Weather pace calculator
Enter the pace you would run in cool, still conditions, then set the temperature and humidity you are actually facing. The calculator returns an apparent temperature, the percentage your pace should slow, and the equivalent pace and race times for those conditions. It runs in your browser, needs no account, and uses the same bands as the CollinghamCo weather and pace guide.
Format mm:ss — for example 5:00
Feels like
23.7°C
Pace cost
+3.5%
Adjusted pace
5:10
Dew point
15.1°C
5K
25:52
10K
51:44
Half
1:49:08
Marathon
3:38:17
Apparent temperature (Steadman, still air) drives the pace cost; dew point is calculated from temperature and humidity. An estimate to plan by, not a guarantee.
Why heat costs you pace at all
Running generates heat far faster than you can radiate it away. At rest you produce roughly the heat of a light bulb; at race effort, closer to ten times that. The body copes by sending blood to the skin for cooling, which means less blood is available for the muscles doing the work. The heart compensates by beating faster, so the same pace now costs a higher heart rate and more perceived effort.
This is why a hot run is a different stimulus rather than a worse version of the same one. Your legs are not weaker; the cardiovascular system is doing two jobs at once. As core temperature climbs above about 39°C, the body also starts reducing power output deliberately as a protective measure. That drop in power output is what appears in your data as a slower pace.
Humidity, dew point and why 26°C can feel worse than 32°C
Sweating only cools you when the sweat evaporates. In dry air it disappears almost immediately; in humid air it drips off your skin and carries very little heat with it. That single mechanism explains why a muggy 26°C day routinely feels harder than a dry 32°C one, and why summer performance in the UK often collapses at temperatures that would be unremarkable in Spain.
Relative humidity is a poor guide on its own, because it is measured against the maximum the air could hold at that temperature. Warm air holds more moisture, so a 60 per cent reading at 30°C contains a lot more water than 60 per cent at 15°C. Dew point is the honest number: below about 13°C most runners barely notice it, between 16°C and 20°C hard sessions become noticeably harder, and above 20°C even easy running raises core temperature quickly.
The calculator reports dew point alongside apparent temperature for exactly this reason. If the dew point is high, treat the session as a heat session regardless of what the air temperature says, and expect heart rate drift over the second half even at a steady pace.
What the calculator does under the hood
Temperature and humidity are combined into an apparent temperature using the Steadman formula in still air, then mapped onto the pace-cost bands published in our weather and pace guide. Nothing is fetched and nothing is stored: the arithmetic happens in your browser the moment you move a slider. The output is an estimate for planning, built from population-level adjustments, so treat a point or two either way as noise and lean on your own history when the two disagree.
How much slower should you actually run?
The bands used here are the ones coaches most commonly apply to road running. They are per-effort adjustments, not excuses: the point is that a hot run is a different stimulus, with a higher cardiovascular cost for the same speed.
| Conditions | Typical pace cost | What to change |
|---|---|---|
| 5–12°C, dry | None — ideal | Run your targets as written |
| 15–20°C | 1–2% | Ignore small shortfalls; keep effort constant |
| 20–25°C | 2–4% | Move key sessions earlier; carry fluid |
| 25–30°C | 4–8% | Convert intervals to effort-based; shorten long runs |
| Above 30°C | 8%+ | Easy running only unless heat-acclimatised |
A four per cent cost sounds trivial until you apply it. A 45-minute 10K becomes 46:48, and a 20-minute 5K becomes 20:48. Over a half marathon in 25°C, the same fitness that produced 1:35 in October produces around 1:40 in July. Knowing that in advance is the difference between a sensible race and a demolition.
Heat acclimatisation: the ten to fourteen day window
The body adapts to heat faster than it adapts to almost anything else. Repeated exposure over ten to fourteen days expands plasma volume, lowers the temperature at which sweating begins, and produces more dilute sweat so you lose less sodium. Heart rate at a given pace drops, and perceived effort falls with it.
You do not need a heat chamber for this. Six to ten easier sessions in the warm part of the day, kept short and controlled, are enough to move the needle for most club runners. The trade-off is real: those sessions add training load, so they belong in a week where nothing else is being pushed. Adaptation fades within a couple of weeks without exposure, which is why preparing for a hot destination race works best in the fortnight before you fly rather than a month out.
Hydration and the warning signs that matter
Dehydration worsens everything above. Losing two per cent of body mass in fluid raises heart rate at any given pace and makes the same effort feel harder. Drink to thirst before and during longer sessions, and weigh yourself before and after a long hot run if you want a personal sweat rate rather than a guess. Electrolytes matter most beyond ninety minutes, or for anyone who finishes a session with salt visible on skin and kit.
Stop and cool down if you get confusion, a pounding headache, dizziness, nausea or goosebumps in the heat. Those are signs of heat illness rather than a hard session, and they are not something to push through. A missed workout costs a day; heat stroke is a medical emergency.
Cold weather is a different problem
Cold barely changes pace. Below about 5°C muscles take longer to warm up, so the first couple of kilometres are slower and the session feels worse than it is. Wind is the bigger factor: it strips heat from exposed skin and adds resistance at the same time, which is why a still 3°C morning can feel easier than a windy 8°C one. Ignore the first few splits, add a layer you can remove, and pace on effort until you are warm.
Turning the adjustment into a useful number
The most practical use of this page is retrospection. Compare two sessions on the same route in different conditions and the raw pace will mislead you; adjust both and the picture becomes obvious. The app does this automatically, matching every synced activity to the conditions at its start time and place. Pair that with the training load calculator to check whether a hot week pushed your load into spike territory.
Frequently asked questions
How much does heat slow you down?+
As a working rule, expect one to two per cent slower per 5°C above roughly 15°C, and two to four per cent once temperatures pass 25°C with meaningful humidity. Above 30°C apparent temperature, the cost rises steeply and most club runners should be running to effort rather than to a pace target.
Why does the calculator use apparent temperature instead of the air temperature?+
Because your body responds to how easily it can lose heat, not to the number on the thermometer. High humidity reduces sweat evaporation, so 24°C at 85 per cent humidity can stress you more than 30°C in dry air. Apparent temperature folds temperature and humidity into one number before the pace cost is applied.
What is dew point and why does it matter more than relative humidity?+
Dew point measures how much moisture the air actually holds, which is what limits evaporation. Relative humidity changes with temperature even when the moisture content stays the same, so it can mislead. A dew point above about 16°C makes hard running noticeably harder, and above 20°C even easy runs raise core temperature quickly.
Should I hold this adjusted pace or run by effort?+
Use the adjusted pace as a ceiling, not a target. On very hot or humid days, heart rate at any given pace runs higher, so effort-based running is safer. The adjusted number is most useful for deciding what a fair performance looks like after the session, and for not panicking when your usual pace is nowhere to be found.
Does the pace adjustment apply to cycling too?+
Not directly. The calculator is built for running, where body weight is carried and cooling depends on air moving past you. On a bike, airflow rises sharply with speed, so heat costs less than the same apparent temperature would while running. Use the number as a rough sense of the conditions, not as a cycling target.
How long does heat acclimatisation take?+
Most of the adaptation arrives in ten to fourteen days of repeated exposure. Plasma volume expands, you start sweating earlier and more dilute sweat is produced, and the same pace costs less. The gains fade again within a couple of weeks without heat exposure, so a single hot race is best prepared for deliberately.
See what weather costs you personally
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