Race Time Predictor: Marathon, Half, 10K and 5K
Enter one race you have actually run and read what it says about every other road distance. Two models, side by side, because they disagree and the gap between them is worth seeing.
A running fitness number worked out from your result, on Jack Daniels' scale. It is not a measured VO2 max, and it is the figure the Daniels column below is built from.
| Distance | Predicted time (Riegel) | Pace | Daniels VDOT equivalent |
|---|---|---|---|
| 5K | 25:00 | 5:00 min/km | 25:00 |
| 10K | 52:07 | 5:13 min/km | 51:53 |
| Half marathon | 1:55:00 | 5:27 min/km | 1:55:04 |
| Marathon | 3:59:47 | 5:41 min/km | 3:57:55 |
Every prediction assumes you have trained for the distance and that you race it on an even pace. A marathon predicted from a 5K is the time your current speed allows, not the time your current endurance will deliver.
A predicted time is where you are now. Your coach builds the running block from here to your race date, in Telegram. 14 free days, no card.
How This Race Time Predictor Works
It takes one finish time, works out how fast you are, and asks what that speed is worth over a distance you have not raced. Two established models answer that question, and this page runs both rather than picking one for you.
Both need the same thing from you: a result you actually ran, on a measured course, in the last few weeks. A time from 18 months ago predicts the runner you were 18 months ago.
Berlin Marathon 2024
Riegel's Endurance Law
Peter Riegel published the formula in 1977 after fitting it to race results across running, swimming and cycling. It says your finish time rises with distance raised to the power of 1.06. Double the distance and your time is multiplied by 2.085 rather than by 2, so your pace slows by a little over 4%.
Written out, the predicted time is your known time multiplied by the ratio of the two distances raised to 1.06. A 25:00 5K becomes a 52:07 10K rather than a flat 50:00, which is 5:00 per kilometre stretching to 5:13, and doubling again gives 1:48:40 over 20km. It is the model behind almost every race time predictor you will find, including this one.
Daniels VDOT
Jack Daniels and Jimmy Gilbert took a different route. They modelled the oxygen cost of running at a given speed, and the share of your maximum you can hold for a given length of time, then combined the two into a single number they called VDOT.
Your VDOT comes out of the result you typed in. Every other distance is then the time that same VDOT produces, which is why the Daniels column is not simply the Riegel column shifted by a fixed amount.
Where the Two Models Disagree
Over 10K the two barely separate. The 25:00 5K above gives 52:07 from Riegel and 51:53 from Daniels, 14 seconds apart. Over the marathon the gap opens up, because Daniels holds a little more back: his curve knows that the share of your maximum you can sustain keeps falling as the hours add up, while Riegel applies one exponent everywhere.
Treat the two as a range rather than as a contradiction. If your training has been short and fast, the Daniels figure is the more honest one. If you have long runs behind you, the Riegel figure is within reach.
Reading This as a Marathon Time Calculator
The marathon is where a predicted time is most useful and least trustworthy at the same time, which is why it gets its own section.
Both models assume you have done the training the distance asks for. Neither one knows whether you have run beyond 30km, whether you have practised taking a gel at race pace, or whether your longest run this year was the 5K you typed in. A marathon predicted from a 5K tells you what your speed is worth. What your endurance is worth is a training question, not a maths one.
Once you have a target time, work backwards into the splits it needs on the running pace chart, which turns a goal finish into a pace per kilometre, a pace per mile and the halfway split you should be passing.
Racing HYROX rather than a road marathon? The HYROX finish time predictor does the same job for the eight runs, the eight stations and the Roxzone.
What to Do With the Number
- Set your race pace from the distance you are actually racing, not from the fastest row in the table.
- Check the prediction against a recent long run. If the pace feels nothing like it, believe the run.
- Re-run it after a hard training block rather than after a taper week, so you are comparing like with like.
- Use the predicted 10K pace as the anchor for threshold work, and your heart rate zones to keep the easy days easy.
- Give yourself a range rather than a single number. The two columns in the table give you exactly that.
One thing to keep in mind. A predicted time estimates what your current fitness is worth on a flat course in good conditions. Heat, hills, wind and a crowded start line all move it, and none of them are in the formula.
Train Better & Race Faster with All Our Tools
HYROX Pace Calculator
Calculate your ideal pace for HYROX events and optimize your performance.
HYROX Time Predictor
Predict your HYROX finish time based on your running and workout performance.
Heart Rate Zone Calculator
Get your five heart rate zones three ways, from your max, your reserve or your threshold.
Running Pace Chart
Convert speed to pace and finish times in km or miles, and work back from a marathon or half marathon goal time.
Race Time Predictor
Turn one recent race result into your predicted 5K, 10K, half marathon and marathon times.
Frequently Asked Questions
Accuracy depends almost entirely on the size of the jump. Predicting a 10K from a 5K is reliable, because the two are raced at close to the same effort. Predicting a marathon from a 5K is an estimate of your speed rather than of your finish time. The predictor cannot see your longest run, whether you have fuelled at race pace, or whether you have ever held a pace for three hours, and over the marathon those are the things that decide the day.
Predicted time equals your known time multiplied by the ratio of the two distances raised to the power of 1.06. Peter Riegel fitted that exponent to race results in 1977 and it has held up since. In plain terms, doubling the distance multiplies your time by 2.085 rather than by 2, so your pace slows by a little over 4%.
VDOT is a running fitness number on Jack Daniels' scale, worked out from a race result. It is not a measured VO2 max, even though it is printed in the same units. A laboratory measures VO2 max on a treadmill with a mask, while VDOT is inferred from how fast you ran. Two runners with the same VDOT should race similar times whatever their laboratory numbers say.
Use them as a range. Over 5K and 10K they agree closely. Over the marathon Daniels is usually the more conservative of the two, and it is the one to lean on if your training has been short and fast. If you have a long-run history behind you, the Riegel figure is within reach.
You can, and the table above will do it, but read it as what your speed is worth rather than as a finish time. A marathon is decided by fuelling, long-run history and pacing discipline over three or four hours, and none of those are in a 5K result. Predict from your longest recent race wherever you have one.
Inside the last six weeks is ideal, and anything beyond three months is a prediction about a runner who no longer exists. Use a race rather than a solo effort where you can. A parkrun counts. A hard training run on your own is usually a little slower than a race over the same distance, so it will under-predict rather than flatter you.