Half marathon pace calculator

A pace calculator will happily convert any goal into a pace, including one you have no business attempting. Picking the input is the hard part, and there are three checks worth running before you commit.

Double your 10K and add 8–12 minutes

This is the crudest test and the most reliable. If you raced 50:00 for 10K in the last couple of months, a half between 1:48 and 1:52 is realistic. The lower end of that window assumes you have done the long runs; the upper end is where most people actually land.

The formal version is Riegel's equation, T2 = T1 × (D2/D1)^1.06, which our race time predictor uses. It fits well over this jump because 10K and the half sit close together on the fatigue curve — the same equation from a 5K to a marathon is far more optimistic than it should be.

Your long-run pace plus about a minute

Half marathon race pace is typically 45–75 seconds per kilometre faster than the pace you run comfortable long runs at. If your Sunday runs sit at 6:30/km, a 5:41/km target for a two-hour half is aggressive but in the right postcode. If they sit at 7:30/km, it is fiction.

The 90-minute test

You will be running for somewhere between 80 and 150 minutes. Have you run for that long, continuously, in training? Not at race pace — just for that duration. If the answer is no, the pace you can hold is not the constraint; the distance is, and no split sheet fixes that.

The table above assumes perfectly even pace, which almost nobody runs and nobody should aim for. It is the reference line, not the instruction.

Nearly every recreational half marathon is run as a positive split: the second half slower than the first, usually by several minutes, because the first five kilometres feel easy and the adrenaline is free. That is not a pacing preference, it is the default failure mode of the distance, and the correction has to be deliberate.

What the calculator's 1% split means

Run the first half 1% slower than average pace and the second half 1% faster. At half-marathon pace that is about three seconds per kilometre either side — small enough to execute, large enough to matter. For a 1:50 target it means 55:33 through 10.5 km and 54:27 home.

Three seconds per kilometre sounds trivial and is not. It is roughly the difference between arriving at 15 km thinking about the finish and arriving at 15 km doing arithmetic about how much you can afford to lose. The closing 6 km of a half is where the race is decided, and the runners passing people there are almost always the ones who were passed at 5 km.

When to break the rule

Downhill first halves, point-to-point courses with a prevailing tailwind, and races that start with a long climb all invalidate an even-split sheet. Pace those by effort for the first third and use the split table only as a drift check — a minute up at 15 km on a flat course is a warning, and on a course that has just dropped 200 m it is nothing.

The one number to memorise

Do not try to hold twenty-one splits in your head. Take three: your 5 km, your halfway, and your 15 km. Everything between them is effort, and the only decision the clock informs is the one you make at 15 km.

A half marathon is 21.0975 km or 13.1094 miles, and races in the same country will cheerfully post markers in either. Two conversions worth being able to do while running:

• Per-km to per-mile: multiply by 1.61. A 5:00/km is 8:03 per mile. The quick mental version is to add 60% — add the pace to itself, halve it, add a tenth.
• Per-mile to per-km: multiply by 0.62. An 8:00 mile is 4:58 per kilometre.

Some useful pairs at half-marathon targets: 1:30 is 4:16/km or 6:52/mile. 1:45 is 4:59/km or 8:01/mile. 1:50 is 5:13/km or 8:23/mile. 2:00 is 5:41/km or 9:09/mile. 2:15 is 6:24/km or 10:18/mile.

The 2:00 half is the one most people are chasing, and 5:41/km is an awkward number to hold. Most runners are better off targeting 5:38 and banking the twenty seconds, because pace discipline at the margin is worse than the arithmetic suggests.