Heat Adjusted Pace Calculator

Running in heat costs you pace at the same effort. Your body sends blood to the skin to shed heat rather than to the working muscles, so the same heart rate buys you less speed. The question is never whether to slow down, it is by how much, and guessing at it is how a steady run turns into an accidental time trial.

This calculator answers that with a number. Enter the pace you hold in cool conditions along with the temperature and humidity you are about to run in, and it returns the pace that represents the same physiological effort today.

It works in three steps. Temperature and humidity give a dew point via the Magnus formula. Dew point and temperature give an approximate wet bulb globe temperature, the standard heat-stress measure used in occupational and sports medicine. WBGT then maps to a percentage pace penalty, which is applied to the pace you entered.

WBGT is used rather than the air temperature because it accounts for the thing that actually limits you. Below about 10°C WBGT the calculator applies no penalty at all, which is the honest answer on a cool day: nothing needs adjusting.

The table below runs the calculator's own model across conditions a road runner will recognise, using a 5:00/km cool-weather pace. The penalty column is the percentage the model applies; the last column is what that costs in seconds per kilometre.

Two things stand out. A warm dry day is cheaper than most runners fear: 25°C at 50% humidity costs about 5 seconds per kilometre, which over a half marathon is under two minutes. And the curve is not a straight line. It stays nearly flat to around 20°C and then climbs steeply, because heat stress is not proportional to temperature.

Over a marathon the top of that table is the difference between a plan and a rescue. At 30°C and 80% humidity, 25 seconds per kilometre is roughly 18 minutes across 42.2 km. Setting out on the cool-weather pace in those conditions does not produce a slightly slower marathon, it produces a walk from 30 km.

Look at the two 30°C rows in the table. The same temperature at 50% and at 80% humidity produces a 4.0% penalty and an 8.4% penalty: the humidity more than doubles the cost, turning 12 seconds per kilometre into 25. This is why a calculator that takes only temperature will mislead you on exactly the days it matters most.

The mechanism is evaporation. You do not cool down by sweating, you cool down by sweat evaporating, and evaporation depends on how much moisture the air can still absorb. Humid air is already close to saturated, so the sweat runs off you instead of taking heat with it. You lose the fluid and get none of the cooling.

Holding temperature constant at 25°C and moving humidity alone shows the same effect in miniature: 40% humidity gives a 1.3% penalty, 60% gives 1.9%, 80% gives 3.3% and 90% gives 4.0%. The thermometer never moved.

Dew point is the single number that captures this best, which is why experienced runners track it instead of relative humidity. For the physiology behind it and how to read a forecast, see how dew point changes your real pace. For the wider picture on training and racing through a hot summer, heat and pace covers the season rather than the session.

Adjust the target before you start, not at the 5 km mark when it is already going wrong. Heat is one of the few running variables you can read off a forecast the night before, so there is no excuse for discovering it mid-run.

On easy runs, do not use the adjusted pace as a target at all. Run to effort or heart rate and let the pace be whatever it is. The calculator's value on an easy day is reassurance that a slower watch reading is the conditions rather than lost fitness.

On interval and threshold sessions the adjusted pace is the target, and it is worth being honest about it. Running the cool-weather split for the first two reps and falling apart on the fourth is a worse session than hitting the adjusted split six times.

On race day, apply the adjustment to your goal pace and commit to it in the first kilometre. Heat penalties compound: you cannot bank time early and pay it back later, because the deficit grows as core temperature rises. Check the forecast for the actual start time rather than the afternoon peak, and remember that a race starting at 08:00 may finish in conditions two hours warmer.

Two related tools help either side of this. The race day weather forecast gives the conditions to type in here, and if your race also has elevation to contend with, the altitude adjustment calculator handles that penalty separately. To turn an adjusted pace back into a finish time, use the pace calculator.