What Is a Marathon Finish Time Calculator?
This calculator predicts your finish time at a new race distance using Peter Riegel's race-time prediction formula — a well-established method from exercise science that's been used by coaches and running calculators for decades. Enter a recent race result and choose the distance you want to predict, from 5K up to marathon.
It works for any endurance running distance, not just the marathon: use it to predict a 10K time from a recent 5K, or a marathon time from a half marathon you just ran.
How to Read Your Results
Predicted Finish Time
The estimated time to complete your target distance, assuming similar fitness, conditions and effort level to your known race.
Known Pace & Predicted Pace (per Mile)
Your average pace for the known race, and the predicted average pace for the target distance. Notice the predicted pace is always a bit slower per mile for longer target distances — that's the formula correctly modeling endurance fade.
Known Time (for Reference)
Simply echoes back the time you entered, useful for double-checking your inputs.
Real-World Example
A runner recently finished a 10-mile race in 1:15:00 and wants to predict their marathon time.
| Value | Result |
|---|---|
| Known distance / time | 10 miles / 1:15:00 |
| Known pace | 7:30 /mile |
| Target distance | 26.2188 miles (marathon) |
| Predicted finish time | ≈3:28:21 |
| Predicted pace | ≈7:57 /mile |
The formula predicts roughly a 27-second-per-mile slowdown for the marathon compared to the 10-mile pace — consistent with how endurance typically degrades over a longer distance.
Tips for Using This Calculator
- Use your most recent, most relevant race result for the best prediction — a 5K time from six months ago is less reliable than a 10K from last weekend.
- Predictions assume proper training for the target distance — Riegel's formula doesn't know if you've actually built up your long runs for a marathon.
- Race-day conditions matter. Heat, hills, and course difficulty can push actual results away from the prediction.
- Predictions get less reliable the further apart the distances are — a 5K-to-marathon prediction has more uncertainty than a 10K-to-half-marathon one.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team