What 5K time predicts a sub-2 hour half marathon?

A 5K time of approximately 26:00-27:00 predicts a sub-2-hour half marathon using the Riegel formula.

Is doubling my 10K time accurate for a half marathon?

No, the half marathon is slightly more than double the 10K distance (21.1 km vs 10 km), and fatigue means you run each km slightly slower. Typically your half marathon time is 2.1-2.2 times your 10K time.

What is a good half marathon time?

The average UK half marathon finish time is around 2:00-2:10. Sub-1:30 is considered fast, sub-1:45 is good for club runners, and sub-2:00 is a common goal for intermediate runners.

RunCalcs

Half Marathon Predictor — Predict Your Half Marathon Time

Predict your half marathon time from a 5K or 10K result using the Riegel formula.

Race Distance

Hours

Minutes

Seconds

How We Calculate This

Riegel Formula for Half Marathon

T2 = T1 × (21.0975 / D1)^1.06

This formula accounts for the natural slowdown over longer distances. The half marathon is 21.0975 km (13.1 miles).

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always consult a coach or medical professional for personalised training advice.