About this calculator
Enter a recent race distance and time, ideally from the last few weeks, run at full effort.
How to use it
- Choose the distance of your recent race.
- Enter your time.
- Read predictions for other distances.
The formula
T2 = T1 × (D2 ÷ D1)1.06
- T1, D1
- your recent time and distance
- T2, D2
- predicted time for another distance
Worked example
A 10 km race in 55:00
- Half marathon: 55:00 × (21.0975 ÷ 10)1.06 = 2:01:21.
- Marathon: 4:13:01.
- 5 km: 26:23.
What the result means
Predictions are most reliable between similar distances; from 10 km to a marathon they assume solid endurance training.
Assumptions
- The race was run at full effort.
Limitations
- Heat, hills and lack of long runs make real marathon times slower.
Frequently asked questions
How accurate is Riegel’s formula?
Usually within a few minutes for distances close to each other.
Why am I slower than predicted in the marathon?
The formula assumes enough long runs; many runners need more endurance work.
Which race should I use?
A recent all-out race, ideally 10 km or a half marathon.
Not medical advice. This calculator is a general screening tool and cannot diagnose any condition. Results are estimates. Please talk to a doctor or a qualified health professional about your health and before making changes to diet, exercise or treatment.
Last reviewed on 1 October 2026. Found a mistake? Tell us.