Race time predictor
Enter a recent race and see the time it points to at every distance from the mile to the marathon, with the pace for each. A 50:00 10K, for example, points to about 1:50:19 for a half marathon and 3:50:01 for a marathon. It uses Riegel's formula with k = 1.06, the same projection our plans use, and assumes you are trained for the distance you are predicting.
How it works
The prediction is Riegel's formula: T2 = T1 × (D2 ÷ D1)k, where T1 is your time over distance D1 and T2 the time for distance D2. With k = 1 you would hold the same pace at every distance; k = 1.06 slows the pace a little each time the distance grows, which is about how trained runners' race times line up from the mile to the marathon. The same projection sets the paces in our training plans.
It assumes you are trained for the distance you are predicting. A good 5K says you have the speed for a fast marathon, not the endurance: that comes from weekly mileage and long runs. So when you give a weekly mileage under 30 miles (48 km) and your race is a 10K or shorter, the marathon uses k + 0.02 and the half the average of the two. The further the target is from your race, the more the number depends on training rather than on today's fitness.
Use the prediction to set a realistic goal, then train for it: the pace calculator gives the splits for a goal time, and the training pace calculator gives the easy, tempo and interval paces this race supports.
Equivalent race times from a 5K
5K times from 15:00 to 35:00 and what each points to, for a runner trained for the distance (k = 1.06).
| 5K | 10K | Half marathon | Marathon |
|---|---|---|---|
| 15:00 | 31:16 | 1:09:00 | 2:23:52 |
| 16:00 | 33:22 | 1:13:36 | 2:33:27 |
| 17:00 | 35:27 | 1:18:12 | 2:43:03 |
| 18:00 | 37:32 | 1:22:48 | 2:52:38 |
| 19:00 | 39:37 | 1:27:24 | 3:02:14 |
| 20:00 | 41:42 | 1:32:00 | 3:11:49 |
| 21:00 | 43:47 | 1:36:36 | 3:21:25 |
| 22:00 | 45:52 | 1:41:12 | 3:31:00 |
| 23:00 | 47:57 | 1:45:48 | 3:40:36 |
| 24:00 | 50:02 | 1:50:24 | 3:50:11 |
| 25:00 | 52:07 | 1:55:00 | 3:59:47 |
| 26:00 | 54:12 | 1:59:36 | 4:09:22 |
| 27:00 | 56:18 | 2:04:12 | 4:18:58 |
| 28:00 | 58:23 | 2:08:48 | 4:28:33 |
| 29:00 | 1:00:28 | 2:13:24 | 4:38:09 |
| 30:00 | 1:02:33 | 2:18:00 | 4:47:44 |
| 31:00 | 1:04:38 | 2:22:36 | 4:57:19 |
| 32:00 | 1:06:43 | 2:27:12 | 5:06:55 |
| 33:00 | 1:08:48 | 2:31:48 | 5:16:30 |
| 34:00 | 1:10:53 | 2:36:24 | 5:26:06 |
| 35:00 | 1:12:58 | 2:41:00 | 5:35:41 |
Questions
- What half marathon time does a 20-minute 5K predict?
- About 1:32:00 for the half and 3:11:49 for the marathon, for a runner trained for those distances. If you run fewer than 30 miles a week, expect nearer 1:33:20 and 3:20:11: speed carries over from a 5K, but the endurance for 13 or 26 miles comes only from the miles.
- How accurate is a race time predictor?
- Between neighbouring distances, such as 5K to 10K or 10K to the half, a hard recent race on a flat course usually lands close. The further apart the two races, the more the prediction becomes a statement of what you could run with the right training, not what you will run next weekend. Use a race run all-out in the last two or three months; a time from a year ago or a tempo run says much less.
- Why is my marathon slower than the prediction?
- Most often because of the training behind it. A 5K or 10K tests speed; the marathon tests whether your legs and fuelling last for three or four hours, and that comes from weekly mileage and long runs. That is why, on under 30 miles a week, this predictor and our plans add 0.02 to the exponent for the marathon. Heat, hills, a fast start and too little to eat on the way cost time on the day as well.
- What 5K time do I need for a sub-4 marathon?
- Run backwards, the same formula says about 25:01 for 5K, if you are trained for the marathon. On low mileage the bar is higher, about 23:59, because the longer race costs more than the formula alone expects. A 5K a little faster than that, plus a full marathon build, gives you some room on the day.
Build a plan from your result
A free week-by-week plan from what you run now, with the pace for every session worked out from your race by this same projection.