health · 2026-05-01
Convert pace to time and distance — predicts marathon time from a 5K PR and shows training pace targets.
| 5K time (minutes) | 22 |
| 5K seconds (extra) | 30 |
| Target distance (km) | 42.195 |
| Current 5K pace (min/mile) | 7.24 |
| Current 5K pace (min/km) | 4.50 |
| Easy pace (min/mile) | 9.41 |
| Tempo pace (min/mile) | 7.97 |
| VO2 interval pace (min/mile) | 6.95 |
Two pieces of running math, packaged together:
T₂ = T₁ × (D₂/D₁)^1.06
You can't run 26.2 miles at 5K pace — fatigue makes you slower as distance grows. Pete Riegel's 1981 formula models this with the 1.06 exponent (the 'fatigue factor'). It's accurate within 2-3% for distances 1.5× to 4× of the reference race.
A 22:30 5K → ~3:30 marathon predicted. Caveat: only accurate if you've trained for the longer distance. A 22:30 5K runner who hasn't built endurance will run 4:00+ marathon despite the prediction.
Jack Daniels' VDOT system maps current race performance to training paces:
The percentages are off the 5K pace because 5K is a near-VO2max effort (~95-98% of VO2 max for trained runners). Calibrating off marathon pace is less reliable because marathon performance is endurance-limited, not VO2-limited.
Run easy days slow enough. Most amateur runners run easy days at tempo pace and tempo days at easy pace — the dreaded "gray zone." The prescribed easy pace from this calculator should feel almost too slow. That's the point.
It predicts your aerobic ceiling. Whether you can hit it on race day depends on training. A 5K-fit runner who hasn't done long runs will fade after mile 16. Use Riegel as 'best case' and adjust down 5-10% if you haven't built distance.
Because aerobic base development happens at 65-75% max HR, not 80-85%. Running easy too fast means you're always slightly fatigued for hard days, so your hard days are also slightly off. Slow the easy down and the tempo gets faster — paradoxical but proven.
Run a 5K time trial on a track or flat course. Alternative: use a recent race at any distance and back-calculate via Riegel. Avoid using a long run pace — it's not race-equivalent and underestimates ability.
Yes, significantly. Add 5-10 sec/mile per 1,000 ft above 4,000 ft elevation, and 10-30 sec/mile in 75°F+ humid conditions. Race-time predictions are for sea-level, cool-day conditions.