Half Marathon Time From 10K

A 10K result is the best single predictor of a half marathon that most runners have lying around: the two races are close enough in duration that Riegel’s exponent barely has to reach. Enter your 10K time and read off the half:

Worked example: 50:00 for 10K

T₂ = 50:00 × (21.0975 ÷ 10)^1.06 = 1:50:19, a pace of 5:14/km (8:25/mile).

10K result	Predicted half	Predicted half pace
45:00	1:39:17	4:42/km
50:00	1:50:19	5:14/km
55:00	2:01:21	5:45/km

The multiplier is always the same — 2.2064 — because the half marathon is always 2.1 times a 10K and the exponent never changes. What changes is what that costs you in seconds per kilometer.

The variable the formula is blind to

Riegel’s equation sees two distances and one time. It does not know whether you have run past 90 minutes since your last 10K. That matters more here than it did stepping up from the 5K, because the half marathon is the first common race distance where carbohydrate availability and simple time on your feet start to shape the last few kilometers.

The good news is that the arithmetic itself holds up at this range. When 2,303 recreational runners were checked against the formula, half marathon predictions came out statistically well calibrated — the same test failed badly one distance further up. So if your race comes in slower than the prediction, the likely culprit is your long-run volume, not the math.

Building the plan around the number

Use the predicted pace as the target for the middle of the race, not the opening kilometer. A practical sequence: run the first 5K about five seconds per kilometer slower than the predicted pace, settle onto it through 15K, and spend whatever is left in the final 6K. Runners who go out at goal pace from the gun almost always finish behind their prediction.

When the half is done, that result becomes the new starting point — see the marathon prediction from a half marathon, and read how accurate the formula really is before you trust the marathon line.