The conversion that always trips people up
Min/km to min/mi is min/km × 1.609344. People learn it as “multiply by 1.6” and end up running 30 seconds per mile off. The calculator avoids that by doing the conversion in total seconds and only formatting once at the end, so you will never see "4:60" — the carry overflow bug that affects half the pace converters online.
Why runners track pace and cyclists track speed
They’re reciprocals (pace × speed = 60 when speed is km/h and pace is min/km). The two communities prefer different units because of how the brain works at different effort durations. A runner has a clear feel for 5:00/km feel; a cyclist has a clear feel for 30 km/h. Use whichever feels natural to you for that sport — but be ready to convert when you read training plans from the other camp.
Race prediction needs a different formula
Flat pace conversion doesn’t work for predicting longer race times — fatigue scales nonlinearly. Pete Riegel’s 1981 formula t₂ = t₁ × (d₂ / d₁)^1.06 is the long-standing standard. A 20-minute 5K predicts roughly a 41:30 10K (20 × 2^1.06 = 41.5 min) and a 3:05 marathon. The calculator shows pace conversions, not race predictions; for race prediction use a separate Riegel or VDOT calculator.
Sources
- Riegel PS. Athletic records and human endurance. American Scientist. 1981;69(3):285-90.
- Daniels J. Daniels’ Running Formula, 4th ed. Human Kinetics, 2021.