Race Time Predictor

The Daniels-Gilbert Formula

The predictor uses the Daniels-Gilbert formula, developed by exercise physiologist Jack Daniels from decades of elite and sub-elite athlete data. The core idea is that a runner's aerobic capacity — expressed as a VDOT score derived from a race performance — produces consistent, predictable equivalences across all standard distances. Enter one result and the formula projects the full table.

VDOT vs VO₂max

VDOT is not the same as a laboratory VO₂max value. It is back-calculated from an actual race performance, so it already encodes both your aerobic ceiling and your running economy — how efficiently you convert oxygen into forward speed. Two runners with identical lab VO₂max values can have very different VDOTs if one has a more economical stride. This is why race-result predictions are more accurate than predictions from a watch estimate, and why watch VO₂max values (which are running-specific) map more closely to VDOT than lab values do.

Why %VO₂max Drops With Distance

The formula encodes a key physiological fact: longer races are run at a lower fraction of aerobic capacity. A 5K is typically run near 95–97% of VO₂max; a marathon sits closer to 75–80%. This is why projecting 5K pace directly to marathon distance doesn't work — and why the confidence interval widens as the target distance grows (±2.5% for 3K–10K, ±4% at the half marathon, ±5.5% at the marathon).

When Are Predictions Less Reliable?

Extrapolations between similar distances (5K → 10K) are the most accurate. Accuracy drops when the input race was run in heat, on hills, or at uneven effort, and when the runner isn't specifically trained for the target distance. Marathon predictions carry the widest range because glycogen depletion, pacing discipline, and fuelling introduce variability that aerobic capacity alone cannot capture. Use the marathon figure as a starting point, not a target set in stone.