What formula does this use?

It uses Pete Riegel's endurance formula: T2 = T1 × (D2 / D1) ^ 1.06, where the exponent 1.06 accounts for fatigue over longer distances.

How accurate is the prediction?

Riegel predictions are most reliable when the input race is recent, run at full effort, and reasonably close in distance to the target. Marathon predictions also assume you have done the appropriate endurance training.

Why is the exponent 1.06 and not 1?

If pace stayed identical at every distance the exponent would be 1.0, meaning time scales linearly with distance. The exponent 1.06 is slightly larger because runners slow down over longer distances as fatigue sets in. Pete Riegel derived 1.06 from real race data; it is the standard value used for road-race prediction.

Can I use a 5K to predict a marathon?

Yes, but the further apart the two distances are the more the prediction depends on your endurance base. A 5K extrapolated to a marathon (more than 8x the distance) assumes you have done marathon-specific long-run training; without it your real time will be slower than the formula suggests.

Does it predict shorter distances too?

Yes. The formula works in both directions, so you can enter a half-marathon and predict a 10K or 5K equivalent. It simply rearranges to T2 = T1 × (D2 / D1) ^ 1.06 for any pair of distances.

What distances can I choose?

You can convert between the four common road distances built in: marathon (42.195 km), half marathon (21.0975 km), 10 km and 5 km. Pick the one you actually raced and the one you want to predict.

Marathon Time Predictor

Q: Is my data sent anywhere?

No — every calculation runs locally in your browser and nothing is uploaded.

If you’ve just raced a 5K, 10K or half-marathon all-out, you can estimate your finish time at any other road distance — including the full marathon — without running it first. This predictor uses the Riegel formula, the equivalent-performance model runners and coaches have relied on for decades, and shows the per-kilometre pace your target time implies.

How it works

Pete Riegel’s formula relates a known time at one distance to a predicted time at another:

T2 = T1 × (D2 ÷ D1) ^ 1.06

where T1 is your known time over distance D1, D2 is the target distance, and the exponent 1.06 captures how pace fades as distance grows. An exponent of exactly 1 would assume identical pace at every distance; 1.06 adds a realistic slowdown. The tool converts your hours/minutes/seconds to total seconds, applies the formula, then divides the predicted time by the target distance to give pace per kilometre.

Example

You run a 5 km in 25:00 (1,500 seconds) and want a marathon prediction:

T2 = 1500 × (42.195 ÷ 5) ^ 1.06 = 1500 × 8.439 ^ 1.06 ≈ 15,090 seconds ≈ 4:11:30, which is a pace of about 5:58 per km.

From	Time	To	Predicted
5 km	25:00	10 km	~51:55
5 km	25:00	Half	~1:55:30
5 km	25:00	Marathon	~4:11:30

Every calculation runs locally in your browser and nothing is uploaded.