Marathon finish time:
turn a pace or a 10K result into a race forecast

Distance times pace

A marathon is 42.195 kilometres, or 26.2188 miles. Once you choose a pace, the finish time follows by multiplication, because pace is simply time per kilometre. If you know your speed instead, the same distance divided by speed gives hours.

The only fiddly part is the arithmetic in minutes and seconds. The cleanest approach is to convert the pace into seconds per kilometre, multiply by 42.195, and then split the answer back into hours, minutes and seconds. A pace of 5:30 per km is 330 seconds.

Worked example: 5:30 per kilometre

A runner plans to hold 5:30 per km, which is 330 seconds, from start to finish. How long will the marathon take?

For a speed-based version, running at 10 km/h gives 42.195 ÷ 10 = 4.2195 hours. The 0.2195 of an hour is 13.17 minutes, so the finish is 4 hours 13 minutes and about 10 seconds.

Pace-to-finish reference table

A useful anchor: a sub-4-hour marathon needs about 5:41 per km, which is 9:09 per mile. Each 30 seconds per km you shave changes the finish time by roughly 21 minutes.

Forecasting from a 10 km result

Most runners do not pick a marathon pace from nowhere; they extrapolate from a recent race. A well-known model, attributed to Pete Riegel, scales a known time T₁ at distance D₁ to a new distance D₂ with an exponent a little above one, which captures the fact that longer races are slower per kilometre.

The model assumes you have trained for the longer distance. Without marathon-specific long runs, a 10 km time will give a prediction that is too optimistic.

Why real marathons slow down

The formulas assume a constant pace. In reality most amateur runners slow in the final third, and the clock keeps running at every aid station. A runner who plans 5:30 per km and then fades by 10 percent over the final 10 km will run those kilometres at about 6:03, and the finish becomes 3 hours 57 minutes 34 seconds, which is 5½ minutes slower than the plan.

• Start the first 5 km at or slightly slower than goal pace; the early adrenaline makes fast feel easy.
• Fuel and fluids from the start, not once you feel the need.
• Expect the last 10 km to feel as hard as the first 32 together.
• Allow a buffer of 2 to 3 percent on the goal time if the course is hilly or hot.

Splits along the course for a 5:30 plan

Even pacing also gives a checkpoint at every distance, which turns the race into a series of small comparisons. These are the elapsed times a runner would see at 5:30 per km, with the halfway point at the official half marathon distance of 21.0975 km.

Two details help in the race. Official results show gun time, from the starting signal, and chip time, from when you crossed the mat. In a big race those can differ by minutes, and your pace plan should use chip time, because that is the time you actually ran.

Second, your watch will almost always show slightly more than 42.195 km, because it records the route you ran including weaving, whereas the course is measured along the shortest line. A watch reading of 42.6 km at the finish does not mean the course was long. Use the marked kilometre signs and the clock for your pace checks, and keep the watch as a guide to effort.

Choosing a goal you can defend

Pick a goal from evidence: a recent half marathon or 10 km at full effort and the long runs you completed in training. A time that requires the best of every day will rarely survive wind, crowds and warm weather.

Many coaches suggest an A goal, a B goal and a safe C goal, each with its own pace. The first reflects a perfect day, the second a good one and the third a finish you will be happy with. If your first 10 km are on the B pace, you can still decide at halfway to move to A or stay.

Anyone new to endurance running or with a health condition should discuss training volume with a clinician or coach before attempting the distance.