Predict Your Marathon Time from a Half Marathon | How to Calculate and Adjust Riegel’s Formula
When you set a goal for your first marathon, your half marathon or 10K time is the best clue you have. Simply doubling your half marathon time gives most people a prediction that is too fast, because pace drops as the distance gets longer. This guide explains how Riegel’s formula predicts a marathon time from a half or 10K result and how to calculate it, gives a prediction table by half and 10K time, shows how to adjust it for a first marathon or lighter training, and covers how to turn a predicted time into a realistic goal.
Why doubling your half marathon time is too fast
Someone who ran a half marathon in 1:45 will, in most cases, find it hard to run a marathon in 3:30. When the distance doubles, running low on carbohydrate fuel, leg muscle fatigue and rising body temperature combine, and you can’t hold the same pace.
Time doesn’t grow in proportion to distance; as distance increases, time grows a little more than proportionally. Riegel’s formula, published in the 1970s by the American engineer Peter Riegel, expresses this relationship in a simple equation, and it is widely used in running time predictors.
How Riegel’s formula works and how to calculate it
The 1.06 exponent means “when the distance doubles, pace slows by about 4%” (2^1.06 ÷ 2 ≈ 1.042). From 10K to a marathon the distance ratio is 4.2195, and 4.2195^1.06 ≈ 4.60. A runner who can do 10K in 50 minutes is predicted at 50 minutes × 4.60 = about 3:50.
- 1
Use a recent race time
Ideally use an official half or 10K time from an all-out race within the last two or three months. Race times are more accurate than training times.
- 2
Work out the distance ratio
Divide the marathon distance, 42.195 km, by the distance you ran: 2 for a half, 4.2195 for 10K, 8.439 for 5K.
- 3
Raise the ratio to the power 1.06 and multiply by your time
On a phone’s scientific calculator, enter something like “2 xʸ 1.06”, then multiply by your time in seconds.
Marathon prediction table by half marathon and 10K time
Alongside the standard exponent of 1.06, the table shows predictions using 1.08 and 1.10 for runners with less training.
| Time run | Exponent 1.06 | Exponent 1.08 | Exponent 1.10 |
|---|---|---|---|
| Half 1:30:00 | 3:07:39 | 3:10:16 | 3:12:55 |
| Half 1:45:00 | 3:38:55 | 3:41:58 | 3:45:04 |
| Half 2:00:00 | 4:10:12 | 4:13:41 | 4:17:14 |
| Half 2:15:00 | 4:41:28 | 4:45:24 | 4:49:23 |
| 10K 45:00 | 3:27:01 | 3:33:03 | 3:39:17 |
| 10K 50:00 | 3:50:01 | 3:56:44 | 4:03:39 |
| 10K 60:00 | 4:36:01 | 4:44:05 | 4:52:22 |
Working backwards with an exponent of 1.06, a sub-4 marathon needs about a 1:55 half or a 52-minute 10K. With 1.10 you need about a 1:52 half or a 49-minute 10K. For sub-3:30 the guide is a half of about 1:40 (1.06) to 1:38 (1.10), and for sub-5 about 2:24 (1.06) to 2:20 (1.10).
For a first marathon or lighter training, use 1.08–1.10
Riegel’s 1.06 was derived from runners who had trained well for both distances. Unlike races up to the half, the marathon is prone to slowing after 30 km, and without enough training you will be considerably slower than predicted.
- You have done several long runs such as 30 km for the marathon: 1.06
- Around 150 km a month, and this is your second marathon or later: 1.07–1.08
- First marathon, or under 100 km a month: 1.08–1.10
- Predicting from a 5K or 10K: the further apart the distances, the bigger the error, so 1.08 or more is safer
Predict with the Marathon Pace Band Maker, then turn it into a pace band
In the “Prediction & Track” tab of the Marathon Pace Band Maker, enter the distance you ran (1500 m, 3 km, 5 km, 10 km, half or 30 km) and your time, and it lists predicted times and per-kilometre pace for the other distances. You can change the exponent from 1.06, so for a first marathon try 1.08 or 1.10 and compare.
Once you have a goal time, build the split for every kilometre in the “Splits & Cutoffs” tab and print it as a pace band. Pace per kilometre for each goal is listed in the sub-3 to sub-6 pace chart.
FAQ
- How do I predict my marathon time from my half marathon time?
- Use Riegel’s formula: marathon = half marathon time × 2^1.06 (about 2.085). A 1:45 half predicts about 3:38:55, and a 2:00 half about 4:10:12.
- What half marathon time do I need for a sub-4 marathon?
- Working backwards with Riegel’s formula (exponent 1.06), you need about a 1:55 half. For a first marathon or lighter training, using 1.10 and aiming for about a 1:52 half is closer to reality.
- Can I predict a marathon time from a 10K?
- Yes: marathon = 10K time × 4.2195^1.06 (about 4.60). A 50-minute 10K predicts about 3:50. Because the distances are far apart the error is larger, and the slower times with an exponent of 1.08–1.10 (about 3:57 to 4:04) are more realistic.
- What does the 1.06 in Riegel’s formula mean?
- It is the exponent describing how much pace drops as distance increases. With 1.06, doubling the distance slows pace by about 4%. The less you train, the bigger the drop, so 1.08–1.10 brings the prediction closer to reality.
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