The short answer
A heat adjusted pace calculator turns your cool-weather pace into the slower pace that costs the same effort in the heat. At 25°C (77°F) with a 15°C (59°F) dew point, a 5:00 per km (8:03 per mile) runner slows to 5:06–5:09 per km by Hadley's chart, or 5:07 by John Davis's model.
Key takeaways
- Hadley's chart and Davis's model agree within about a per cent in typical summer conditions.
- Use dew point or humidity with the temperature for the hour you'll run, not the day's high.
- No calculator follows a race that warms up; adjust most for the late kilometres.
- All of them are averages; acclimatisation and sun can move your number either way.
Feature check
What each one actually shows a runner, side by side.
| Tool | Best for | Price (as of September 2026) | Own route | Min:sec pace | Dew point | Heat stress |
|---|---|---|---|---|---|---|
| Hadley's chart | A quick adjustment you can work out in your head | Free | – | ◐ | ◐ | ? |
| Running Writings (John Davis) | A race plan from a published model | Free, open source, no sign-up | – | ✓ | ◐ | ? |
| RunnersConnect | A quick training answer from your local forecast | Free result; email for a bonus chart | – | ✓ | ◐ | ? |
| McMillan heat adjustment | Adjusting race times and training paces together | Price not shown | – | ✓ | ? | ? |
| Wyndo | A pace range with the research cited | Free, no account | – | ✓ | ◐ | ◐ |
| RunDida | Kilometres and °C first, with a heat-risk level | Free | – | ✓ | ? | ? |
| Runna Adapt for Heat | Training plans that adjust sessions for heat automatically | Runna subscription: $19.99 a month or $119.99 a year; 7-day free trial | – | ✓ | ? | ? |
| RunFast Map your run Ours | Seeing where on your route the heat will bite, at your pace | Free, no account | ✓ | ✓ | ✓ | ✓ |
✓ yes ◐ partly – no. Facts from each maker's own pages, as of September 2026.
On this page
- What does a heat-adjusted pace calculator tell you?
- Which heat-adjusted pace calculators can you use?
- How do Hadley's chart and Davis's model compare?
- What do RunnersConnect, McMillan, Wyndo and RunDida add?
- How is Runna's Adapt for Heat different?
- What do all these calculators miss?
- Which heat-adjusted pace calculator should you use?
- Questions runners ask
- Sources and method
Words this guide uses
- Heat-adjusted pace
- The pace in the heat that costs the same effort as your target pace in cool weather.
- Temperature plus dew point
- Coach Mark Hadley's rule of thumb, adding the air temperature and dew point in °F and reading a slowdown from a chart.
- Pace-timed forecast
- The weather for each stretch of your route at the time you will actually run it.
What does a heat-adjusted pace calculator tell you?
A heat adjusted pace calculator tells you how much slower to run in heat and humidity for the same effort. Here is one worked example, a 5:00 per km (8:03 per mile) target on a 25°C (77°F) day with a 15°C (59°F) dew point.
| Method | Slowdown (%) | Pace per km | Pace per mile |
|---|---|---|---|
| Cool-weather target | 0 | 5:00 | 8:03 |
| Hadley's chart (sum 136) | 2–3 | 5:06–5:09 | 8:12–8:17 |
| Davis's model (Running Writings) | 2.5 | 5:07 | 8:15 |
It is a practical tool for running in heat and humidity. Each one takes a single set of conditions. To see how the heat changes along your own route, use the free Map your run.
Which heat-adjusted pace calculators can you use?
Seven are worth knowing, as of September 2026. Runna's comes with a subscription; the rest are on free web pages.
| Calculator | Weather it asks for | Units | Based on | Cost and sign-up | Worth knowing |
|---|---|---|---|---|---|
| Hadley's chart | Air and dew point | °F only | Coach's rule of thumb (2013) | Free chart | Half for intervals; no hard running above a sum of 180 |
| Running Writings (John Davis) | Air with humidity or dew point, or heat index | °F or °C; miles or km | Model of 3,891 marathoners in 754 races | Free, no sign-up | Data sparse above 35°C (95°F) |
| RunnersConnect | Air and dew point, or your forecast | °F or °C; miles or km | Its coaches' formula | Free result; email for a chart | Easy, workout or race setting |
| McMillan | Air and humidity, now and target | °F or °C | McMillan's own method | Public web page | Race times and training paces |
| Wyndo | Air, dew point and sky, or the forecast | °F or °C; miles or km | Four cited studies | Free, no account | A range; stops above 28°C (82°F) WBGT |
| RunDida | Air and humidity | °C or °F; km | What it calls the Galloway model | Free | Heat-risk level |
| Runna Adapt for Heat | Feels-like temperature, session length and goal | In the app | Runna's own method | Runna subscription | Adjusts your plan's sessions |
How do Hadley's chart and Davis's model compare?
They agree closely in typical summer conditions. Davis's figure sits inside Hadley's range, usually towards its low end.
