Half Marathon to Marathon Time Calculator
Enter your recent half marathon finish time to estimate your marathon finish, pacing, and key checkpoints.
How to Estimate a Marathon from a Half Marathon
If you have a recent half marathon result, you already have one of the best data points for predicting marathon performance. A half marathon captures your current fitness, endurance, and pacing discipline better than short races.
This calculator helps convert your half marathon time (21.1 km / 13.1 mi) into a marathon estimate (42.2 km / 26.2 mi). It gives you a realistic target that can guide training, race strategy, and pacing plans.
What Formula Is Used?
1) Riegel Formula (Recommended)
The Riegel model is a common endurance prediction equation:
T2 = T1 × (D2 / D1)1.06
For half marathon to marathon, distance roughly doubles, so the multiplier is about 2.087, not exactly 2. This reflects natural fatigue over longer distances.
2) Simple Double
A quick method: just double your half marathon time. This can work for highly trained runners with strong endurance and good fueling, but it is often too optimistic for first-time marathoners.
3) Double + 10 Minutes
A conservative rule used by many coaches for newer marathoners or tougher courses. It adds a margin for late-race fatigue.
Example Conversion
If your half marathon is 1:45:00:
- Simple Double: 3:30:00
- Riegel Estimate: about 3:39:08
- Double + 10: 3:40:00
This range is useful. Your true marathon potential depends on your long-run consistency, fueling, weather, elevation, and pacing execution.
Quick Reference Table
| Half Marathon Time | Estimated Marathon (Riegel) | Approx Pace per Mile |
|---|---|---|
| 1:30:00 | 3:07:50 | 7:10 /mi |
| 1:40:00 | 3:28:40 | 7:58 /mi |
| 1:50:00 | 3:49:29 | 8:45 /mi |
| 2:00:00 | 4:10:19 | 9:33 /mi |
How Accurate Are Marathon Predictions?
Predictions are estimates, not guarantees. Two runners with the same half marathon time can finish very differently in a marathon because of training specifics.
Big factors that change your result
- Long runs: Weekly long-run progression improves durability.
- Fueling practice: Carbs during long runs reduce late-race slowdown.
- Pacing discipline: Going out too fast can add 10+ minutes.
- Heat and humidity: Warm weather often slows marathon pace.
- Course profile: Hills and turns can significantly affect time.
How to Turn Your Prediction into a Real PR
Build endurance first
Keep your easy mileage consistent. Add long runs gradually and include marathon-pace segments in key workouts.
Practice race fueling
Train your gut. Use the same gels and hydration strategy you plan for race day.
Use smart pacing
Start controlled. Run the first 10K slightly easier than goal pace, settle in through 30K, then race the final 12K if you still feel strong.
Bottom Line
A half marathon is an excellent predictor for marathon potential. Use the calculator above to set a realistic target, then let your training consistency and race-day execution bring that number to life.