Race time calculator: adjust for hills and heat
Enter the time you could run on a flat course in cool weather, choose a race, and see what that is likely to mean on its real course and in its typical race-day weather.
Estimated finish at the Boston Marathon (Marathon, 19 Apr 2027)
3:51:28
5:29 /km · +6:28 compared with flat and cool
| Flat course, cool weather | 3:45:00 | |
| Hills | 248 m up, 386 m down | +6:22 |
| Heat | typically 10→14°C, 52% humidity | +0:06 |
How the estimate works
Hills. We use the “effort distance” convention from the International Trail Running Association (ITRA): every 100 m of climbing costs about as much as 1 km of flat running. Descents help, but less than climbs hurt, so we credit a third of that.
Heat. From the race’s typical weather (10 years of history from the start time), about 0.3% slower per °C of average temperature above 12°C, and a quarter more when humidity is 75% or higher.
Altitude. About 1% slower per 300 m of average altitude above 1,000 m.
These are simple rules of thumb, not a prediction: fitness, pacing, heat acclimatization, trail surface and the weather on the day all matter. Use the result to set a realistic range, not a single number.