Race finish time calculator

Use a race you have just run to predict another distance. It works best when you have trained for the new one.

Your details

Distance in
Race you ran
Your time
Distance you are aiming for
Your age (optional, for age grade)
Sex
Another recent race, distance (optional)
Its time (optional)

Predicted finish time

2:01:21

for 13.11 miles (21.10 km)

  • Pace for that5:45 minutes per kilometre
  • Per mile9:15
  • Speed6.5 mph (10.4 km/h)
  • Even splits25% in 30:20, 50% in 1:00:41, 75% in 1:31:01
  • Same fitness over other races1 mile 7:56; 5K 26:23; 10K 55:00; marathon 4:13:01
  • Training paces per km (VDOT 36.0)easy 6:22 to 7:35; tempo 5:33 to 5:48; intervals 5:00 to 5:13
  • Age gradeadd your age and sex to see it

A good guide if you have trained for the distance. Heat, hills and a slow start all add time.

The working

Riegel's formula

New time = old time × (new distance ÷ old distance)1.06. A 55 minute 10K predicts 3,300 × 2.109751.06 = 2:01:21 for a half marathon, at 5:45 per kilometre.

Why 1.06

Longer means a little slower

If pace stayed the same, the power would be 1. Peter Riegel found that across world records the extra slowing fits a power of about 1.06. It is a neat rule that holds best between about 3 and 50 km.

Use it well

Getting a fair prediction

  • Use a race, not a training run
  • Use a recent result, within about 8 weeks
  • Predicting up more than 4 times the distance is a best case
  • Hot or hilly days need a slower plan

Read with care

What this cannot tell you

  • Your trainingIt does not know if you have done the long runs.
  • UltrasBeyond the marathon, stops and terrain matter more than pace.
  • CourseElevation and surface can move times a lot.

Questions

Common questions

How accurate is the Riegel formula?

Usually within a few minutes up to the half marathon, if you have trained for it.

Can I predict a marathon from a 5K?

You can, but it is often optimistic. A half marathon is a better guide.

What is a good finish time?

One you are happy with. Finishing is the main thing.

Sources

Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.

Before you push on

New to exercise, or got a heart, lung or joint condition? Have a word with your GP or practice nurse first. Stop and get help if you feel chest pain, faintness or unusual breathlessness, and call 999 if it is severe.

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