Race Time Predictor
Use your recent race results to predict your potential performance at other distances.
Recent Performance
Distance
Time
When race predictions are useful
Predictions work best when the input race is recent, the course was honest, and the target distance is not wildly different from the race you entered.
5K to 10K
Usually reasonable if you train consistently and do not fade badly after 5K.
5K to marathon
Often optimistic unless you already have long-run durability. Use it as a ceiling, not a promise.
10K to half marathon
A useful range for most runners because the distances are close enough to share fitness.
Half marathon to marathon
Good for setting a marathon target, but only if fueling and long runs are already in place.
About the Riegel formula
Predictions use the Riegel formula: T₂ = T₁ × (D₂/D₁)^1.06, a widely-used model developed by researcher Peter Riegel. The exponent 1.06 accounts for the natural slowdown over longer distances due to aerobic fatigue. Predictions are most accurate when your input race distance is within 2-3x of the target distance.
Use this result next
Connect this calculator with the next training decision.