Race Time Predictor
A 1:55:00 half marathon predicts a 3:57:48 marathon by VDOT (3:59:46 by Riegel). Enter one recent race and this race time predictor estimates your time at every distance from the mile to 50K, two ways, side by side.
Both are estimates that assume you train for the new distance. The notes below say where each one tends to go wrong.
VDOT prediction
3:57:48
Riegel 1.06
3:59:46
FromHalf
In1:55:00
VDOT38.3
From a Half marathon in 1:55:00, the predicted marathon time is 3:57:48 by VDOT and 3:59:46 by Riegel.
What are your predicted race times?
Every chart distance from the result above. The Riegel column uses exponent 1.06; the VDOT column uses the Daniels and Gilbert equations. Each distance links to its pace chart, where you can print a pace band for the goal you choose.
Pace unit
| Distance | Riegel | VDOT |
|---|---|---|
| Mile | 7:317:31/mi | 7:237:23/mi |
| 5K | 25:008:03/mi | 24:598:03/mi |
| 8K | 41:098:17/mi | 40:578:14/mi |
| 10K | 52:078:23/mi | 51:528:21/mi |
| 15K | 1:20:068:36/mi | 1:19:548:34/mi |
| 10 mile | 1:26:198:38/mi | 1:26:088:37/mi |
| Half marathonyour result | 1:55:008:46/mi | 1:55:008:46/mi |
| Marathon | 3:59:469:09/mi | 3:57:489:04/mi |
| 50K | 4:47:029:14/mi | 4:42:549:06/mi |
Estimates, not promises. Both assume you have trained for the longer distance.
How does the race time predictor work?
Riegel scales your time by the ratio of the distances raised to the power 1.06 (Riegel 1981, American Scientist 69:285-290). With your result and target substituted in:
T2 = T1 × (D2 ÷ D1)^1.06
T2 = 1:55:00 × (42.195 km ÷ 21.098 km)^1.06
T2 = 1:55:00 × 2.0000^1.06 = 1:55:00 × 2.0849
T2 = 3:59:46
VDOT works out the fitness score your race implies, then finds the time at the new distance that gives the same score. The VDOT calculator shows both equations with your numbers and the training paces that go with them, and the methodology page documents every constant.
Where is each prediction weaker?
Fitted to world-best performances, so it assumes elite-level endurance. It tends to be optimistic at the marathon for less-trained runners, who slow more over the last 10K than the exponent allows.
It also has no idea how fit you are: it scales any time the same way, whether it came from a 15:00 5K or a 35:00 one.
Built from trained runners' data, so it accounts for how long a race lasts. It is still an estimate: it assumes average running economy and the training to match the distance, and it is less reliable for very short races and ultras.
Both methods assume equivalent training for the target distance. If your long runs and weekly mileage are well below what the race needs, add time.
What does a 25:00 5K predict?
A worked example, so you can see how far the two methods spread at longer distances. At the marathon they differ by 1:51.
| Distance | Riegel | VDOT |
|---|---|---|
| Mile | 7:317:31/mi | 7:247:24/mi |
| 5Kthe result | 25:008:03/mi | 25:008:03/mi |
| 10K | 52:078:23/mi | 51:538:21/mi |
| Half marathon | 1:55:008:46/mi | 1:55:048:47/mi |
| Marathon | 3:59:479:09/mi | 3:57:559:04/mi |
Estimates, not promises. Both assume you have trained for the longer distance.
Frequently asked questions
How do you predict a marathon time from a half marathon?
Scale the half time up with Riegel's formula or match it on the VDOT tables. A 1:55:00 half predicts 3:59:46 by Riegel and 3:57:48 by VDOT. Doubling the half (3:50:00) is too optimistic for almost everyone, because pace slows as distance grows.
What is the Riegel formula?
T2 = T1 × (D2 ÷ D1)^1.06. Peter Riegel published it in American Scientist in 1981 from world-best times. The exponent above 1 builds in the slowdown over longer distances: double the distance and the time grows by a factor of 2.085, not 2.
Which is more accurate, Riegel or VDOT?
Neither wins everywhere. They agree closely between nearby distances, such as 5K to 10K. For the marathon, VDOT usually comes out a little faster than Riegel from a 5K, 10K or half (3:57:48 against 3:59:46 from a 1:55:00 half), and the gap grows for slower runners. Neither knows your weekly mileage, so use the slower of the two as your starting goal unless your training says otherwise.
Why is my predicted marathon time faster than I can run?
Both formulas assume you have trained for the marathon as well as you trained for the race you entered. A runner on 20 to 30 miles a week who races a fast 10K will usually run the marathon slower than predicted, because endurance, fuelling and pacing fail before aerobic fitness does.
What race should I use for a prediction?
A recent, all-out race on a measured, flat course. The closer it is to the target distance, the better: a half marathon predicts a marathon far better than a mile does.
What does a 25:00 5K predict for the marathon?
3:59:47 by Riegel and 3:57:55 by VDOT. Treat that as the ceiling for a well-trained marathoner, not a first-marathon goal.