Hadley's chart is a coaching rule of thumb from 2013, not a peer-reviewed model. Davis's calculator is a statistical model fitted to marathon results from the dataset Mantzios and colleagues published in 2022.
Best forA quick adjustment you can work out in your head
Strengths
- Add air temperature and dew point in °F, then read off a slowdown
- Half the adjustment for interval sessions
- Fits in your head, with no signal needed
Limits
- A coaching rule of thumb (2013), not a peer-reviewed model
- °F only
- One set of conditions, not a race that warms up
Checked September 2026
Best forA race plan from a published model
Strengths
- A model of 3,891 marathoners in 754 races (Mantzios et al. 2022)
- Heat index, humidity or dew point; °C or °F; per km or per mile
- Free and open source
Limits
- Data sparse above 35 °C (95 °F)
- Data reach marathoners of about 3:30
- One set of conditions, not a race that warms up
Checked September 2026
| Air, °C (°F) | Dew point, °C (°F) | Hadley sum | Hadley (%) | Davis (%) |
|---|---|---|---|---|
| 20 (68) | 12 (54) | 122 | 1–2 | 1.2 |
| 25 (77) | 15 (59) | 136 | 2–3 | 2.5 |
| 25 (77) | 20 (68) | 145 | 3–4.5 | 3.3 |
| 30 (86) | 22 (72) | 158 | 4.5–6 | 4.7 |
His data reach marathoners of about 3:30, and the model may slightly understate the heat's effect on an individual, because heat-tolerant runners rise up the results on hot days.
What do RunnersConnect, McMillan, Wyndo and RunDida add?
Convenience and context.
- RunnersConnect pulls your local forecast and asks how hard the session is. Its own example takes an 8:30 per mile (5:17 per km) easy pace at 85°F (29°C) with a 70°F (21°C) dew point to about 8:50 per mile (5:29 per km).
- McMillan adjusts training paces as well as race times, with notes on acclimatisation.
- Wyndo cites four studies and gives a range, but doesn't model wind, acclimatisation or shade.
- RunDida works in kilometres and °C first. It names its method the "Galloway Temperature Adjustment Model".
Best forA quick training answer from your local forecast
Strengths
- Air and dew point, or your local forecast
- An easy, workout or race setting
- °F or °C; miles or km
Limits
- Its coaches' formula, not a published model
- Email needed for the bonus chart
- One set of conditions
Checked September 2026
Best forAdjusting race times and training paces together
Strengths
- Race times and training paces
- Air and humidity, now and on the target day
- Notes on acclimatisation
Limits
- McMillan's own method, not a published model
- Humidity rather than dew point
- One set of conditions
Checked September 2026
Best forA pace range with the research cited
Strengths
- Cites Ely 2007, El Helou 2012, Mantzios 2022 and Vihma 2010
- Gives a range, not one number
- Air, dew point and sky, or the forecast
Limits
- No wind, acclimatisation or shade
- Stops above a WBGT of about 28 °C (82 °F)
Checked September 2026
Best forKilometres and °C first, with a heat-risk level
Strengths
- Works in km and °C first
- Shows a heat-risk level
Limits
- Method named only as the 'Galloway model'
- Humidity rather than dew point
- One set of conditions
Checked September 2026
How is Runna's Adapt for Heat different?
It adjusts a training plan automatically, instead of answering a one-off question. Runna launched it on 29 July 2026 for subscribers.
Best forTraining plans that adjust sessions for heat automatically
Strengths
- Adjusts session pace targets from the forecast
- Uses feels like, session length and goal
- A colour-coded hourly forecast
Limits
- One hour and one place, not a race route
- Needs a Runna subscription
- Uses humidity rather than dew point, per one independent test
Checked September 2026
According to 9to5Mac, it scales back pace targets using the forecast feels-like temperature, the session's length and its goal. A colour-coded hourly forecast helps you pick a cooler time. It is built for training sessions, not for planning a race along a route.
What do all these calculators miss?
The heat changes during the run. A long race warms up behind you, and a slower runner meets more of it.
At the 2021 Berlin Marathon in Germany, a four-hour runner started in 16.5°C (61.7°F) air and later met 23.4°C (74.1°F), by RunFast's analysis of ERA5 reanalysis (reanalysis, not a thermometer at the start line). With the day's peak dew point of 14.9°C (58.8°F), Davis's model gives a very different answer at each end.
| Moment | Air, °C (°F) | Davis (%) | Pace per km | Pace per mile | Hadley (%) |
|---|---|---|---|---|---|
| Even four-hour pace | n/a | 0 | 5:41 | 9:09 | 0 |
| At the start | 16.5 (61.7) | 0.4 | 5:43 | 9:12 | 1–2 |
| Warmest stretch | 23.4 (74.1) | 2.2 | 5:49 | 9:21 | 2–3 |
None of them knows your route's sun, shade, hills or wind either. A pace-timed forecast, the weather for each stretch at the time you'll run it, fills that gap. Replay the race on the Berlin race page.
