Every runner who's raced a warm-weather marathon knows the feeling: goal pace that felt easy in March training runs turns into a fight by mile 10 on an 80-degree race day. The pace didn't change. The cost of holding it did.
Why heat slows you down
Running generates heat that your body needs to shed, mostly through sweat evaporation. As ambient temperature rises, your cardiovascular system has to split effort between delivering oxygen to working muscles and delivering blood to the skin for cooling — two jobs competing for the same limited output. Past a certain point, that competition slows you down even if your legs and lungs feel fine, because your heart is doing double duty.
A commonly cited coaching rule of thumb: expect roughly 1% slower pace for every 5°F above about 60°F, and expect that number to get worse — not just additive — as humidity climbs, since sweat that can't evaporate isn't cooling you at all.
Running the numbers
The Heat Pace Adjustment Calculator applies the temperature side of this rule to your goal pace.
Worked example: A 7:30/mile goal pace at a race-day temperature of 80°F — 20 degrees above the 60°F baseline — suggests roughly a 4% slowdown, putting realistic pace closer to 7:48/mile. That's before layering in humidity, which the simple temperature-only rule doesn't capture, and which can push the real number well past what this calculator alone suggests on a muggy morning.
What to actually do with this number
Don't fight the adjustment — plan around it. If race morning is running 15-20 degrees warmer than your training conditions, adjust your goal pace down before the gun, not at mile 18 when you're already overheated and behind on fluids. Heat acclimation over 1-2 weeks of deliberately training in warm conditions can close some of this gap, but it doesn't erase it. On a genuinely hot race day, running by effort and pulling back the target pace going in beats chasing a goal time set for 55-degree conditions.