What it costs.
Place & time
Your coordinates go from your browser to the weather API. They never touch our servers.
Conditions
Entered by hand
You
It does two jobs: turning percentages into seconds, and placing your heat sensitivity.
The breakdown
Orders of magnitude, not measurements: these lines tell you what to expect and which factor weighs most, not your exact second per kilometre.
- Heat measured
- +10s/km
- Wind - brakes literature
- +2s/km
- Shoes literature
- +0s/km
- Shelter literature
- +0s/km
- Surface literature
- +0s/km
- Total
- +11s/km
Ely 2007 gives the slope, Mantzios 2022 the window: across 1258 races you run best between 7.5 and 15 °C WBGT, and you pay on both sides. A recreational runner loses three to four times what an elite loses - your reference pace places you.
Wind carries heat away, which pulls the WBGT down. That gain already sits inside the heat line; we break it out so you can see it.
Beaumont & Polidori 2025, a runner simulated in wind. An out-and-back does not cancel: the headwind costs more than the tailwind gives back. You see the average of the two halves, at constant effort.
Two measurements, two mechanisms. Hoogkamer 2018: about 4 % better running economy with a carbon plate, against a standard shoe of the same weight. Hoogkamer 2016: 100 g more per shoe costs 1.11 % of economy, which came back as 0.78 % of race time - the figure our own solve returns. The pace bought is always smaller than the saving.
A few percent of your energy goes into the air, and the faster you run the more there is. Well sheltered in a pack you get most of it back. Counted against running alone.
Road and track are the baseline. Past that, published values scatter badly: a « trail » is not one thing.
Zero is your ordinary run: standard shoes, on your own, on road, inside the optimal thermal window (WBGT 7.5 to 15 °C). What you pick gives time back; the environment can only take it away, since weather is never better than its own optimum.
Heat is the only line that moves with duration: strain accumulates, and a short run never pays it in full. Guy 2014 bounds the ratio - at the World Championships the marathon lost about 3 % in the heat, the 5000 and 10000 m about 2 %, and the sprints actually gained. Their short races are run far harder than a marathon, so at equal pace we stay below that ratio.
Beyond 4:16/km the sensitivity stays that of the slowest runner Ely measured, so your real loss is probably larger than what's shown.
An estimate, not a measurement
Real WBGT is read off a calibrated globe thermometer. Here it is estimated from four weather variables, to about 1 °C in ordinary conditions. It describes the environment's strain, not yours: training, acclimatisation, clothing and hydration all change how you respond without changing the number. This is not medical advice.