Tool · Pace & time
Race time predictor
Enter a recent finish time over one distance and get your predicted times for the 5K, 10K, half marathon and marathon, plus any target distance you like. The math relies on Jack Daniels’ VDOT, the same method that powers RunMorph plans.
These times are theoretical potentials based on Jack Daniels’ VDOT model. They assume training suited to the distance: your marathon potential only materializes if you’ve built the specific endurance for it. Adjust for weather, course profile and how fresh you feel on race day. In your RunMorph account, the half and full marathon are also adjusted to your actual training volume (weekly distance and longest run): at low mileage, they come out slower there than here.
How to predict your race times
Predicting a race time boils down to one simple question: if I run this time over this distance, what am I worth over the others? Two approaches are the go-to references.
1. The VDOT method (Jack Daniels), used here
Your performance is first turned into a VDOT index, which matches your equivalent VO₂max given how long the effort lasted. The model relies on two equations from Daniels and Gilbert: the oxygen cost of a given speed, and the fraction of VO₂max you can sustain for a given duration. From that VDOT, it looks for the speed that produces exactly the same index over each target distance. That speed gives the predicted time.
Worked example: a 10K in 45:00 gives a VDOT of about 45 and a field maximal aerobic speed close to 9.4 mph (a pace of 6:21/mi). The model then predicts a half marathon around 1:39:44 and a marathon around 3:27:15, provided you have the specific endurance for those distances.
2. Riegel’s formula (the classic alternative)
Older and simpler, Pete Riegel’s formula extrapolates one time from another by raising the ratio of the distances to the power of 1.06:
Example: from a 10K in 45:00 (t1 = 2,700 s, d1 = 6.214 mi), the half marathon (d2 = 13.109 mi) is estimated at t2 = 2,700 × (13.109 / 6.214)^1.06, about 1:39:17. The 1.06 exponent captures the drop in speed as the distance grows, and it lands very close to VDOT (1:39:44) in this example. VDOT refines the principle with a physiological model, which is why the calculator uses it by default.
Either way, the pace for a given speed is pace (min/mi) = 60 ÷ speed (mph).
3. The limits: from 1.5 km to the marathon
The calculator predicts distances from 1.5 to 42.2 km (0.9 to 26.2 miles), the range in which Daniels’ model was built. Beyond the marathon, fatigue, stops and elevation matter more than speed: for a trail race or an ultra, estimate your time with the effort-km calculator, which accounts for elevation gain and fatigue.
Equivalence chart: one fitness level, four distances
A few race equivalents calculated with the VDOT model. Each row brings together times that match the same fitness level. Read it left to right to see your potential over longer distances.
| 5K | 10K | Half marathon | Marathon |
|---|---|---|---|
| 20:00 | 41:28 | 1:31:50 | 3:11:17 |
| 22:30 | 46:40 | 1:43:27 | 3:34:43 |
| 25:00 | 51:53 | 1:55:04 | 3:57:55 |
| 27:30 | 57:08 | 2:06:38 | 4:20:54 |
| 30:00 | 1:02:23 | 2:18:10 | 4:43:39 |
These equivalents are theoretical potentials at the same fitness level. The calculator above starts from your actual race time to give you your own projections, target distance included.
Frequently asked questions
How do I predict my race time from another race?
Start from a recent performance over a known distance (a 10K, for example). Jack Daniels’ VDOT model turns it into a performance index, then projects the time you could run over other distances. It’s more reliable than simply extrapolating an even pace, because the model accounts for the unavoidable loss of speed as the distance grows.
Are the predictions reliable?
They’re reliable as a theoretical potential, especially for distances close to your reference race (a 10K predicts a 5K or a half well). The further you move away, the more the prediction assumes specific training: your marathon potential is only within reach if you’ve built the matching endurance. Your actual time also depends on the weather, the course profile and how fresh you are on race day.
What’s the difference between VDOT and Riegel’s formula?
Riegel’s formula is a simple rule: t2 = t1 × (d2 / d1)^1.06. It gives a good first approximation. Jack Daniels’ VDOT is a more complete physiological model, going through equivalent VO₂max and the fraction you can sustain for a given effort duration. This calculator uses VDOT by default because it matches real performances better across all distances.
Which reference race should I use to predict my marathon?
The most accurate is a long, recent race: a half marathon predicts a marathon better than a 5K, because the effort duration is closer. If all you have is a 10K, the marathon prediction remains a potential to confirm with a specific endurance block.
What are the VDOT and maximal aerobic speed shown with the result?
VDOT is a performance index (in ml/kg/min) from the Jack Daniels method: the VO₂max equivalent to your performance, on the same scale as RunMorph’s VO₂max. The maximal aerobic speed shown is your field MAS, the speed you could hold for about 6 minutes all-out, derived from the same VDOT. Both underpin the race predictions and the calibration of your training paces.
Can this tool predict an ultra or a trail race?
No: Daniels’ model works from 1.5 km up to the marathon (26.2 miles). Beyond that, fatigue, stops, nutrition and elevation matter more than speed, and a simple extrapolation would give times far too fast. For a trail race or an ultra, use the effort-km calculator: it turns elevation gain into an equivalent distance and estimates your time with fatigue and stops.
Turn this prediction into a training plan
A predicted time only comes true with training built for the distance. RunMorph builds your plan session by session, periodized all the way to your goal race.
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