Race time predictor and pace calculator
Enter one recent race and see what it predicts for 5K, 10K, half marathon and marathon, plus the paces to train at.
From your 5K in 25:00, if you train for the distance.
Race predictions
| Distance | Time | Pace /mi |
|---|---|---|
| 1 mile | 7:24 | 7:24 |
| 5KYour race | 25:00 | 8:03 |
| 10K | 51:53 | 8:21 |
| Half marathon | 1:55:04 | 8:47 |
| Marathon | 3:57:55 | 9:04 |
Training paces
The same five zones Zurge prices run plans with. Effort is out of 10.
| Zone | Pace /mi | Effort | Used for |
|---|---|---|---|
| Easy | 9:44 or slower | 4 | Most of your running, and every long run |
| Steady | 8:51-9:17 | 6 | Longer efforts close to marathon pace |
| Threshold | 8:29-8:38 | 7 | Tempo runs and threshold repeats |
| Interval | 7:39-7:52 | 8 | Repeats of 3 to 5 minutes |
| Repetition | 7:11-7:21 | 9 | Strides and short fast repeats |
Fitness number
Every time and pace above comes from this one number, a VDOT in Jack Daniels' terms: the oxygen uptake (ml per kg per minute) your race implies, given how long you held the pace. It is close to, but not the same as, a lab VO2 max. A higher number is a faster runner. One point is worth about 20 seconds over 5K for a 20 minute runner, and about a minute for a 35 minute runner.
How the predictions work
The predictor uses Jack Daniels' running formula, built on the oxygen equations Daniels and Gilbert published in 1979. One equation gives the oxygen cost of running at a given speed. The other gives the share of your maximum you can hold for a race of a given length: close to all of it for a few minutes, much less for a few hours. Your race time and distance go in, and out comes one fitness number, the VDOT.
Every prediction is the time that same number gives for another distance, and every training pace is a set share of it. Because they all come from one number, your predictions and your paces always agree. It is the same math Zurge uses to set the paces in its run plans, so the numbers here match the app.
What the predictions assume
- You train for the distance.A 5K runner's marathon prediction is optimistic without the long runs and weekly mileage a marathon needs. The closer the distance to your race, the better the guess.
- Similar conditions. Heat, humidity, hills, wind and altitude all slow the clock. The prediction is for a flat course on a cool day.
- Even pacing. Starting too fast costs more time at the end than it gains early, most of all in a half or a full marathon.
Times from under 1,500 meters are refused, because a shorter effort leans on sprint speed. So are times that land outside the runners the formula was fitted to (a VDOT below 20 or above 85).
Using the training paces
- Easy: most of your running and every long run. The pace is a limit; slower is fine.
- Steady: marathon pace, give or take a few seconds per kilometer.
- Threshold: comfortably hard, the pace you could hold for about an hour in a race. Tempo runs and longer repeats.
- Interval: repeats of 3 to 5 minutes with jog recoveries.
- Repetition: strides and short fast repeats, with full recovery between them.
Questions
How accurate are race time predictions?
For a distance close to the one you raced, and one you have trained for, most runners land within a few percent. The further you predict from your race, the more it depends on training: a 5K time says a lot about your 10K and much less about your marathon.
Why is my marathon prediction faster than I can run?
The prediction assumes you have done the training for that distance, mostly long runs and steady weekly mileage. A 5K runner without that base usually fades in the last 10 km. Race a half marathon and use that time for a truer marathon number.
Which race should I enter?
A recent all-out race or time trial on a flat, measured course, ideally 5K to half marathon. A mile works, but it leans on speed. A training run, a hilly course or a hot day gives a slower number than you can really run.
What is VDOT?
A fitness number from Jack Daniels' running formula. It is the oxygen uptake your race implies once you allow for how long you held the pace. It is close to a lab VO2 max but not the same thing, and it is useful because one number sets both race predictions and training paces.
Why is easy pace only a limit?
Easy running has a speed limit, not a target. Go as slowly as you like on easy days and long runs; the number shown is the quickest those runs should be.
How often should I update my paces?
After a new race, or when quality sessions keep coming in clearly faster than the range. Paces that are too fast make hard days harder and easy days not easy enough, so it pays to use a current time rather than your best ever.