Calculator

Running heat pace calculator

Rule of thumb

Air temperature 77°F

Relative humidity 70%

Target pace, per mile or kilometre

Minutes and seconds, for whichever unit you train in — the adjustment is a percentage, so it works for either.

5:09–5:14 Adjusted pace — about 11 seconds slower for the same effort.
Adjust the pace and the session is fine to run.

Dew point: 66°F
Temperature + dew point: 143
Suggested slow-down: 3–4.5%

The commonly published temperature-plus-dew-point adjustment, in °F. Coaching practice rather than a physiological model. The row matching your inputs is highlighted.
Temperature + dew point	Slow down by
0–100°F	No adjustment
100–110°F	0–0.5%
110–120°F	0.5–1%
120–130°F	1–2%
130–140°F	2–3%
140–150°F	3–4.5%
150–160°F	4.5–6%
160–170°F	6–8%
170–180°F	8–10%
Above 180°F	Hard running not recommended

The numbers above are the answer for the values the page loaded with. Changing the controls needs JavaScript — without it the calculator cannot recalculate, so read the figures as one worked example rather than a live result.

How much slower should you run in the heat?

Add temperature and dew point together in Fahrenheit and read the slow-down off the published table. At 77°F with 70% humidity the dew point is 66°F, a sum of 143, which calls for 3 to 4.5% slower — about 9 to 14 seconds a mile off a five-minute pace. Enter your own numbers above.

Why the two numbers get added together

Running produces a great deal of heat, and almost all of it leaves through sweat evaporating. Two things decide whether that works: how hot the air is, and how much water it is already carrying. Temperature alone misses the second one entirely, which is why 95°F in Arizona and 77°F in Florida can cost a runner the same amount.

Adding temperature to dew point is a crude way of combining them, and it works well enough to have become standard in coaching. It is not a physical quantity — nobody claims 143 means anything on its own. It is an index that happens to sort days in roughly the order runners experience them.

Worked example: the same session, two climates

Phoenix, 6pm: 95°F, 20% humidity → dew point 48°F → sum 143
Orlando, 6am: 77°F, 70% humidity → dew point 66°F → sum 143

3–4.5% Same adjustment for both. Eighteen degrees apart on the thermometer, identical demand on the runner.

What to change besides pace

The percentage handles the clock. These handle the rest of it:

Start earlier rather than later. The dew point barely falls in the evening, while the temperature does — but the ground has been absorbing heat all day. Dawn is usually the best window, and the hottest part of the day explains why 6pm is not the mirror of 6am.
Take the effort target, not the pace target. On a threshold session, run the effort and let the pace be whatever it is. Trying to force the pace in heat converts a threshold run into an interval session.
Shorten the reps, not the rest. Cutting recovery in the heat is the fastest way to end a workout early.
Give it two weeks. Most of the adaptation to heat happens in 10 to 14 days of exposure, and it is substantial: plasma volume rises, sweating starts sooner and at a lower salt concentration. Heat acclimatization covers what changes and how quickly it is lost.

Where the table stops

At a sum above 180°F the guidance is no longer a percentage. That corresponds to conditions like 95°F with 70% humidity — a heat index around 125°F, which is the top of the Danger band. At that point the honest adjustment is not a slower pace but a different plan.

Running in the heat covers the decision in full, including the signs that a run should be abandoned mid-session.

Common questions

Why dew point instead of relative humidity?

Because relative humidity swings through the day even when the moisture in the air is unchanged: as the temperature climbs from dawn, humidity falls without a drop of water leaving. Dew point is the actual water content expressed in degrees, so it stays roughly flat and lets you compare a 6am run with a 6pm one.

How reliable is this adjustment?

It is a coaching rule of thumb with wide adoption, not a validated physiological model. Individual variation is large — acclimatisation, fitness, body size and sweat rate all matter — so treat the range as a starting point and let effort, not the watch, make the final call.

Does the adjustment apply to easy runs too?

Less so. The table is aimed at quality sessions where you are trying to hit a pace. On an easy run the right adjustment is simply to run by feel and accept whatever pace that produces, which will usually be slower than the table suggests.

What about running in dry heat?

Dry heat is genuinely easier to run in, and this table reflects that automatically: at 95°F with 20% humidity the dew point is around 48°F, giving a sum of 143 — the same as a muggy 77°F day. Both call for the same adjustment, which is the point of adding the two numbers.

When should I move the session indoors?

When the sum passes 180°F the published table stops giving a percentage and recommends against hard running. That is also roughly where a heat index sits in the Danger band, so it is a consistent line to hold.

Sources

NIOSH (2016). Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments — Acclimatisation and heat-stress background
NWS Weather Prediction Center — The Heat Index Equation — For the heat index shown alongside

Published 30 July 2026