5K to Marathon Time: Predicted Finish Times for Every Common Race

Last updated: May 2026

If you have a recent 5K result, you can calculate a predicted finish time at any other race distance using the Riegel formula — the standard prediction model used by coaches and running calculators worldwide. This page provides pre-calculated prediction tables so you can read off your expected times at 10K, half marathon, and marathon without doing any math.

5K to Marathon Prediction Table

All predictions use the Riegel formula: T₂ = T₁ × (D₂ ÷ D₁)^1.06. For a 5K input, D₁ = 5 km. Marathon distance = 42.195 km.

The Alternative: 10K as a Predictor

A 10K result gives a more accurate marathon prediction than a 5K because the distances are closer — the Riegel formula becomes less reliable the further apart the input and target distances are. If you have both a 5K and 10K result, use the 10K.

A widely used rule of thumb from RunnersConnect: multiply your 10K time in minutes by 4.65 to get a reasonable marathon estimate. For a 50-minute 10K: 50 × 4.65 = 232.5 minutes ≈ 3:52:30. This heuristic typically aligns closely with Riegel predictions.

Why Your Marathon May Differ from the Prediction

Training Specificity

The most important caveat: the Riegel formula predicts what your aerobic fitness allows at marathon distance. It does not predict what your endurance training can sustain. A runner with a 25:00 5K who has never run longer than 10 miles will not run a predicted 3:59 marathon — the long-run adaptation, glycogen capacity, and race-specific fitness aren’t there. The prediction assumes you’ve completed the training required for the target distance.

As a general guide: a marathon prediction from a 5K is reliable when the runner has completed at least one 18–20 mile long run and has consistent weekly mileage of 40+ miles for at least 8–12 weeks prior to the race.

Pacing Strategy

The formula also assumes optimal pacing. Runners who go out too fast in the first half routinely finish 10–30 minutes slower than predicted due to glycogen depletion. The prediction is achievable with controlled early pacing — it is not achievable when the first half is run at or faster than goal pace.

Course and Conditions

The formula assumes flat roads in neutral weather. Hills, heat, humidity, wind, and trail surfaces all add time beyond what the formula predicts. Adjust your goal time upward (add time) for any of these factors.

Using the Prediction to Set a Goal

The Riegel prediction is a starting point, not a guarantee. For a first marathon, target 5–10 minutes slower than predicted — the training may not have been perfectly specific, and it’s better to finish strong than to blow up chasing a number. For experienced marathoners with race-specific training, the prediction is often accurate to within 5–10 minutes.