How this is calculated
Pace = total time ÷ distance. A 5 km run in 25 minutes is 25 ÷ 5 = 5:00 per kilometre, or about 8:03 per mile.
Equivalent race times use Riegel's endurance formula: T₂ = T₁ × (D₂ ÷ D₁)^1.06. The exponent of 1.06 encodes how much pace slows as distance increases — if you could hold pace perfectly, the exponent would be 1.
Worked example from a 25:00 5K: the predicted 10K is 25 × 2^1.06 ≈ 52:07, the half marathon about 1:55:30, and the marathon about 4:00:40. Notice the marathon prediction is more than eight times the 5K time, not the six-and-a-bit that a naive scaling would give.
Why longer races cannot be run at short-race pace
Short races are limited by how much oxygen you can process and how much lactate you can tolerate. Longer ones are limited by fuel and by the accumulating cost of ground contact — glycogen depletion and muscular fatigue become the binding constraints rather than cardiovascular capacity.
The marathon sits at the far end of this, which is why it punishes inadequate preparation so dramatically. The Riegel prediction is a ceiling on what your current fitness allows, not a promise. A runner who has never exceeded 15 km in training will not hit their predicted marathon time regardless of how fast their 5K is.
Using pace in training
Most training should be considerably slower than race pace. The widely used guideline is that around 80% of weekly volume should sit at an easy conversational effort, with the remaining 20% at genuinely hard intensities — a distribution that consistently outperforms spending every session at moderate effort.
Threshold pace, roughly the effort you could hold for an hour, is close to 10K race pace for most recreational runners. Interval work sits nearer 5K pace or faster. Knowing your current pace across distances is what makes those targets concrete rather than guesswork.
How to use the pace calculator
- Enter the distance. Any distance in kilometres. Use a recent race or a hard effort rather than an easy run.
- Enter your finish time. Hours, minutes and seconds. The more recent the effort, the more meaningful the predictions.
- Read the equivalent times as ceilings. They show what your current fitness permits if you have trained for that distance. Without the appropriate mileage, the longer predictions will overstate what you can do.