Running Race Predictor
Enter a recent race or all-out time trial, and the calculator estimates what you'd run at other common distances — 5k, 10k, half marathon, and full marathon. It uses Pete Riegel's formula, a well-established method based on how runners predictably slow as distance increases. The further the predicted distance is from the one you entered, the rougher the estimate gets. A half marathon predicting your marathon is solid. A 5k predicting your marathon is a ballpark — useful for goal-setting, not race-day pacing.
Race Time Predictor
Enter a recent result - see your other distances.
Predicted times
Based on your result - your race is highlighted.
| Distance | Time | Pace |
|---|---|---|
| (your race) |
Estimates use Riegel's formula and assume equivalent training and effort. The further the prediction is from your race distance, the rougher it gets.
How it works
Plug in a recent race time, see what you might do at other distances
The formula takes one thing you know — how long it took you to cover a specific distance — and uses it to estimate how long other distances would take, based on the natural relationship between speed and fatigue.
It works because runners don't slow at random as distances get longer. There's a consistent, measurable pattern to it. Riegel's formula captures that pattern and applies it across distances.
The key word is recent. A race from two years and a significant fitness change ago won't tell you much about where you're at now. A flat-out effort from the last three months will. Solo time trials work too — just make sure you actually pushed it. A comfortable training run will make everything look rosier than it is.
A note on what "all-out" means: it doesn't have to be a race. A parkrun you genuinely raced, a time trial on a flat route, a track session where you went for it — all of those work. A Sunday long run at a conversational pace doesn't.
Why it's useful
A reasonable sense of where you're at, without racing every distance
The obvious use case is goal-setting. If your 5k is 28 minutes, is a sub-2-hour half marathon realistic? What about a sub-60 10k? The calculator gives you an honest answer based on your current fitness rather than wishful thinking.
It's also genuinely useful for pacing when you're stepping up to a new distance. If you've never raced a half marathon but you have a solid recent 10k, plugging that in gives you a sensible starting pace. Not a guaranteed finish time, but a much better guide than guessing — and a much safer strategy than going out at a pace that feels easy for the first 5k and costs you everything in the last three.
For runners targeting multiple distances in a season, it helps you see how they relate to each other. Improving your 5k by two minutes doesn't just affect your parkrun — it shifts the predictions across every other distance too.
What it can't tell you is as important as what it can. The formula doesn't know your training history, your longest run, whether you've fuelled properly, or how your head holds up in the late stages of a long race. Those things matter — especially at marathon distance. Use the number as a starting point, not a guarantee.
Frequently asked
Race predictor questions
How accurate is this?
Reasonably, with caveats. Predictions tend to get less accurate the further the predicted distance is from the one you input. A 5k time predicting your 10k will be close. The same 5k predicting your marathon is a much rougher guess — too much happens between 25 minutes of running and four hours of running for any formula to capture properly. Treat marathon predictions as "ballpark", not "this is what you'll run".
Should I use a recent training run or only races?
A flat-out race effort is best. A solo time trial works too, as long as you genuinely pushed it. If you put in a comfortable training run, the predictor assumes that pace is sustainable for longer than it really is, and you'll get over-optimistic numbers for everything else.
Why is the marathon prediction so much slower than I expected?
Because marathons are hard in ways that shorter races aren't — fuelling, pacing, the wall, all of it. The formula also assumes you've trained appropriately for the distance you're predicting. If you've raced a fast 5k but never run further than 10k, the predicted marathon time is what's possible if you do the marathon training — not what you'd run on Sunday off the back of your current weekly mileage.
What if I've improved a lot since my last race?
Then the prediction will be a bit out of date. The predictor only knows what you tell it — if your last race was six months ago and you've trained hard since, your real current fitness is probably better than what shows up here. Time trial a 5k if you want a fresh number.
What's the most useful distance to enter?
The closer your input distance is to the one you're predicting, the better. If you're targeting a half marathon, a recent 10k is a good input. For the marathon, a half marathon time is significantly more reliable than a 5k. The formula works across all combinations, but accuracy drops the bigger the gap.
Can I use this to set a goal for a race I've never done?
Yes — that's one of the most practical uses. Just treat the output as a realistic ceiling with good preparation, not a certainty. First-timers at any distance tend to run slower than the formula suggests, because the formula assumes you've trained specifically for that distance and can execute a controlled effort. If it's a new distance for you, give yourself a buffer.