How long is a marathon, really?

The exact length of a marathon

World Athletics Technical Rule 55.1 lists the standard road distances and gives the marathon as 42.195 km. The international mile is exactly 1,609.344 m (NIST lists the factor). So:

42,195 / 1,609.344 = 26.21876 miles

The old imperial description, 26 miles 385 yards, works out to 42,194.988 m: about a centimetre short of the metric figure, so the two definitions agree for any practical purpose. The rounded 26.2 miles leaves out 30.19 m, or 33.0 yards. The half marathon has the same problem in miniature: 21,097.5 m is 13.10938 miles, not 13.1.

The race distances converter shows every standard road and track distance in kilometres, miles, yards and laps from the same two definitions.

How far off is a pace chart built on 26.2 miles?

Take a 4:00:00 goal, which is 14,400 seconds. Dividing by the rounded distance gives 14,400 / 26.2 = 549.62 seconds, or 9:09.6 per mile. Dividing by the true distance gives 14,400 / 26.21876 = 549.22 seconds, or 9:09.2 per mile. The difference is 0.39 seconds per mile. Hold the slower pace for the full 26.21876 miles and you cross the line at 549.62 x 26.21876 = 14,410.3 seconds, which is 4:00:10.

I ran the same arithmetic for six goals with the site's pace engine:

The error grows with the goal time because the missing 30 m take longer to cover at a slower pace. For anyone chasing a round number, though, 8 to 16 seconds can be the whole margin: a runner who is exactly on a 26.2-mile chart at every marker finishes a few seconds over the time printed at the top of it.

The marathon pace chart on this site divides by 42,195 m, so its per-mile and per-km paces are the true ones. The half marathon chart does the same with 21,097.5 m; a 13.1-mile chart there would finish 3.9 seconds late at 1:30, 5.2 at 2:00 and 6.4 at 2:30.

The kilometre version of the same mistake is bigger

Runners who pace in kilometres sometimes round the other way and think of the marathon as 42 km. The 195 m dropped there are worth far more than the 30 m dropped from 26.2 miles:

The 3:30 row is the classic trap: 5:00 per km sounds like a 3:30 pace, but 5:00 x 42.195 = 3:30:58.5. The true 3:30 pace is 4:58.6 per km. If you use the pace calculator with the marathon preset, it uses the exact distance either way.

What the course rules say

Rule 55.3 of the World Athletics Technical Rules sets how the 42.195 km is laid out. Three parts of it matter for pacing:

• The course is measured along the shortest possible route an athlete could follow within the part of the road allowed for the race. That line cuts every corner and follows every bend on the inside.
• The length must not be less than the official distance. For the top international competitions, the measurement uncertainty must not exceed 0.1%, which the rule itself spells out as 42 m for the marathon, and the course should be certified in advance by an approved measurer using the calibrated bicycle method.
• Note (ii) recommends a short course prevention factor so that a re-measurement never finds the course short. For bicycle measurement the factor is 0.1%, meaning each kilometre has a measured length of 1,001 m.

The rule also says the distance in kilometres shall be displayed to all athletes (55.4), and that a course measurement certificate is valid for five years.

The short course prevention factor in seconds

The prevention factor is a recommendation, so not every race uses it, but when one does, the shortest route is about 0.1% longer than 42.195 km:

42,195 x 1.001 = 42,237.2 m, or 26.2450 miles on the measured line

At true 4:00 pace those extra 42.2 m take 14,400 / 42,195 x 42.2 = 14.4 seconds. At 3:00 pace it is 10.8 seconds and at 5:00 pace 18.0 seconds. Stack that on top of a 26.2-mile chart and a runner who is exactly on schedule at every marker finishes 18.5 seconds late at 3:00, 24.7 seconds late at 4:00 and 30.9 seconds late at 5:00, while running perfectly to plan along the shortest legal line.

The point is not that courses are long by mistake. They are built to be at least the full distance, with a deliberate margin. A chart that assumes exactly 26.2 miles has no margin at all.

Why your watch reads 26.4 or more

Nobody runs the measured line. It hugs every inside corner, and on a crowded road you go round other runners, drift to the drinks tables and run the middle of the road on bends. Each of those adds distance the course does not count. GPS adds a second, separate effect: the watch samples your position at intervals and each fix has some error, so the track it draws is not your exact path either. I am not quoting a typical figure for either effect, because it depends on the course, the crowd, the buildings and the device.

What can be computed is what that extra distance costs. Every 0.1 mile you cover beyond the measured line takes 41.2 seconds at 3:00 pace, 54.9 seconds at 4:00 pace and 68.7 seconds at 5:00 pace.

And here is the trap it creates. Suppose you pace off the watch's current-pace display and hold the true goal pace on it the whole way, but the watch ends up recording more than 26.21876 miles. You run the extra distance at goal pace and finish late by the full amount:

Each cell is the true goal pace multiplied by the watch distance. A watch that reads 26.5 at the finish turns a perfectly paced 4:00 into 4:02:34. To finish on 4:00:00 with that watch total you would have needed to show 9:03.4 per mile on the watch, not 9:09.2: a gap of 5.8 seconds per mile, fifteen times larger than the 26.2 rounding.

Pacing to the markers instead

The course markers sit on the measured line, so the elapsed time at each one is the number that matches a correct pace chart. A pace band lists the target elapsed time at every mile or kilometre, built from 42,195 m, so you compare the race clock (or your watch's elapsed time) with the band as you pass each marker rather than trusting the watch's distance. If you are behind, the band tells you by exactly how much, regardless of what the GPS thinks you have run. Manual laps at the markers do the same job on a watch.

If you want a deliberate cushion, the honest way to build it is in time, not distance: choose a goal a little under the one you need and print the band for that. The marathon chart accepts any goal time, and if you plan to run the second half faster, the guide to negative splits shows what that does to each marker.

Limits of these numbers

Every figure here assumes steady pace. Real races include hills, wind, drinks and crowded starts, which change individual splits far more than half a second per mile; the course pace planner handles hills from a GPX file. The prevention-factor figures apply only to courses laid out with the recommended 0.1% margin, and a course can be measured slightly longer than that within its tolerance. The watch table is a calculation of what extra recorded distance would cost, not a statement of how far any particular watch is out.

Exact paces for any goal: marathon pace chart, pace band generator, race distances. The marathon pace calculator uses the full 42.195 km.

Sources: World Athletics Book C, C2.1 Technical Rules (1 Nov 2019), Rule 55 Road Races (TR 55.1, 55.3 and notes, 55.4); NIST Guide to the SI, Appendix B.9: conversion factors (mile = 1,609.344 m). Rules checked 7 October 2026. Found a problem? Tell us.

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