Free calculator

Half marathon pace calculator

This calculator gives you a predicted half marathon finish time, race pace per km and per mile, and your easy and tempo training paces from one recent 5 km or 10 km time.

How it's calculated

It uses the Riegel formula, first published by Peter Riegel: predicted time = known time × (21.0975 ÷ known distance)^1.06. The exponent of 1.06 accounts for slowing slightly as distance increases. A 55:00 10 km predicts 2:01:21; a 25:00 5 km predicts 1:55:00.

Race pace is the predicted time divided by 21.0975 km. Training paces are fixed multiples of race pace: easy runs at 1.22 ×, long runs at 1.18 ×, tempo at 0.96 × and 1 km intervals at 0.90 ×. They keep easy days easy and put the hard work in a narrow, useful range.

The prediction assumes you train for the distance. If your longest recent run is well under 15 km, you will probably run slower than predicted. A 10 km time predicts more reliably than a 5 km time, and a time from the last 8 weeks is better than an old personal best. Heat, hills and wind also slow real race pace.

Want the paces turned into a full plan? The half marathon plan builder uses the same paces to write every run from today to your race date.

Frequently asked questions

What 10K time do I need for a sub 2 hour half marathon?

About 54:20 or faster by the Riegel formula, or a 5 km of about 26:05. That predicts a finish just under 2:00:00 at 5:41 per km, assuming you complete the long runs in training.

How accurate is the Riegel formula?

It is a reasonable estimate for trained runners racing between 5 km and the marathon. It tends to be optimistic if you have not built up your long runs, and less reliable from very short races. Retest mid-plan to keep the prediction honest.

Should I use my 5K or my 10K time?

Use the 10 km if you have a recent one. It is closer to the half marathon distance, so the prediction needs less extrapolation. Use a 5 km if that is your only recent, all-out effort.

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General fitness information, not medical advice. Check with a doctor if you have a medical condition, are pregnant or have an injury.