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Marathon Finish Time Calculator

Predict Your Marathon Finish Time

Use your recent race time to predict your marathon finish. Our calculator uses two scientifically-validated models — Riegel (optimistic) and Cameron (conservative) — to give you a realistic range. See your predicted pace per mile and per km, get your finisher percentile, check if you qualify for Boston, and get VDOT-based training zones to prepare for race day.

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Try an Example

Pick a scenario to see how the calculator works, then adjust the values

Sub-4 Marathon

Target a sub-4-hour marathon finish with even splits

Key values: 42.195 km · 5:41/km pace · 3:59:59

First Marathon

Comfortable pace for a first-time marathon finisher

Key values: 42.195 km · 6:30/km pace · ~4:34

10K Race Pace

Calculate pace for a target 10K finish time of 50 minutes

Key values: 10 km · 50:00 target · Pace-based

Documentation

This calculator is also known as Marathon Finish Time Calculator.

Read the complete guide

How Race Time Prediction Works

Race time predictions use mathematical models that account for the physiological cost of running longer distances. The two most respected models are: The Riegel formula (T2 = T1 x (D2/D1)^1.06) uses a universal fatigue factor of 1.06, meaning pace slows approximately 6% for each doubling of distance. It tends to be optimistic for recreational runners predicting marathon times from shorter races. The Cameron model uses a distance-specific correction function that penalizes large distance extrapolations more heavily. It's generally more conservative and realistic for amateur runners. Our calculator shows both predictions, giving you a range from optimistic to conservative. A good strategy: train for the Cameron prediction, dream about the Riegel one.

Prediction Accuracy by Source Distance

How the source race distance affects prediction reliability:

Category	Value
5K to Marathon	Least reliable. Large extrapolation. Predictions can be 5-10% optimistic because 5K fitness doesn't guarantee marathon endurance. Use as a rough guide only.
10K to Marathon	Moderately reliable. Still a significant extrapolation. Accuracy within 3-5% for well-trained runners with adequate long-run preparation.
Half Marathon to Marathon	Most reliable. Closest distance ratio. Accuracy within 2-3% for runners with marathon-specific training. The gold standard for marathon prediction.
Previous Marathon	Highly accurate if recent (within 3 months) and conditions were similar. Apply small adjustments for fitness changes, course difficulty, and weather.

Examples

Predicting Marathon Time from a Half Marathon PR

A runner with a recent half marathon time of 1:45:00 wanted to know what marathon time to target.

The Riegel model predicted 3:39:24 (8:22/mile pace), while the Cameron model predicted approximately 3:44:00 (8:33/mile pace). This 5-minute range gives the runner a realistic target window. Given the half-to-marathon extrapolation is the most reliable, the runner targeted 3:42:00 as a compromise. The calculator also showed this would place the runner around the 52nd percentile of male finishers and would be approximately 12 minutes away from the BQ standard for a 35-year-old male (3:05:00).

Key takeaway: Using both prediction models gives you an honest range. Target the conservative end for your race plan, and if you feel strong in the second half, you can always speed up toward the optimistic prediction.

Using Predictions Wisely

Get the most out of race time predictions:

Use a half marathon time from the last 3 months for the most accurate marathon prediction
Target the Cameron (conservative) prediction for your race plan — you can always run faster if feeling great
If predicting from a 5K, add 5-10% to account for the large distance extrapolation
Run at least one 20+ mile training run at your predicted marathon pace before committing to it as a goal
Consider that weather, course elevation, and race-day conditions can shift your result by 2-5%

Frequently Asked Questions about Marathon Finish Time Calculator

How accurate is predicting a marathon time from a 5K?

Predicting a marathon from a 5K has the widest margin of error — typically 5-10%. The main issue is that 5K fitness tests your VO2max and speed, while marathon performance depends heavily on glycogen storage, fat utilization, muscle durability, and mental endurance — none of which are measured by a 5K. A fast 5K runner who hasn't trained for the marathon distance will almost certainly run slower than predicted. For the most reliable prediction, use a half marathon time. If you only have a 5K, add 5-10% to the Riegel prediction for a more realistic target.

What is the difference between Riegel and Cameron predictions?

The Riegel formula uses a single fatigue factor (1.06) for all distances, making it simple but potentially optimistic for large extrapolations. The Cameron model uses a distance-specific correction function that more heavily penalizes the physiological challenges of longer distances. In practice: for predicting 10K from 5K, both models give similar results. For predicting a marathon from a 5K, Cameron is typically 3-8% slower (more conservative) than Riegel. Most coaches recommend using the Cameron prediction as your realistic target and treating the Riegel prediction as a best-case scenario.