Key Takeaways
- The Riegel formula predicts race times with 95%+ accuracy for trained runners
- Best predictions come from using a recent race within 6 weeks
- Marathon pace is typically 10-15% slower than 10K pace per mile
- A 50-minute 10K predicts approximately a 3:51 marathon
- Use the closest race distance to your target for most accurate results
What Is a Marathon Time Predictor?
A marathon time predictor is a tool that estimates your finish time for a marathon (26.2 miles / 42.195 km) based on your performance at a shorter distance. This calculator uses the Riegel formula, developed by researcher Peter Riegel in 1977, which remains the gold standard for race time prediction.
The predictor works on the principle that running pace naturally slows as race distance increases due to fatigue, glycogen depletion, and the cumulative stress on muscles. The Riegel formula captures this relationship mathematically, allowing accurate predictions across distances.
The Riegel Formula Explained
T2 = T1 x (D2 / D1)1.06
The 1.06 exponent is the key insight - it represents how pace degrades as distance increases. A value of 1.0 would mean pace stays constant (unrealistic), while higher values indicate faster degradation. Research has shown 1.06 works well for trained recreational runners, though elite runners may use 1.04-1.05 and newer runners might need 1.07-1.08.
Example Calculation
If you ran a 10K in 50 minutes, here's how to predict your marathon time:
- T1 = 50 minutes (3,000 seconds)
- D1 = 10 km
- D2 = 42.195 km (marathon)
- T2 = 3000 x (42.195 / 10)^1.06
- T2 = 3000 x 4.2195^1.06
- T2 = 3000 x 4.603
- T2 = 13,809 seconds = 3:50:09
How to Use This Marathon Predictor
- Select your known race distance - Choose from 5K, 10K, 15K, half marathon, or enter a custom distance
- Enter your finish time - Use hours:minutes:seconds format
- Choose target distance - The race you want to predict
- Click "Predict Time" - View your predicted times for all major distances
Pro Tip: Use Your Most Recent Race
For the most accurate prediction, use a race time from the past 4-6 weeks. Your fitness level changes over time, so older race results may not reflect your current ability. Time trial results work too, but races tend to push you harder and produce better benchmark times.
How Accurate Is the Riegel Formula?
Studies have shown the Riegel formula predicts race times within 3-5% for most trained runners. However, accuracy depends on several factors:
- Training consistency - Runners with steady training see better predictions
- Race conditions - Heat, humidity, hills, and wind affect results
- Experience level - First-time marathoners often finish slower than predicted
- Distance gap - Predicting 10K to marathon is more accurate than 5K to marathon
Race Pace Conversion Table
| 10K Time | Half Marathon | Marathon | Pace/Mile |
|---|---|---|---|
| 35:00 | 1:17:30 | 2:42:35 | 6:12 |
| 40:00 | 1:28:36 | 3:05:26 | 7:05 |
| 45:00 | 1:39:41 | 3:28:18 | 7:58 |
| 50:00 | 1:50:46 | 3:51:09 | 8:50 |
| 55:00 | 2:01:51 | 4:14:00 | 9:42 |
| 60:00 | 2:12:56 | 4:36:52 | 10:33 |
Training Tips Based on Your Predicted Time
Sub-3 Hour Marathon (Elite)
Requires a 10K time under ~38 minutes. Training should include tempo runs at 6:00-6:30/mile pace, weekly long runs of 18-22 miles, and weekly mileage of 60-80+ miles.
3:00-3:30 Marathon (Advanced)
Requires a 10K time of 38-45 minutes. Focus on threshold runs at 7:00-7:30/mile, long runs of 16-20 miles, and 45-60 miles per week.
3:30-4:00 Marathon (Intermediate)
Requires a 10K time of 45-52 minutes. Build endurance with 35-50 miles per week, include tempo runs, and practice race-pace running.
4:00+ Marathon (Beginner)
Focus on completing the distance safely. Build to 30-40 miles per week, emphasize time on feet over pace, and include walk breaks if needed.
Frequently Asked Questions
Several factors can cause this: insufficient long run training, going out too fast, inadequate fueling during the race, hot weather conditions, or being a first-time marathoner. The Riegel formula assumes optimal conditions and proper preparation. Most runners improve on subsequent marathons as they learn pacing and nutrition strategies.
Yes, but with caveats. Treadmill running is often easier due to consistent pace, no wind resistance, and controlled temperature. Set the incline to 1-2% to simulate outdoor conditions. Race times are usually 1-3% faster than equivalent treadmill efforts, so adjust your prediction accordingly.
The half marathon provides the most accurate prediction because it's closest in distance and energy system demands. A recent half marathon time typically predicts your marathon within 2-3%. 10K times are also excellent predictors and more accessible for regular racing.
The Riegel formula works for all ages, but older runners (55+) may see slightly slower marathon times than predicted due to increased muscle fatigue over longer distances. Masters runners often perform well in the half marathon relative to the marathon compared to younger runners.
Add approximately 30 seconds per mile for each 100 feet of elevation gain per mile on the course. For example, the Boston Marathon has significant hills and typically produces times 3-7 minutes slower than flat courses for the same fitness level. Include hill training in your preparation.
The Riegel formula becomes less accurate beyond marathon distance because fatigue factors change dramatically. For 50K-100K ultras, consider using a higher exponent (1.08-1.12) or specialized ultramarathon predictors. Terrain, elevation, and aid station strategy become major factors in ultras.