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Race Time Predictor
A recent 5K or 10K tells you more than you think.
Race Time Predictor
Enter a recent race time to predict your finish time at any other distance.
Your recent result
Predict my time for
Predicted Marathon finish
3:59:47
An estimate only, based on Pete Riegel's race-time prediction formula (T2 = T1 × (D2/D1)^1.06). Real performance over a much longer distance depends heavily on endurance and training specificity that a shorter race time alone can't capture — treat this as a starting point, not a guarantee.
Use It On A Real Race
About This Tool
Run a recent 5K, 10K, 15K, 10 mile or half marathon? This tool uses Pete Riegel's race-time prediction formula — the same math behind most online race calculators — to estimate your finish time at any other distance, marathon included. It's an estimate based on your current fitness, not a guarantee: endurance and pacing specific to the longer distance still matter, especially the further apart the two distances are.
How To Use It
- Select the distance and enter the time from your most recent race.
- Select the distance you're predicting a time for.
- Your predicted finish time updates instantly, using the Riegel formula (T2 = T1 × (D2/D1)^1.06).
- Treat predictions from a 5K or 10K to a marathon as a rough guide — the closer the two distances, the more reliable the estimate.
How a race prediction is made
This uses Pete Riegel's formula, which has been the standard for this job since he published it in 1977: T2 = T1 × (D2 ÷ D1)^1.06. Your new time is your old time scaled by the ratio of the distances, raised to a power slightly above one.
That exponent is the whole idea. If it were exactly 1.0, doubling the distance would exactly double the time and pace would never change with distance, which is obviously wrong. At 1.06 the formula says you slow down slightly as distance grows, by an amount fitted to a large body of real race results.
What equivalent performances look like
Generated by the same formula the calculator uses, so any row is checkable.
| 5K | 10K | Half marathon | Marathon |
|---|---|---|---|
| 18:46 | 39:08 | 1:26:20 | 3:00:00 |
| 21:54 | 45:39 | 1:40:43 | 3:30:00 |
| 25:01 | 52:10 | 1:55:07 | 4:00:00 |
| 28:09 | 58:42 | 2:09:30 | 4:30:00 |
| 31:17 | 1:05:13 | 2:23:53 | 5:00:00 |
Where the prediction stops being reliable
The formula assumes appropriate training for the target distance, and that assumption is where most disappointment comes from. It is worth being specific about the failure modes.
- 1 Predicting a long race from a short one, on low mileage. A 25:01 5K genuinely does correspond to a 4:00 marathon — for someone who has trained for a marathon. Off 20 miles a week it corresponds to a hard afternoon and a long walk.
- 2 Predicting across very different distances. The further apart the known and target distances, the more the estimate depends on qualities the shorter race never tested.
- 3 Predicting from a time trial rather than a race. Solo efforts are almost always slower than the same fitness in a race, so the prediction will understate you.
- 4 Predicting from a stale result. Fitness moves; a result from last season predicts last season.
- 5 Predicting a marathon at all, really. It is the distance where fuelling, pacing and durability matter most, and none of those show up in a 10K.
The reliable direction
Predictions downwards — using a marathon to predict a 10K — tend to be more trustworthy than predictions upwards, because you have already demonstrated the endurance and are being asked about speed you already possess. Upwards predictions ask about endurance you may not have built yet.
Which result to use
The half marathon is the best single predictor of marathon performance, and it is not close. It is long enough to test endurance and fuelling, short enough to be run properly on marathon training, and it sits at a point on the curve where the extrapolation is short.
For everything shorter, a recent 10K is usually the most useful input. It is aerobically dominated without being decided by fuelling, and most people race one honestly.
Want the full picture?
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Frequently Asked Questions
How accurate is a race time predictor?
Accurate for similar distances (5K to 10K, for example) and when your training matches the target distance — the formula assumes you've trained appropriately for the distance you're predicting, so a 25:01 5K genuinely does correspond to a four-hour marathon, but only for someone who's done marathon training, not someone running 20 miles a week. It gets less reliable the further apart the distances are, since a 5K doesn't test the endurance or fuelling a marathon needs. Use it as a starting point for goal-setting, not a guarantee.
What formula does this race time predictor use?
Pete Riegel's formula: T2 = T1 × (D2/D1)^1.06, where T1/D1 is your known time and distance and T2/D2 is the distance you're predicting. Published in 1977 and still the standard — the 1.06 exponent rather than 1.0 encodes the fact that you slow down slightly as distance increases, fitted to a large body of real race results.
Can I predict a marathon time from a 5K?
Yes, the calculator supports it, but treat the result cautiously — a marathon depends heavily on endurance and fueling that a 5K effort doesn't test at all. A 10K or half marathon time will generally give you a more reliable marathon prediction.
Which race distance best predicts a marathon?
The half marathon, by a clear margin. It is long enough to test endurance and fuelling, short enough to run properly on marathon training, and close enough to 26.2 miles that the extrapolation is short. A 10K is the best input for everything below that.