Hot, muggy air costs you pace whether you plan for it or not. This runs the coach-standard temperature plus dew point heat-stress model against your own easy, tempo, and race paces, so instead of guessing you get an exact percent to add and a plain read on how serious today actually is.
⏵ TODAY'S AIR
DIAL IN THE WEATHER
Temperature
°F
Moisture
°F
Dew point is the better read on how heavy the air feels, use it when your weather app shows it. No dew point handy? Humidity percent estimates one for you.
Your paces
Easy/mi
Tempo/mi
Race/mi
Any field can be blank. Enter as M:SS (like 9:30) or decimal minutes (like 9.5).
⏵ THE READ
142° HEAT-STRESS SUM
Temp + dew point
82°+60°
F, sum drives the adjustment
Pace add
+6percent
Risk: high
What this means
Add 5 to 6 percent to your pace today. Heat stress is building, slow down and drink more.
⏵ YOUR ADJUSTED PACES
TODAY'S NUMBERS
Easy
9:3010:04/mi+34s
Tempo
7:307:57/mi+27s
Race
6:457:09/mi+24s
Shareable line
Heat-stress sum 142 (82°F temp + 60°F dew point): Add 5 to 6 percent to your pace today. Heat stress is building, slow down and drink more.
⏵ This is the generic version
This tool takes one temperature and dew point reading you type in. Summit Line runs the same model against your race-day forecast and your actual trained paces, and factors it straight into your fueling and pacing plan so race morning is not the first time you have thought about the heat.
No major running app models heat-adjusted pacing well, most either ignore the weather entirely or bury a vague warning somewhere. Here is the actual logic behind the number above.
Temperature alone lies about heat stress
A 90 degree day in the desert and a 90 degree day in the swamp are not the same run, and pure temperature cannot tell them apart. What actually costs you pace is how well your sweat evaporates, and that depends on how much moisture is already sitting in the air. Dry heat lets sweat evaporate and cool you, wet heat traps it against your skin and does almost nothing, so the same thermometer reading can mean two completely different days on the road or trail.
That is why coaches lean on the temperature plus dew point sum instead of temperature by itself. Add the two together and you get a single number that tracks real heat stress far better than either one alone, because it folds moisture into the read directly instead of leaving it as an asterisk.
The sum, and what it means
Below a sum of about 100 (say, 70 degrees with a 30 degree dew point), the air barely matters and this calculator returns no adjustment. From about 100 up to 170, the cost climbs steadily, roughly half a percent to a bit over one percent slower for every 10 points the sum climbs, and that is what shows up here as an add on your easy, tempo, and race pace.
Past about 180, the model stops being mainly about pace and starts being about safety. Heat illness risk accelerates faster than any pace formula can capture at that point, so this calculator flags racing as not advisable rather than handing you a number to chase. That flag is the whole point of building this against a real physiological model instead of a vague weather warning.
Dew point beats humidity percent
Relative humidity is relative to the current temperature, so a 70 percent reading means almost nothing on its own, it is comfortable at 50 degrees and miserable at 90. Dew point has no such asterisk: it is an absolute measure of moisture in the air, so a 65 degree dew point feels sticky whether the thermometer says 75 or 95. That is why every serious version of this heuristic runs on dew point, and why this calculator estimates one for you from humidity when that is all you have, rather than working off humidity directly.
Rule of thumb for reading a bare dew point number: under 60 feels comfortable, 60 to 65 starts to feel humid, 65 to 70 is sticky, and above 70 is the kind of heavy, soaking air that slows everybody down, fit or not.
Percent, not a flat number of seconds
This calculator adjusts by percent rather than a flat number of seconds per mile, because the right add scales with how fast you are already moving. A 6 percent add on a 9:30 easy pace is about 34 seconds. The same 6 percent on a 6:45 race pace is about 24 seconds. Both are the same underlying heat cost, they just land differently depending on the effort, which is exactly why the calculator runs the math separately against your easy, tempo, and race pace instead of handing back one generic number.
Heat pace FAQ
What is the temperature plus dew point rule?
It is a quick heat-stress heuristic coaches use out on the roads and trails: add the air temperature in Fahrenheit to the dew point in Fahrenheit and read the sum. Below about 100 the air is not much of a factor. Push past 130 or so and the cost gets real. Cross about 180 and most coaches will tell you racing hard is not a good idea at all. Dew point matters more than a bare humidity percent because it is an absolute read on how much moisture is actually in the air, not a percentage that means something different at 50 degrees than it does at 90.
Why dew point instead of humidity percent?
Humidity percent is relative to the air temperature, so 70 percent humidity at 50 degrees is nothing and 70 percent humidity at 90 degrees is brutal, same number, completely different day. Dew point does not have that problem. It is the actual temperature the air would need to cool to for water to start condensing out of it, so a higher dew point always means more moisture in the air, full stop. Below about 60 dew point feels comfortable, above 65 it starts to feel sticky, and above 70 it is the kind of heavy air that soaks your shirt standing still.
How many seconds per mile should I add for the heat?
This calculator turns the temp plus dew point sum into a percent, not a flat number of seconds, because the right add scales with how fast you are already running. A percent add on an easy pace is a small number of seconds, the same percent on a hard tempo or race pace is a bigger one. Enter your own easy, tempo, and race paces above and the calculator does that math for each one.
I don't know today's dew point, what do I do?
Flip the toggle to humidity percent. The calculator estimates a dew point from your temperature and humidity using a standard meteorological approximation, close enough for pacing purposes. If your weather app shows an actual dew point reading, use that instead, it is more accurate than any estimate.
At what point should I just not race hard in the heat?
Once the temp plus dew point sum crosses roughly 180, most coaches call that racing-not-advisable territory, and this calculator will say so plainly. That is not about being slow, it is about heat illness risk climbing faster than any pace adjustment can account for. If a race falls on a day like that, the smart move is to run by feel, not by goal pace, and to treat finishing upright as the actual win.
Does this replace acclimatization?
No. A well heat-acclimatized runner handles the same temp plus dew point sum better than someone who has not trained in the heat, this calculator does not know your acclimatization status, only the air. Two weeks of deliberate heat exposure meaningfully changes how your body handles a hot race, this tool is for pacing on a given day, not a substitute for building that adaptation ahead of time.
Take heat pacing from one guess to a real forecast
Summit Line runs this same heat-stress model against your actual race-day forecast, your trained paces, and your fueling plan, so the plan you rehearsed is the plan that survives a hot start line.
This calculator uses the coach-standard temperature plus dew point heat-stress sum (Fahrenheit), banded into a percent slowdown that rises from roughly 0.5 to 1 percent per 10 points of sum below about 170, and steepens above that toward a racing-not-advisable flag near 180. When only humidity is available, dew point is estimated from temperature and relative humidity using a standard meteorological approximation. It is a pacing heuristic, not a medical tool, always use your own judgment (and stop) if you feel signs of heat illness.