How accurate is the race time predictor?

The Riegel formula is a good estimate for well-trained runners, but longer distances need specific endurance training to hit the predicted time.

Which result should I enter?

Use a recent, all-out race result for the most realistic prediction.

How much error should I expect from a race time prediction?

For distances close together, like 10K to half marathon, the Riegel formula is typically within 1-3% of race day reality for a well-trained runner. The error grows to 5-10% or more when predicting a marathon from a 5K or 10K, because the extrapolation is larger and marathon-specific endurance matters more than raw speed.

Can I predict a marathon from a 5K?

Yes, but treat it as a rough ceiling rather than a target: a 5K only tests speed over about 12 minutes, while a marathon tests fuelling and endurance over three-plus hours, so the formula has to extrapolate further and is less reliable. A 10K or half-marathon result predicts a marathon more accurately than a 5K does.

Why is my real marathon slower than the prediction?

The Riegel formula assumes you have trained specifically for the marathon distance - long runs, fuelling practice and pacing discipline - not just that you are fit at shorter distances. Without that endurance-specific preparation, most runners fade in the last 10K and finish 5-10% slower than the formula predicts, so treat the prediction as an upper bound rather than a guarantee.

Can I use a training run or treadmill time as input?

No - use a genuine race or time-trial result run at maximum effort on a measured course; a comfortable training run or treadmill session underestimates your race fitness and the tool will predict a time that is too slow. A parkrun or official race result is ideal because both the distance and the effort are verified.

Race Time Predictor — Estimate Your Finish Time

How to use the race time predictor

Enter the distance and time from a recent, all-out race effort — not a training run. Use an official race result where possible; a parkrun is ideal because the distance is accurate and the effort is genuinely competitive. The tool immediately shows predicted finish times for the 5K, 10K, half marathon and marathon.

What the formula does

The predictions use Peter Riegel's formula: your known time is scaled to a new distance using an endurance exponent of 1.06. This exponent reflects the fact that longer distances are disproportionately harder — not because of speed, but because of the cumulative energy cost of sustaining effort over more time.

The formula is most reliable when the distances being compared are within a factor of about three of each other. Predicting a half marathon from a 10K is more accurate than predicting a marathon from a 5K, because the extrapolation is smaller.

How the math works

The predictor runs Peter Riegel's formula directly, with no rounding or simplification:

T₂ = T₁ × (D₂ ÷ D₁)^1.06

T₁ is your race time at the known distance.
D₁ is that known distance, and D₂ is the target distance you want a prediction for.
1.06 is the endurance exponent — the same number the tool uses for every prediction, whether it's scaling a 5K up to a marathon or a half marathon down to a 10K.

Plug in round numbers to see what the exponent does: if D₂ is exactly twice D₁, the distance ratio is 2, and 2^1.06 ≈ 2.085 — about 4.2% more than a flat doubling would give. That extra 4.2% is the cost of running twice as far. Nobody can hold 5K pace for twice the distance, so the time has to grow a bit faster than the distance does. The exponent packs that entire pace-fade effect into a single number, one that Riegel fitted from real race and world-record times across distances rather than derived from a physics equation — it's an empirical average, not a law.

Because it's a fitted average, the exponent works best over a moderate distance gap and gets shakier the further apart D₁ and D₂ are. That's why this tool always shows all four standard distances (5K, 10K, half marathon, marathon) from a single input — so you can see the predicted pace gradually slow as the target distance grows, and judge for yourself how far you're extrapolating from your known result. For the full derivation, the data Riegel used, and why the formula tends to run optimistic at the marathon distance specifically, see the race time prediction guide.

Worked example

Say your most recent race is a 10K finished in 52:00 (5:12/km), and you want a half-marathon prediction. The half marathon is a standard 21.0975 km, so here's what the tool does with those two numbers, step by step:

Set up the known values. D₁ = 10 km, T₁ = 52:00 = 3,120 seconds. D₂ = 21.0975 km.
Find the distance ratio. D₂ ÷ D₁ = 21.0975 ÷ 10 = 2.10975.
Raise the ratio to the endurance exponent. 2.10975^1.06 ≈ 2.2064 — noticeably more than a flat 2.10975× would be, because the exponent adds the extra fatigue cost of covering 11 more kilometres.
Multiply by the known time. T₂ = 3,120 seconds × 2.2064 ≈ 6,884 seconds.
Convert to a clock time. 6,884 seconds = 1:54:44.
Check the implied pace. 6,884 seconds ÷ 21.0975 km ≈ 326.3 seconds/km, or 5:26/km — about 14 seconds/km slower than the 10K pace of 5:12/km that produced it, which is a realistic fade for stepping up from 10K to half-marathon distance.

Enter the same 10K in 52:00 into the predictor above and it returns exactly this 1:54:44 half-marathon prediction, alongside 5K and marathon estimates computed the same way from the same input. From there, take the 1:54:44 into the splits calculator to turn it into an exact per-km pacing plan for race day.

Choosing the best input

For the most useful predictions, pick a result from the last 8–12 weeks that genuinely reflects a maximum effort over a measured course. Avoid a comfortable long run or a training session run at an easy pace — the predictor assumes you raced hard, and a conservative input produces predictions that are too slow.

If your fitness has changed significantly, the most recent result gives the best forecast. A half marathon from six months ago reflects who you were then; last week's parkrun reflects who you are now.

What to do with the output

The predictions give you a realistic target range for a new race distance. Use them to:

Set a goal time before signing up for a longer or shorter race.
Check whether your current fitness supports the time you have in mind.
Plan your race-day pacing — take the predicted time into the splits calculator to get the exact per-km pacing plan.

Accuracy and its limits

No formula is perfect. The Riegel model assumes consistent training across distances and tends to be optimistic for marathon predictions unless the runner has done the marathon-specific long runs to back it up. For a first marathon especially, treat the predicted time as an upper bound and plan conservatively — most debut marathons are run 5–10% slower than the formula suggests.

For a deeper look at how the formula works and where it tends to over- or under-predict, read the race time prediction guide.

Race Time Predictor