Running Calculator
Enter a recent race time and predict your 5K, 10K, half marathon and marathon finish times with the Riegel formula — plus even pace splits for race day.
How prediction works
The Riegel formula (T2 = T1 × (D2/D1)^1.06) scales a known performance to other distances. It is well validated for trained runners across 5K to marathon. Enter your best recent time at any of the four distances and all four predictions update, each with its required average pace.
Using the pace splits
The marathon splits table shows even pacing every 5 km — the strategy behind most personal bests. Aim to hit each split within a few seconds; banking time early usually backfires after 30 km.