Put in your race conditions and your goal. It returns the estimated WBGT, what the heat will cost you, and the pace you should start at.
Values observed on race morning at airport weather stations. Edit them by hand for your own day's forecast.
Race guide, with the profile, the cut-offs and how to train for it: Miami Marathon.
The WBGT is estimated with the Australian Bureau of Meteorology model: WBGT = 0.567 × Tair + 0.393 × e + 3.94, where e is the vapour pressure derived from the dew point. It carries no solar radiation and no wind term, so it approximates a shaded value: in direct sun the real WBGT is higher. Treat it as a floor, not a central estimate.
The penalty applies the published coefficient of 0.3–0.4% per degree of WBGT above 15 °C, measured across 1,258 races and 7,867 athletes. For mid-pack it multiplies by 2 to 3, from large-marathon data where 300th place lost 7.9% while 25th place lost 2.6%.
The acclimation discount is the softest of the three: it assumes a complete block compresses the penalty by 30 to 50%. That sits inside the published range, but it is an estimate, not a measured coefficient.
The combination of the three is derived, not measured. No validated WBGT-to-pace table has ever been published. This calculator is the explicit arithmetic of real coefficients — which is different from a number out of nowhere, and different from a guaranteed result.
You now know what it will cost. What the calculator cannot tell you is what to do with the weeks you have left, how to cool yourself during the race, or the point at which heat stops being a pacing problem and becomes a safety one.
Read the full guide to heat and performance — twelve sections, with the plan sorted by how long you have.