Running pace calculator

Your pace per mile and per kilometer from distance and time, and what it means for a 5K, a 10K, a half and a marathon.

Your pace per mile

8:03 /mi

6.21 mi in 50:00 · 7.46 mph

5:00 /km
Half marathon 1:50:19

If you run 6.21 mi in 50:00, your pace is 8:03 per mile (5:00 per km), a speed of 7.46 mph.

At the same effort, Riegel’s formula predicts a half marathon in about 1:50:19, at 8:25 per mile.

I’m not a doctor or a dietitian. This is an estimate, for information only. For decisions about your health, check with a professional.

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Race predictions with Riegel’s formula
Race	Distance	Time	Pace per mile
5K	3.11 mi	23:59	7:43
10K	6.21 mi	50:00	8:03
Half marathon	13.11 mi	1:50:19	8:25
Marathon	26.22 mi	3:50:01 (less reliable)	8:46

How it works

Pace = time ÷ distance. Per mile, it’s the pace per kilometer × 1.609344, because a mile is exactly 1.609344 km. Speed is the other way around: distance ÷ time.
Race predictions use Peter Riegel’s formula (1981): time for the new distance = your time × (new distance ÷ your distance)^1.06. The 1.06 means you slow down a little as the distance grows: twice the distance takes about 2.08 times as long.
Race distances are the official ones: 5 km, 10 km, the half marathon at 21.0975 km (13.1094 miles) and the marathon at 42.195 km (26 miles 385 yards).

Worked example

A 10K in 50:00: 50 minutes ÷ 10 km = 5:00 per km, and 5:00 × 1.609344 = 8:03 per mile. 10 km in 50 minutes is 12 km/h, or 7.46 mph. Half marathon: 50:00 × (21.0975 ÷ 10)^1.06 = 50:00 × 2.206 = 1:50:19, at 8:25 per mile. The same math gives a marathon in 3:50:01, just past the 230 minutes Riegel’s data covered.

Sources

Text checked on September 30, 2026

Good to know

A prediction assumes you’re as well trained for the new distance as for the one you ran: a quick 10K doesn’t mean you’re ready for a marathon, and the formula can’t know the difference.
The 1.06 is a rule of thumb drawn from world records. Researchers note that each runner’s own fatigue factor is different and changes with training.
Riegel’s data covered races from a few minutes to about 230 minutes. Outside that range, predictions are rougher, so the calculator flags them.
Your pace here is an average over the whole distance; hills, heat and wind change it along the way.

Frequently asked questions

How do I convert pace per km to pace per mile?

Multiply by 1.609344, because a mile is 1.609344 km: 5:00 per km is 8:03 per mile. To go the other way, divide: 8:00 per mile is 4:58 per km.

How accurate are race time predictions?

They’re a rule of thumb. Riegel’s 1.06 comes from world records, and researchers point out that each runner’s fatigue factor is different and changes with training. Use the prediction as a starting point for a goal pace, not a promise.

How long is a half marathon?

21.0975 km, or 13.1094 miles: exactly half a marathon, which World Athletics fixes at 42.195 km, or 26 miles 385 yards.

What pace do I need for a sub-2-hour half marathon?

About 9:09 per mile, or 5:41 per km, gets you there in exactly 2 hours, so you need to be a little faster than that. Type 13.11 miles and 2:00:00 above to see it.

What’s the difference between pace and speed?

Pace is time per distance, like minutes per mile: it’s what runners use. Speed is distance per time, like miles per hour. 8:03 per mile is 7.46 mph.