A race time predictor uses your recent race performance to estimate how fast you can run other distances. The most widely used model — the Riegel formula — accounts for the physiological reality that pace slows as distance increases, giving runners a realistic goal for their next race without guesswork.
Your Recent Race Result
How to Use the Race Time Predictor
The race time predictor takes one known race result and uses proven mathematical models to estimate your finish times for other distances. Whether you have a recent 5K and want to know your marathon potential, or you finished a 10K and want a realistic half marathon goal, this tool gives you a data-backed answer in seconds.
Step 1: Choose Your Unit System
Toggle between miles and kilometers at the top of the tool. Switching units instantly updates all distance presets and pace outputs to your preferred system.
Step 2: Enter Your Known Race Distance
Click a preset button — 5K, 10K, Half Marathon, or Marathon — or type any custom distance. Custom distances are useful if your recent race was a 4-mile road race, a 15K, or a trail event at an unusual distance. The tool works with any distance as long as you know your finish time.
Step 3: Enter Your Finish Time
Enter your finish time in hours, minutes, and seconds. Use a recent race where you ran your full effort — not a fun run, a training run, or a race where you were injured or held back. The more accurately the input reflects your current fitness, the more accurate the predictions will be.
Step 4: Read the Prediction Table
The results table shows predictions from three models for each standard race distance:
- Riegel — the industry standard (T2 = T1 × (D2/D1) ^ 1.06), developed by Pete Riegel in 1977 and still widely used by coaches and athletes.
- Cameron — a refined model that adjusts the exponent based on distance, often giving slightly faster marathon predictions than Riegel for trained runners.
- VO2max — uses Jack Daniels' VDOT methodology to estimate your aerobic capacity and derive equivalent performances at each distance.
The average of all three models is shown as the recommended pace in the quick-view cards below the table.
Understanding Training Pace Zones
The training pace zones section shows recommended paces for different types of training runs, derived from your Riegel-predicted marathon time. Easy runs build your aerobic base and account for the majority of weekly mileage. Tempo runs improve your lactate threshold. Interval pace develops speed and VO2max. Using these zones ensures you train smart rather than running every workout too hard.
How Accurate Are the Predictions?
For recreational runners with balanced training, the race time predictor is typically accurate within 2–5% when predicting between similar distances. Accuracy is best when predicting the next distance up (e.g., 10K from 5K, half from 10K). Predicting a marathon from a 5K time spans a very large range and should be treated as a rough benchmark, not a firm goal. Individual factors like heat, course profile, fueling, and race-day execution all affect actual performance.
Frequently Asked Questions
Is this race time predictor free?
Yes, the race time predictor is completely free with no limits or signup required. All calculations run locally in your browser using client-side JavaScript, so no personal data is ever sent to a server.
Is my data safe and private?
Yes, all calculations happen entirely in your browser. Your race times and personal details are never transmitted to any server or stored anywhere. Nothing is saved after you leave the page.
What is the Riegel formula and how does it work?
The Riegel formula is T2 = T1 × (D2 / D1) ^ 1.06, where T1 is your known finish time, D1 is the distance you raced, and D2 is the target race distance. The exponent 1.06 captures the physiological reality that pace slows as distance increases. It was developed by Pete Riegel and published in 1977, and remains the most widely used race prediction model.
What is the Cameron formula?
The Cameron formula is a more complex prediction model that adjusts the fatigue exponent based on the distances involved. It tends to give slightly faster predictions for longer races compared to the Riegel formula, and is generally considered more accurate for predicting marathon times from shorter race efforts.
How accurate are these race time predictions?
Predictions are most accurate when extrapolating between similar distances (e.g., predicting a 10K from a 5K time), typically within 2–5% for runners with balanced training. Accuracy decreases when predicting across very different distances (e.g., marathon from a 1-mile time) or when training is highly specialized. The predictions assume equivalent fitness and effort levels for both races.
Which prediction formula should I use?
The Riegel formula is the standard and works well for most runners across all distances. The Cameron formula is often preferred for longer race predictions (half marathon and marathon). The VO2max-based method provides a physiology-grounded estimate. Comparing all three and using the average is a good strategy for realistic goal-setting.
Can I use a custom race distance as my input?
Yes, the tool accepts any race distance in miles or kilometers. In addition to the standard presets (5K, 10K, Half Marathon, Marathon), you can type in any distance such as a 4-mile race, a 15K, or a 25K trail run to generate predictions for all four standard race distances.
Why does predicted marathon pace get slower than my shorter race pace?
This is a fundamental physiological reality captured by the Riegel formula. As race distance increases, your body cannot sustain the same intensity, so average pace must decrease. The formula's exponent of 1.06 quantifies this slowdown. A runner who can run 5K at 6:00/mile typically cannot sustain that pace for 26.2 miles — a realistic marathon pace might be 7:00–7:30/mile for the same runner.