Enter one recent race result and predict your finish time at 5K, 10K, 15K, half marathon or marathon, using the Riegel endurance formula.
A race time predictor uses a known result from one distance — say a recent 5K — to estimate your finish time at a different distance, based on a well-documented mathematical relationship between endurance and race length. It is not a guarantee: your actual result on the day still depends on training specificity, pacing, terrain, weather and fuelling. Treat the prediction as a solid training-plan target, not a promise.
| Distance | Predicted Time | Pace (min/km) |
|---|
Race time prediction rests on a simple observation: once you know how fast someone can hold a given effort over one distance, you can make a reasonable estimate of how that effort would hold up over a longer or shorter one. The tool used here is the Riegel formula, a widely-cited piece of running science that links finish time to distance through a single exponent.
To show the formula in action across the standard race ladder, here is what a 25:00 5K result predicts at 10K, half marathon and marathon distance. These are the same numbers the worked example above builds on, extended out to the longer distances.
| Distance | Predicted Time | Predicted Pace |
|---|---|---|
| 5K (5.0 km) — known result | 25:00 | 5:00 /km |
| 10K (10.0 km) | 52:07 | 5:13 /km |
| Half Marathon (21.1 km) | 1:55:01 | 5:27 /km |
| Marathon (42.2 km) | 3:59:48 | 5:41 /km |
If pace stayed perfectly constant regardless of distance, the exponent in the formula would simply be 1, and doubling the distance would exactly double the time. In practice it does not work that way. Every extra kilometre draws further on glycogen stores, generates more accumulated muscular fatigue, and asks more of your body's ability to regulate heat and hydration. Endurance is drawn down faster than distance increases in a straight line, which is exactly why the exponent sits a little above 1.0 rather than at exactly 1.0. That small excess — 0.06 — is doing a lot of work: it is the formula's way of encoding that pace naturally eases off as a race gets longer, even for a well-conditioned, evenly-trained runner.
This is also why the gap between your known result and your target matters. A short hop, such as 5K to 10K, only asks the formula to extrapolate over a small change in distance, so the fatigue-related error stays small. A long reach — using a 1-mile time trial to predict a marathon, for instance — asks the same formula to project four decimal places of exponent growth over a 26-fold increase in distance, and small inaccuracies compound. The further apart your known and target distances are, the more the prediction should be read as a rough guide rather than a precise number.
This calculator assumes your training and physiological make-up transfer evenly between the known and target distances — which, for most recreational and club runners doing general aerobic training, is a reasonable assumption. It breaks down at the extremes. A runner who has spent months doing nothing but short, sharp 5K-specific sessions typically has less of the deep aerobic base a marathon demands, so they will often run slower than this formula predicts over 42.2 km. The reverse also holds: a marathon-trained runner with big weekly mileage but little top-end speed work will often out-run this formula's 5K prediction. Longer target distances also open the door to more real-world variables the formula cannot see — pacing discipline, race-day weather, hills, and how well you fuel and hydrate over two, three or four hours. Use the prediction as a sensible starting target for your race plan, not as a locked-in outcome.
How accurate are race time predictors?
They are a genuinely useful estimate, not a guaranteed outcome. Over a modest distance gap — 5K to 10K, for example — the Riegel formula tends to land close to what well-trained runners actually achieve. The accuracy drops the further apart your known and target distances are, and it does not account for pacing mistakes, weather, hills or how well you have specifically trained for the target distance.
Can I predict my marathon time from a 5K result?
Yes, the formula will produce a number, but treat it cautiously. A 5K result mostly reflects speed and short-duration effort, while a marathon depends heavily on aerobic endurance, fuelling and pacing over several hours. If your training has been 5K-focused with limited long runs, your actual marathon result is likely to be slower than this formula predicts. A 10K or half marathon result is a more reliable base for a marathon prediction.
What is the Riegel formula?
It is a formula published by Pete Riegel in 1977 that estimates a finish time at one distance from a known finish time at another: T2 = T1 × (D2/D1)^1.06, where T1 and D1 are your known time and distance, and T2 is the predicted time at target distance D2. The 1.06 exponent reflects that runners naturally slow slightly as distance increases, even at consistent effort.
Why do race predictions get less accurate over longer distances?
The formula extrapolates from a single data point, and the further you stretch that extrapolation, the more room there is for real-world factors to throw it off. Predicting 5K to 10K only stretches the formula across a small gap. Predicting a mile time out to a marathon stretches it across a 26-fold increase in distance, where fuelling, pacing discipline and aerobic endurance — none of which the formula can see — start to dominate the result.
Does this calculator work for walking or trail running?
The formula was developed from road and track running data, so it is most reliable for flat, run-the-whole-way road or track efforts. Trail races with significant elevation change, technical footing, or heat, and walking efforts with a very different fatigue profile to running, will generally not match the prediction as closely.
What if my known race was run in hot weather or on a hilly course?
The prediction will inherit that bias. A known time that was slowed by heat, wind or hills will produce a target prediction that is proportionally slower too, and vice versa for a fast, wind-assisted, downhill result. For the most reliable prediction, use a known time from a fair, reasonably flat course run in mild conditions.
How is race pace different from race time prediction?
A pace calculator answers "how fast am I going right now, and what does that mean for a given distance at that same pace." A race time predictor goes a step further: it accounts for the fact that pace itself changes as distance changes, using the 1.06 exponent to model that natural slowdown, rather than assuming pace stays flat.
Should I use my best-ever time or my most recent time as the known result?
Your most recent time, run at or near your current fitness, will generally give a more realistic prediction than an older personal best from a different training block. Fitness changes over months, and the formula only knows what you tell it — feeding it a stale result will produce a stale prediction.