Race Time Predictor
5K → marathon is optimistic without marathon training. This is not even-pace math — use the running pace calculator for pace × distance — and it is not a branded training table.
Worked example
5K in 25:00
10K 52:07 · half 1:55:00 · marathon 3:59:47
Tmarathon = 1500 × (42.195 / 5)1.06 seconds, displayed as H:MM:SS. Even-pace finish times live on the running pace calculator. Print mile, km, or 400m clocks on the split calculator.
Formulas
Riegel, 1981, American Scientist, “Athletic Records and Human Endurance” (popularized in Runner’s World, 1977):
T2 = T1 × (D2 / D1)1.06
Cameron column (approximation): exponent = 1.06 − 0.0026 × D1 in miles. Riegel is the primary prediction.
- 5K = 5.000 km · 10K = 10.000 km · 50K = 50.000 km
- half = 21.0975 km · marathon = 42.195 km
- 1 mile = 1.609344 km
Official Boston qualifying times live on the Boston qualifier calculator. Two all-out races to critical speed (not Riegel) on the critical speed calculator. Even pace for one statute mile on the mile pace calculator.
FAQ
How does this predict race times?
It uses Riegel’s 1981 formula from American Scientist, Athletic Records and Human Endurance — the same power-law model popularized in Runner’s World in 1977. T2 = T1 × (D2/D1)^1.06. Enter a recent race distance and finish time; equivalent times update live. A second column uses a Cameron-style exponent that depends on the known distance.
Why not just multiply pace × distance?
Even pace assumes you hold the same speed the whole way. Longer races add fatigue, so equivalent times are slower than pace × distance. For even-pace finish times with no fatigue model, use the running pace calculator — not this page.
How accurate is 5K → marathon?
Treat it as optimistic unless you trained for the marathon. A fresh 5K says a lot about speed and less about whether you can hold that fitness for 42.195 km. Use the prediction as a starting point, not a guarantee. The Cameron column is usually a bit more conservative on large jumps.
What is the 1.06 exponent?
It is Riegel’s fatigue exponent: time grows slightly faster than distance. An exponent of 1.00 would be even pace. 1.06 is the published constant from the 1981 American Scientist paper (and the 1977 Runner’s World popularization). Distances use 5K = 5.000 km, 10K = 10.000 km, half = 21.0975 km, marathon = 42.195 km, 50K = 50.000 km, and 1 mile = 1.609344 km.
Does this use branded training tables?
No. SplitLab does not use branded training tables or lookup charts. The primary column is Riegel’s published power law. The second column is a Cameron-style approximation with exponent 1.06 − 0.0026 × D1 in miles. Neither is a coaching plan.
Miles vs kilometers?
The US/Metric toggle is the same control as the pace and marathon calculators, and it stays on your device. Distances in the formula are converted internally with 1 mile = 1.609344 km. Predicted times are the same in either unit system; even pace is shown as both min/mile and min/km.