How to use it
- Choose what to work out: pace, finish time or distance.
- Fill in the other two. Distances can be in kilometers, miles or meters, and the buttons fill in the standard race distances (the marathon is 42.195 km and the half marathon 21.0975 km). Pace is minutes and seconds per mile or per kilometre.
- Read pace per mile and per km, finish time, distance, speed and the 400 m lap time, then the split table (every km or every mile) and the estimates for other race distances.
How it calculates
Everything is kept as exact fractions of seconds and meters, using the exact international mile of 1,609.344 m, and is rounded half-up only when it is shown: times and paces to the second, speeds and distances to two decimals. That matters for splits. A 10K in 50:00 is a pace of 8:02.7 per mile, so the table shows 6 miles at 48:17; adding up six rounded 8:03 miles would give 48:18. It matters for the marathon too: at 9:00 per mile the full 42.195 km takes 3:55:58, ten seconds more than 26.2 × 9:00, because the marathon is 26.219 miles rather than 26.2.
Predicting other race times
The estimates use Peter Riegel’s endurance formula, time = known time × (new distance ÷ known distance)b. Most calculators use b = 1.06. In his 1981 American Scientist paper Riegel fitted world records to straight lines on log-log axes; for men’s running the fit was t = 2.299 d1.07732 (minutes and kilometers), and he described the fit as holding for efforts of about 3.5 to 230 minutes. The page shows both exponents. For a 50:00 10K they predict a marathon of 3:50:01 (1.06) and 3:55:49 (1.077), and the page warns when the known effort is outside the 3.5 to 230 minute range. Neither knows how much you have trained for the longer race.
What we tested
On October 15, 2026 we ran 3,004 cases (random distances in km, miles and meters, the race presets, times with fractional seconds, all three modes) through the page’s own functions and compared every output, including each split row, with an exact Python fractions reference: 0 differences. Two deliberately broken versions were caught: one with the mile as 1,609 m differed in 2,688 cases, and one that cut off seconds instead of rounding differed in 3,000. In Chromium: default 10K in 50:00 shows 8:03 per mile and 5:00 per km; a marathon at 9:00 per mile finishes in 3:55:58; one hour at 5:00 per km covers 12.00 km; at 1280 and 390 px with 0 page errors and no sideways scrolling.
Sources
- National Institute of Standards and Technology, Guide for the Use of the International System of Units (SP 811): the international foot is 0.3048 m exactly, so the mile (5,280 ft) is 1,609.344 m.
- Peter S. Riegel, “Athletic Records and Human Endurance,” American Scientist 69(3): 285–290 (1981); the men’s running coefficients are as read by A. Long, Riegel’s Figure 3 and Riegel’s Formula (Northern Kentucky University, 2020).
Frequently Asked Questions
Built by Derek Giordano · Part of Ultimate Design Tools