What the conditions do.
Place & time
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Conditions
Entered by hand
You
It does two jobs: turning percentages into seconds, and placing your heat sensitivity.
The breakdown
Orders of magnitude, not exact splits: these figures give you an idea of what impacts your session most; they remain orders of magnitude to adapt to your conditions (example: tailwind, etc.).
- Heat measured
- +10s/km
- Wind that slows literature
- +2s/km
- Shoes literature
- +0s/km
- Shelter / group session literature
- +0s/km
- Surface literature
- +0s/km
- Total
- +11s/km
Don't underestimate the heat! The ideal temperatures for performance sit between 7.5 and 15 °C WBGT*. Outside that zone your paces take a hit, and a recreational runner loses three to four times more than an elite. Ely (2007), Mantzios (2022). Heat weighs all the more as the session gets longer: if it's too hot, better to favour low intensity (easy endurance), or interval sessions with short reps.
Wind carries heat away, so it pulls the WBGT down. That gain is already counted in the heat line above; we break it out here to make it visible.
A tailwind does not make up for a headwind: the headwind costs you more energy than what you gain with the wind at your back. We show the headwind / tailwind average, at constant effort. If your session is headwind only, expect a bigger loss; if you only have tailwind, you'll see gains. Beaumont & Polidori (2025).
Two studies, two different mechanisms. Hoogkamer (2018): about 4 % better running economy with a carbon plate, compared to a standard shoe of the same weight. Hoogkamer (2016): every extra 100 g per shoe costs 1.11 % of running economy, i.e. 0.78 % of race time - which our calculation finds on its own.
Well sheltered in a group, you can shield yourself from the wind and also benefit from the drafting effect. Try not to isolate yourself on your session or race (when possible) so you spend less energy at the same pace.
Road and track are the baseline. On a trail or a path, your energy cost degrades.
The reference is your ordinary run: standard shoes, on your own, on road, inside the optimal thermal window (WBGT 7.5 to 15 °C). Be aware of all these parameters so you can adapt your sessions and stay in the right training intensity.
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 an exact measurement!
This is not medical advice. You stay in charge of your session; be careful if conditions are not suitable for sport.
*WBGT (Wet Bulb Globe Temperature): an indicator measuring the heat-stress risk for the human body, which is far more relevant than air temperature alone, which doesn't tell the whole story.