How to use it
- Enter the Air temperature and the Dew point from the forecast, in the unit you chose under Temperature unit.
- Type your Planned pace and choose Kilometre or Mile under Pace is per.
- Read Adjusted pace for the range of slower paces, and the Temperature plus dew point sum that gives it.
- Open the table to see every band, up to the limit above 180 °F where hard running is not advised.
How the sum and the slowdown are calculated
Coaches add the air temperature and the dew point, both in degrees Fahrenheit, to get one number for the day. A sum of 100 or less needs no adjustment. Each band above that adds a slowdown range, from 0% to 0.5% for a sum of 101 to 110, up to 8% to 10% for a sum of 171 to 180. Above 180 the table advises against hard running.
This is not medical advice. In strong heat, slow down or stop when you feel dizzy, sick or confused, and get help if the symptoms do not pass.
Sum (°F) = air temperature (°F) + dew point (°F)
Adjusted pace = planned pace × (1 + slowdown ÷ 100)
Celsius to Fahrenheit: °F = °C × 9 ÷ 5 + 32- Celsius values are converted to Fahrenheit before the two numbers are added, because the table is written in Fahrenheit.
- The sum is rounded to a whole degree before its band is chosen.
What this table does not include
- Wind, sun on the course, and the change of humidity between the start and the finish. The table ignores them, and they can change the real effect a lot.
- Your fitness and how well you have adapted to heat. The same band gives the same range for a 5 km and a marathon, although the table is a rough planning figure for any distance.
- Any figure from a medical source. Heat illness depends on the person, and the pace range does not measure it.
Where the figures come from
The sum table is the one coaches quote for hot and humid runs. The source below attributes it to Coach Mark Hadley and quotes all of its bands. It is a rule of thumb: we did not find a measured data set that tests each band for each runner.
A 2007 study of seven large North American marathons (Ely and colleagues) found that marathon times slowed as the wet-bulb globe temperature rose from 5 to 25 °C. That study used wet-bulb globe temperature, not dew point, so it does not give this table.
Worked example: 25 °C with a 20 °C dew point
25 °C is 77 °F, and a dew point of 20 °C is 68 °F. The sum is 77 + 68 = 145 °F.
A sum of 145 falls in the band 141 to 150, which gives a slowdown of 3% to 4.5%. For a planned pace of 6:00 per km, 360 seconds × 1.03 = 370.8 seconds (6:11) and 360 × 1.045 = 376.2 seconds (6:16), so the adjusted pace is 6:11 to 6:16 per km.
| Sum (°F) | Slowdown | Adjusted pace per km |
|---|---|---|
| 141 to 150 | 3% to 4.5% | 6:11 to 6:16 |
Frequently asked questions
Why is the dew point used instead of the relative humidity?
The dew point tells you how much moisture the air holds, and it is the figure coaches add to the air temperature to get one number for the day. Relative humidity is a percentage that depends on the temperature as well, so it is harder to add to a temperature.
Is the same adjustment right for a 5 km and a marathon?
The table gives one slowdown range for each sum, whatever the race length. Treat it as a rough planning figure for any distance, and not as a measured value for your race.
Can this replace an adjustment from my coach?
No. A coach who knows your fitness and your heat history can give a better figure. The tool gives a planning range from a common rule of thumb.
What does it show when the sum is above 180?
It shows that hard running is not recommended, because the table advises against it above 180 °F. In that case no adjusted pace is given, only the advice.
Should I enter Celsius or Fahrenheit?
Either one works. Choose the unit of your forecast, and the tool converts Celsius to Fahrenheit before it adds the two numbers, as the table requires.
Does the tool keep what I type?
No. The sum and the adjusted pace are worked out in your browser, and nothing you enter is sent to us or saved on your device.
Sources
- Triple Threat Life: how to adjust run pace in the heat (the sum table, quoting Coach Mark Hadley)
- Ely, Cheuvront, Roberts and Montain (2007): Impact of weather on marathon-running performance, Medicine and Science in Sports and Exercise 39(3): 487-493
Figures and text last checked on Tuesday, November 10, 2026.