Marathon Time Predictor
The calculator below uses an established method, The Riegel Formula, to estimate your marathon finishing time based on recent race time. This formula takes into account the normal fatigue and slowing down that happens as you get into the later stages of a marathon. You can also adjust the prediction based on how much you think you'll fatigue.
Marathon Time Estimate
Enter a recent race or time trial to get an idea of your marathon finishing time.
Predictions for shorter distances can be less reliable at marathon distance and beyond. For best results, use a half marathon or longer as your base race.
We'd recommend a more recent race - your estimate may not be accurate if your fitness has changed since then.
Update your predictions based on how much you think you'll slow down. A higher number means more fatigue and slowing down in the late stages of the marathon.
Predicted times
Based on your result - your race is highlighted.
| Distance | Time | Pace |
|---|---|---|
| (your race) |
Estimates use Riegel's formula (T₂ = T₁ × (D₂/D₁)). The further the prediction is from your race distance, the rougher it gets. Adjust the fatigue factor for more conservative long-distance estimates.
How it works
How to predict your marathon time
Predicting your marathon time isn't an exact science, but there are ways to get a good idea of where you might finish.
The general idea for these predictions is to take a recent race time of yours, preferably for a distance close to the marathon like a half marathon for example, and then multiply your time by something called The Riegel Formula.
This formula has a built in way to account for the normal fatigue and slowing down as you get further into the marathon, so you can get a pretty close estimate of your time.
The calculator above uses this formula to help you predict your marathon finishing time.
A rough rule of thumb that often works better for beginners than the standard formula: take your half marathon time, double it, and add 20 minutes. If the calculator is spitting out something significantly faster than that, the fatigue factor is probably worth nudging up.
Why it's useful
How to get an accurate marathon time prediction
The number the calculator gives you is a starting point, not a finish line. Here's how to make it as useful as possible:
Use a half marathon time if you have one. It's close enough to marathon distance that the formula works well. A 5k to marathon prediction is crossing a lot of distance — aerobically, nutritionally, and mentally — that the maths can't fully capture.
Be honest about your training. If you haven't run further than 16k in training, the standard prediction will likely be optimistic. Bump the fatigue factor up. Most first marathons involve some degree of slowing in the back half — it's not failure, it's just reality for most people at this distance.
Recency matters. A race from eight months ago and three injury setbacks doesn't tell you much about what you'll run next month. If your fitness has changed significantly, the prediction changes too — a recent time trial is worth doing before you lock in a race goal.
The marathon is different from everything else. A 5k and a 10k are mostly aerobic efforts with some discomfort. A marathon adds fuelling complexity, muscular fatigue that accumulates across four-plus hours, and a mental challenge that shorter distances don't prepare you for. The formula is unaware of all of that. You shouldn't be.
Frequently asked
Race predictor questions
Do these marathon calculators work for your first marathon?
In a perfect world, yes. Unfortunately, most runners find that they'll fatigue and slow down much more than the standard formula accounts for. If you've never done a marathon before, especially if you've not got a history of long distance running, In would use these estimates as an idea of your time, not a sure thing.
Can I use a 5K time for the marathon estimate?
Of course you can, but just beware that the formula used for these calculations are less accurate that way. Ideally, enter a recent half marathon race performance for best results.
Why does the calculator show a faster time than I expect to run?
The standard Riegel formula was developed from race data that skews towards well-trained runners running controlled efforts. If you're new to the marathon, haven't done much running above 20k, or know from experience that you tend to fade in longer races — the default formula will likely be optimistic. That's exactly what the fatigue factor is there to fix. A setting between 1.10 and 1.40 is realistic for most recreational first-timers.
What fatigue factor should I use for my first marathon?
Honest answer: 1.20–1.40 for most people doing their first marathon with solid but not elite training. If your weekly mileage has been on the lower end (under 50k per week), you've done few runs over 25k, or you're not confident about your pacing discipline on the day — lean towards 1.40 or higher. It's much better to start conservatively and run a strong second half than to blow up at kilometre 30.
Should I try to hit the predicted time on race day?
Use it for pacing guidance, not as a target to chase from the gun. If the calculator gives you a 4:20 at the default fatigue setting, that's a reasonable best-case. Start at a pace that feels almost embarrassingly easy for the first 10k — you'll thank yourself later. The marathon punishes optimism early and rewards patience.