The 13.1 mile arithmetic
A half marathon is 21.0975 km, which is why the pace for a 1:45 goal is 4:59 per kilometre and not a round 5:00. The difference sounds trivial until you multiply it by 21 km: running 5:00 instead of 4:59 costs about 30 seconds, which is the gap between a personal best and a near miss.
Checkpoints that matter
The 5 km, 10 km and 15 km splits in the table are the ones most courses mark. The 10 km split is the best early indicator: if you are 40 seconds ahead of target at 10 km, you are probably 60–90 seconds ahead of what you can hold, so ease off rather than banking more time.
Half marathon pace versus other distances
For most runners, half marathon pace is 5–8 seconds per kilometre slower than 10K pace, and 10–15 seconds per kilometre faster than marathon pace. If the number here looks unrealistically fast, check it against a recent 10K using the race predictor.
Frequently Asked Questions
What is a good half marathon time?
For recreational runners, 1:45–2:15 is a common range; sub-1:30 is a strong club standard. The right target is the one your recent race times support.
How many 5 km splits are there?
Four full ones plus the final 1.1 km. The table shows the cumulative time at each 5 km marker.
Can I use this for a 10 mile race?
Enter the distance in the finish time calculator instead — 10 miles is 16.09 km, so a half marathon pace would overstate the distance.
Related tools
Privacy & your data
SwimRunPace calculates everything in your browser. Distances, times and body weights are never uploaded — there is no account, no server-side record and no training history stored between visits.