Race Time Predictor
Enter a recent race result and predict your finish time at any distance.
How it works
This calculator uses the Riegel formula, the most widely-used race time prediction model in running. It estimates your performance at longer distances by applying an exponential fatigue factor to your known race result. The formula accounts for the fact that pace naturally slows as distance increases, and you can't hold your 5K pace for a marathon.
For the most accurate predictions, use a recent race result (within the last 2-3 months) from a race where you gave a full effort. Training runs and time trials tend to underpredict.
When the Riegel formula breaks down
The formula assumes proportional fitness across distances, which isn't always true. A speed-focused 5K runner will get an optimistic marathon prediction because marathon performance depends heavily on endurance training that doesn't show up in shorter races. Similarly, a high-mileage marathoner may run a faster marathon than their 5K would predict.
The prediction is most accurate when your input distance is close to your target distance. A half marathon predicts a marathon better than a 5K does. If you only have a 5K, treat the marathon prediction as a best-case ceiling, not a target.
Half marathon to marathon conversion
The most common use of this calculator: doubling a half marathon time and adding 10-15 minutes. The Riegel formula does something similar but more precisely—it applies a fatigue exponent of 1.06, which means your marathon is roughly 2.09x your half marathon time. A 1:30 half predicts a 3:08 marathon. A 1:45 half predicts a 3:40.
The catch: this only works if your training supports the full distance. Running a 1:30 half on 30 miles per week doesn't guarantee a 3:08 marathon. The half shows your fitness; the training determines whether you can hold it for twice the distance.
Want to convert your predicted finish time into a target pace? The Running Pace Calculator handles the math — enter your distance and time to get your pace per mile and per km.
Once you know your predicted finish time, use it to figure out your fueling plan. Read How Many Gels Do You Need for a Marathon? for the exact math by gel brand and finish time.
Frequently Asked Questions
How do I predict my marathon time from a half marathon?
The Riegel formula multiplies your half marathon time by approximately 2.09 to predict your marathon finish. A 1:30 half predicts a 3:08 marathon. A 1:45 half predicts a 3:40. This is the most accurate single-race predictor, but it assumes your training supports the full distance—running a fast half on low mileage does not guarantee the predicted marathon time.
What is the Riegel formula for race time prediction?
The Riegel formula (T2 = T1 × (D2/D1)^1.06) predicts race times by applying an exponential fatigue factor to a known result. The 1.06 exponent accounts for the fact that pace naturally slows as distance increases. It is the most widely used prediction model in running and is most accurate when the input race distance is close to the target distance.
Is a 5K time a good predictor of marathon performance?
A 5K gives a rough ceiling but is less accurate than a half marathon for predicting marathon times. The Riegel formula assumes proportional fitness across distances, which breaks down when distances are far apart. A speed-focused 5K runner will get an optimistic marathon prediction because marathon performance depends heavily on endurance training that does not show up in shorter races. Use a half marathon result when possible.
How does predicted race time affect fueling strategy?
Your predicted finish time determines how many hours you need to fuel and at what intensity. A faster marathon (under 3:00) requires fewer total gels but higher carb rates (90-120g/hr) because the effort is more intense. A slower marathon (4:00+) means more hours of fueling at a lower rate (60-80g/hr) but more total gels overall. Use your predicted time to build a race-specific gel plan.