Enter a recent race result and see your equivalent effort at other distances.
Predictions use Riegel's formula (T2 = T1 × (D2/D1)1.06), a widely used estimate for equivalent-effort race times. Actual results vary with training, terrain, and race-day conditions.
It's an estimate of equivalent aerobic effort, not a guarantee. Predictions are most reliable between distances of similar duration (e.g. 5K to 10K) and get less reliable the further apart the distances are, since longer races depend more on endurance-specific training, fueling, and pacing discipline that a single result can't capture.
T2 = T1 × (D2/D1)1.06, a widely used formula from exercise scientist Pete Riegel that predicts an equivalent-effort finish time at a new distance from a known time at another distance. The 1.06 exponent accounts for endurance naturally declining a bit as distance increases.
Predicting a marathon from a short race (like a 5K) tends to be optimistic, because the marathon also demands endurance-specific training, fueling, and pacing that a 5K result doesn't measure. Treat long-distance predictions from short races as an upper-bound estimate, not a target.
It's a reasonable starting point, especially between similar distances, but factor in your specific training for the goal race, the course, and conditions before committing to a goal pace.