Best forSeeing where on your route the heat will bite, at your pace
Strengths
- Your own GPX, FIT or TCX route, or four big races
- Feels like, dew point and WBGT heat stress for every kilometre at your pace
- A map, a km-by-km table, a share image and a 3D flyover
Limits
- Not a pace calculator: no single adjusted pace
- Forecasts reach 10 days ahead
- Race-day history for four races so far
- No phone app yet: the apps are coming
Checked September 2026
Which heat-adjusted pace calculator should you use?
Use one whose method you trust, and feed it the conditions for the hottest part of your run.
- For a race plan: Running Writings, with Wyndo as a second opinion for the range.
- For daily training: RunnersConnect for a quick answer, or Runna if you already use it.
- With no signal or app: Hadley's chart, which fits in your head.
Whatever the number says, official race flags and medical advice come first. For why the dew point matters, see does humidity affect running pace; for apps, see the best running weather apps.
Questions runners ask
Are heat-adjusted pace calculators accurate?
They are good averages, not personal predictions. John Davis tested his own model on famous hot races. It put Luke Kibet's 2007 World Championships win in Osaka at 2:08:55 in ideal weather, against a best of 2:08:52. For Berhane Adere at the hot 2007 Chicago Marathon it gave 2:28:35, well short of her 2:20:42 best. Your own heat tolerance matters.
Should I use a heat calculator for easy runs?
Usually not. RunnersConnect advises running easy days by conversational effort and letting the pace land where it lands. Save the calculated adjustment for workouts and races, where starting too fast has a real cost. Coach Mark Hadley adds that interval sessions need only about half his continuous-run adjustment, because you cool a little during the recoveries.
Can I use a heat-adjusted pace calculator in Celsius?
Yes, most of them, as of September 2026. Running Writings, RunnersConnect, McMillan, Wyndo and RunDida all switch between °C and °F. Hadley's chart is in Fahrenheit only: convert both numbers first (°F = °C × 9/5 + 32) and then add them. A 25°C (77°F) day with a 15°C (59°F) dew point gives a sum of 136.
Does a heat-adjusted pace calculator work above 30°C?
Treat the answer with care. John Davis warns his data are sparse above 35°C (95°F), because marathons are almost never held above about 32°C (90°F). Wyndo stops giving paces above a WBGT of about 28°C (82°F) and defers to race flags. Hadley's chart says hard running is not recommended once temperature plus dew point passes 180.
Does heat adjustment change as you acclimatise?
Yes. McMillan's coaching notes say to use the higher end of the adjustment early in summer and less later, as you adapt. John Davis says a well-adapted runner may beat his model, and an unadapted one may fare worse. The 2015 consensus led by Sébastien Racinais says most heat adaptation comes in the first week of training in the heat.
See the heat at your pace.
Drop in your route (GPX, FIT or TCX), pick the day and your pace, and see the feels like, dew point and heat stress for every kilometre, timed to when you’ll be there. Free, in your browser, no sign-up.
Sources and method
- Maximum Performance Running (Mark Hadley) (2013). Temperature + dew point for pace adjustments
- Running Writings (John Davis) (2026). Heat and humidity pace calculator for runners
- RunnersConnect (2026). Running heat pace calculator
- McMillan Running (2026). McMillan heat adjustment calculator
- Wyndo (2026). Heat-adjusted pace calculator
- RunDida (2026). Heat adjustment pace calculator
- 9to5Mac (2026). Runna now automatically adapts training paces based on heat and humidity
- Medicine & Science in Sports & Exercise (ACSM) (2022). Effects of weather parameters on endurance running performance: discipline-specific analysis of 1258 races (Mantzios et al.)
- British Journal of Sports Medicine (2015). Consensus recommendations on training and competing in the heat (Racinais et al.)
- Copernicus Climate Change Service (ECMWF) (2026). ERA5 hourly data on single levels from 1940 to present
Calculator features, inputs and limits are from each tool's own page, checked in September 2026; we compared them, we did not test them against race results. Hadley figures come from his published chart (air °F plus dew point °F, rounded). Davis figures were computed with the model in the Running Writings calculator's published JavaScript (effort mode), with relative humidity from the dew point by the Magnus formula. Paces are rounded to the second; per-mile paces are per-km paces times 1.609344. The Berlin example uses RunFast's analysis of ERA5 reanalysis (Copernicus Climate Change Service) along the course for a four-hour finish on 26 September 2021: the start air, the warmest air met, and the day's peak dew point for both rows (reanalysis, not a thermometer at the start line).
Wes Thomson
Wes Thomson builds RunFast, which times the heat, humidity and dew point to a runner's own pace along the route, and its sister Windline, live wind forecasts for cyclists. Both run on the same forecast engine. He writes these guides with one focus: what the weather will do to your run, kilometre by kilometre, and how to plan around it.