this page, 80°F air with the dew point at 65°F, sum to 145 and call for roughly a 3–4.5% slowdown, which stretches a 9:00 mile to about 9:16–9:24. Once the sum passes 180, the chart stops suggesting percentages and simply says skip the hard running. Dew point does the heavy lifting here because it tracks the absolute moisture in the air: when the air is already saturated, sweat sits on your skin instead of evaporating, and evaporation is your main cooling system. The output is deliberately a band rather than one pace, since acclimation, body size, and fitness change how hard the same conditions hit each runner.
Sources
- Hadley temperature + dew-point method Mark Hadley, Maximum Performance Running — heat & humidity pace-adjustment chart: temperature (°F) + dew point (°F) → percentage slowdown band.
- Why dew point, not just temperature Dew point quantifies absolute humidity; high humidity limits evaporative sweat cooling, raising heat strain independent of air temperature.
- Heat and endurance performance Endurance running pace declines as heat stress rises, with larger effects over longer races and for less heat-acclimated runners.
FAQ
When should I use the heat adjusted pace calculator?
The night before a warm race, or before any quality workout in summer. Pull the forecast temperature and dew point for your start time, enter your cool-weather pace, and you have a realistic target band before you toe the line — instead of discovering the slowdown at mile four.
What inputs matter most?
The dew point, and it is the one runners skip. Temperature alone misses the humidity half of the equation — a 75°F morning with a 70°F dew point can cost you more than a dry 85°F afternoon. Use the dew point for your actual start hour, not the daily average, and make sure the baseline pace really is a cool-weather pace.
How should I read the slowdown band?
As a bracket, not a verdict. Well heat-acclimated runners typically live at the fast end of the band; if you have trained through a cool spring and the race turns hot, plan for the slow end. Mild conditions may cost under 1%, while a truly hot, humid day can take 5% or more off your pace.
Does this work in °C and kilometers?
Yes — the temperature and dew point fields toggle between °F and °C, and the pace fields accept per-mile or per-kilometer. The Hadley chart itself is defined in Fahrenheit, so Celsius entries are converted before the sum is computed; the band you get back is identical either way.
What dew point is comfortable for running?
Below roughly 55°F most runners barely notice the moisture. From 55–65°F it starts to register, 65–70°F feels genuinely hard, and past 70°F the air is oppressive no matter how fit you are. Those thresholds are why an innocent-looking forecast can still call for a meaningful pace adjustment.
When is it too hot for hard running?
When temperature plus dew point tops about 180, the method stops prescribing a percentage and recommends skipping hard efforts entirely. At that level, downgrade the session to easy effort, move it to dawn, or take it indoors — and treat dizziness, chills, or confusion as reasons to stop immediately.
The slowdown bands here come from a coaching chart applied to the forecast you enter; your own response to heat will vary with acclimation, fitness, and health. Running hard in hot, humid weather carries real risk of heat illness — ease off or stop if you feel unwell, and treat this page as planning information, not medical guidance.