Race Time Predictor

Written byOleh Soloninupdated 25 September 2026

Known distance, km

Hours

Minutes

Seconds

Target distance, km

Result

Predicted time3:27:01
Pace at the target4:54 min/km
Pace at the known distance4:30 min/km
Slowdown, %9.02%
Average speed, km/h12.23
What matters
This is a Riegel estimate, not a guarantee: it is most reliable when the two distances are close and the training matches the target.

This result is informational only and is not professional advice.

Time grows faster than distance

Predicts a time over a longer distance from a result at a shorter one, using Riegel's formula.

If pace held, a marathon would take exactly the four-and-a-bit times a 10K does. In practice it takes longer: pace inevitably falls as distance grows, and it falls predictably.

Pete Riegel described that with a power law and an exponent of 1.06. The formula is simple, has held up for half a century, and remains a working guide — provided the training matches the target distance.

How to use

Enter a distance you have already raced, in kilometres.
Enter your time for it in hours, minutes and seconds.
Enter the target distance: 42.195 for a marathon, 21.0975 for a half.
The result gives the predicted time, the target pace and how much slower it is than your known pace.

Marathon from a 10K

Riegel's formula: target time = known time × (target / known distance)^1.06. Going from 10K to marathon always costs 9 percent of pace: a 50-minute 10K predicts a 3:50:01 marathon at 5:27 per km instead of 5:00.

Every row comes from the calculator on this page.
10K time	Predicted marathon	Marathon pace	10K pace
40:00	3:04:00	4:22	4:00
45:00	3:27:01	4:54	4:30
50:00	3:50:01	5:27	5:00
55:00	4:13:01	6:00	5:30
1:00:00	4:36:01	6:32	6:00

Marathon from a half

A half marathon is half the distance and sits well inside the range where the formula holds; the slowdown here is 4.25 percent. A 1:45 half predicts 3:38:55.

Every row comes from the calculator on this page.
Half marathon time	Predicted marathon	Marathon pace
1:30:00	3:07:39	4:27
1:45:00	3:38:55	5:11
2:00:00	4:10:12	5:56
2:15:00	4:41:28	6:40

What this calculator shows

The predicted time over the target distance
Pace at the target and pace at the known distance
The percentage slowdown between them

What to keep in mind

A Riegel prediction is an estimate: the further the target distance from the one you raced, the further the real result can fall from it.
It assumes training for the target distance: without long runs, a real marathon comes out slower than predicted.

FAQs

How does Riegel's formula work?

Time is multiplied by the ratio of distances raised to the power 1.06. The exponent above one captures the unavoidable drop in pace: 9 percent from 10K to marathon, 4.25 percent from a half.

What marathon can I run off a 50-minute 10K?

Riegel says about 3:50, at 5:27 per kilometre. Straight pace conversion would give 3:31, but 10K pace cannot be held for a whole marathon.

How accurate is a marathon prediction?

It is optimistic unless your training is marathon training. Vickers and Vertosick's 2016 study of recreational runners found the formula underestimates marathon times on average, and more so for those running low weekly mileage.

Which race predicts a marathon best?

A half marathon: it is closest in the kind of effort and sits inside the range where the formula holds. A 5K is more than four times shorter.

Can I predict the other way?

Yes, the formula works both ways. A 45-minute 10K gives a 21:35 5K and a 1:39:17 half, though shorter races lean more on speed.

Why does a 22-minute 5K predict such a fast marathon?

Because the formula stretches it over eight times the distance: the prediction is 3:31:00. A 5K measures speed rather than endurance, so for a marathon that figure is too optimistic.

Worked example

From a 10K to a marathon

Input: 10 km in 45:00, target 42.195 km

Output: 3:27:01 at 4:54 per km against 4:30

Note: A nine percent slowdown is the price of the distance by the formula. Holding a flat-out 10K pace for a whole marathon invites a heavy slowdown; where it arrives, the formula does not say.

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Race Time Predictor

Result

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