Race time predictor (Riegel formula)

Enter a recent race or time-trial result. Using the formula published by Peter Riegel (1981), this race time predictor estimates a realistic time for other distances.

Source: T2 = T1 × (D2/D1)^1.06, Riegel PS (1981): Athletic Records and Human Endurance. American Scientist 69(3), 285–290 (PubMed 7235349). Updated: 27 September 2026.

Distance you ran

Your time

As mm:ss or h:mm:ss, e.g. 24:30 or 1:52:10.

Target distance

Optional, e.g. 15 km or 30 km (1 mile = 1.609344 km).

My ToolboxYour inputs are saved in this browser only.

Result

Predicted time for the target distance: 1:09:10 (15 km)
Required pace: 4:37 min/km · 7:25 min/mi
Predictions: Distance Prediction Pace min/km Pace min/mi
5K 21:35 4:19 6:57
10K (your input) 45:00 4:30 7:15
Half marathon 1:39:17 4:42 7:34
Marathon 3:27:01 4:54 7:54
15 km (target) 1:09:10 4:37 7:25; The prediction assumes suitable training for the target distance and is not a guarantee. It is not a substitute for medical advice.

Distance	Prediction	Pace min/km	Pace min/mi
5K	21:35	4:19	6:57
10K (your input)	45:00	4:30	7:15
Half marathon	1:39:17	4:42	7:34
Marathon	3:27:01	4:54	7:54
15 km (target)	1:09:10	4:37	7:25

How it is calculated

Riegel’s formula

In 1981 the engineer Peter Riegel analysed world records over many distances (American Scientist 69, pp. 285–290). Time grows slightly faster than distance:

How to predict your race time

Choose the distance you ran and enter your time.
Choose the target distance, such as half marathon or marathon.
Read the predicted time based on the Riegel formula.

T2 = T1 × (D2 ÷ D1)^1.06

T1 is your time over distance D1, T2 the predicted time over distance D2. The exponent 1.06 reflects the fact that you slow down over longer distances.

Worked example

10K in 50:00 (3,000 s). Marathon: 42.195 ÷ 10 = 4.2195; 4.2195^1.06 = 4.6002; 3,000 s × 4.6002 = 13,800.6 s ≈ 3:50:01 (5:27 min/km, 8:46 min/mi). Half marathon: 2.10975^1.06 = 2.2064; 3,000 s × 2.2064 ≈ 1:50:19. 5K: 0.5^1.06 = 0.4796 → 23:59.

How accurate is the prediction?

Valid range: Riegel gives running times of about 3.5 minutes to about 230 minutes. Beyond that the curve drifts.
Training: A good 10K time only translates into a realistic marathon time if you have also trained the long runs. Many recreational runners are slower in the marathon than predicted.
Conditions: Heat, hills and form on the day change the result.

The results are an estimate, not a guarantee, and do not replace medical advice.

Related: work out the pace you need with the pace calculator, set training zones with the heart rate zone calculator, and estimate fitness with the VO2 max calculator.

Frequently asked questions

How does the Riegel formula work?

Your known time is multiplied by the distance ratio raised to the power 1.06: T2 = T1 × (D2 ÷ D1)^1.06. Doubling the distance therefore multiplies the time by 2^1.06 ≈ 2.085.

What marathon time does a 50-minute 10K predict?

About 3 hours 50 minutes according to Riegel – assuming marathon-specific training with long runs.

Which race should I use as the basis?

Ideally a recent race over a distance close to your target – for a marathon, a half marathon is a better basis than a 5K.

Why is my real marathon slower than the prediction?

The formula is based on record performances. Recreational runners often lack the endurance training and experience for very long distances, and beyond about four hours the formula becomes less accurate.

What half marathon time does a 25-minute 5K predict?

With Riegel: 25 × (21.0975 / 5)^1.06 ≈ 115 minutes, about 1:55 – provided you have trained for the longer distance.

Sources and legal basis

T2 = T1 × (D2/D1)^1.06: Riegel PS (1981): Athletic Records and Human Endurance. American Scientist 69(3), 285–290 (PubMed 7235349)
World Athletics Competition Rules: World Athletics – Book of Rules (Competition Rules, road races: half marathon 21.0975 km, marathon 42.195 km)

As of: 27 September 2026 · Editorial policy