Race time predictor
Enter a recent race result and see your predicted times from 1 mile to 50K, using Riegel's proven formula. A quick reality check for your next goal.
Predicted times
| Distance | Time | Pace |
|---|
How the prediction works
Guaruna uses Pete Riegel's formula: T2 = T1 × (D2 / D1)1.06. In plain terms, it takes how fast you ran one distance and scales it to another, adding a little slowdown for longer efforts. It's the same maths behind most race calculators and it's remarkably accurate when your fitness and the conditions are similar.
Getting a realistic number
Use a recent, all-out result over a distance not too far from your target — predicting a marathon from a 5K is a stretch, predicting it from a half is solid. The fatigue factor (1.06 by default) controls how much you fade as distance grows; bump it up if you fade hard late, drop it toward 1.05 if you're an endurance specialist.
Got your target time? Build your pacing with the race planner, or dial in effort with heart-rate zones.
Frequently asked questions
How accurate is the race time prediction?
It uses Riegel's formula and is accurate when your fitness and the conditions are similar and the input distance is not too far from the target.
What formula does it use?
Riegel: predicted time equals your known time times the distance ratio raised to the power 1.06. You can adjust that fatigue factor.
Can I predict a marathon from a half marathon?
Yes, and a half marathon is a far more reliable input than a 5K for a marathon prediction.