10K to Half Marathon: What Half Can You Run?

Here is the quick answer. Your half marathon time is about 2.21 times your 10K time. So a 50:00 10K points to about a 1:50 half. A 45:00 10K points to about 1:40. Your half pace will be roughly 11 to 18 seconds per kilometer slower than your 10K pace. The half is more than twice the distance of a 10K, so you slow down a bit more.

The table below turns common 10K times into a half marathon time and pace. Find the row closest to your 10K.

Your 10K	Projected half	Half pace /km
40:00	1:28:33	4:11
45:00	1:39:44	4:43
50:00	1:50:52	5:15
55:00	2:01:56	5:46
60:00	2:12:56	6:18

What half marathon can I run off my 10K?

Take your 10K time and multiply it by about 2.21. That gives a fair half marathon guess for most runners.

The half is 21.1 km. That is a little more than two 10Ks. So your half should be a bit more than double your 10K — not exactly double. The extra distance means a slightly slower pace.

The 10K is a strong race to predict from. It is close in distance to the half. When the gap between two races is small, the guess is more accurate (RunnersConnect, 2024). Your 10K predicts your half better than your 5K does.

How big is the endurance gap between a 10K and a half?

It is bigger than it looks. The half takes most runners more than twice as long as a 10K. That extra time is all aerobic. It asks your body to hold a strong pace while slowly running low on energy.

In pace terms, the half is about 11 to 18 seconds per kilometer slower than the 10K:

• A 45:00 10K is 4:30 per km. The half is about 4:43 per km — 13 seconds slower.
• A 50:00 10K is 5:00 per km. The half is about 5:15 per km — 15 seconds slower.
• A 60:00 10K is 6:00 per km. The half is about 6:18 per km — 18 seconds slower.

Faster runners lose fewer seconds per km. Slower runners lose a bit more. This steady slowdown with distance is a measured "fatigue factor," and it is regular enough to predict (Vickers & Vertosick, 2016).

How many long runs do I need to hit it?

This is where the half is won or lost. Your 10K shows your speed. Your long runs show if that speed will last for 21 km.

A large study of half-marathon and marathon runners found a clear link: more weekly running and a longer long run go with faster race times (Fokkema et al., 2020). The long run is the key workout.

Here is a simple guide:

1. Build your long run. A common intermediate half plan grows the long run to about 12 miles before race day (Hal Higdon, intermediate half). The longer your longest run, the better your last 5 km will feel.

2. Run 4 to 5 days a week. Easy miles on the other days build the aerobic engine that carries the pace.

3. Add one tempo run. A steady, "comfortably hard" run near half pace teaches your body to hold it.

If your long run is short, you will likely finish near the slow end of the prediction. If you have done several long runs of 16 to 19 km, aim for the fast end.

How do I predict my half marathon?

Enter your most recent 10K and let PIRX do the math. You will get a projected half time and the pace to train at — free, no signup, no watch.

PIRX's proprietary machine-learning prediction engine learns your style instead of using one fixed formula. It shows a supported range, not a fake-exact number, so you know how sure it is. Add a second race, like a recent 5K or a longer run, and the range gets tighter.

This is what makes PIRX different. Strava tells you what you did. Garmin shows your data. PIRX tells you why it happened and what's next — with validated prediction accuracy of 98% across PIRX users.

New here? Start with the complete guide to predicting your race time. Coming up from shorter races? See the 5K to 10K conversion or check your speed with the 10K Time Predictor. Thinking past the half? Read what marathon you can run off your half. When you are ready, project your half below.

Sources

• Vickers, A.J. & Vertosick, E.A. (2016). An empirical study of race times in recreational endurance runners. BMC Sports Science, Medicine and Rehabilitation. (2,303 recreational runners; the "fatigue factor" k.)
• Fokkema, T. et al. (2020). Training for a (half-)marathon: training volume and longest endurance run related to performance. Scandinavian Journal of Medicine & Science in Sports.
• RunnersConnect (2024). How accurate are race calculators? (Predictions are most accurate when the distance gap is small.)
• Hal Higdon. Intermediate 1 half marathon training program.