How to Use the Race Equivalent Calculator
Use a completed race or hard effort as the known performance, then choose the distance for the predicted equivalent time. The initial selections are 5K for the known distance and 10K for the target distance.
- For each custom distance, choose km or mi beside its input. Known and target distances can use different units.
- For the known performance, choose a preset or select Custom distance... and enter a positive value. Enter Known race time as hours:minutes:seconds or minutes:seconds, such as 00:25:00 or 25:00.
- Choose the target distance. Use another preset or Custom distance... with a positive value.
- Optional: open Advanced: fatigue exponent. Leave the field blank to use
1.06, or enter a value from0.5through2.0. - Select Calculate. A result appears only after validation succeeds; otherwise, an error replaces the result.
- Use Copy result or Download result (.txt) when a result is available.
- Select Reset to clear the entered numeric values, restore 5K and 10K, hide the result, and clear the exponent.
Formula and Fatigue Exponent
The calculator uses Pete Riegel's formula: T2 = T1 × (D2 ÷ D1) ^ k. T1 and D1 are the known time and distance, D2 is the target distance, T2 is the predicted time, and k is the fatigue exponent. The default is 1.06.
You can change k in Advanced: fatigue exponent. Values from 0.5 through 2.0 are accepted. Values below 1.00 or above 1.15 show a warning, but they do not stop the calculation.
Distances, Units, and Custom Values
Preset choices include 5K, 10K, 15K, 10 Mile, 20K, Half Marathon (21.0975 km), 30K, Marathon (42.195 km), 50K, 50 Mile, 100K, and 100 Mile. Presets are converted internally to kilometers. Custom fields accept positive numeric values.
Each custom distance has its own km/mi dropdown. The dropdown defines the unit of the entered value; both distances are converted to kilometers for calculation. Results use the target distance unit.
Validation and Reading the Result
Known and target distances must be positive. Enter a positive duration as hours:minutes:seconds or minutes:seconds. Seconds must be below 60; the minute component must also be below 60 when hours are provided. If validation succeeds, the result shows predicted target time, pace, and speed. The table also lists both distances in the target unit and kilometers, known time, pace, speed, distance ratio, exponent, the ratio raised to the exponent, and predicted values.
Limitations and Data Sources
- The estimate uses one known performance and a generic exponent.
- It assumes similar training, terrain, weather, course difficulty, and effort for both distances.
- Predictions across very different distances, such as 5K to marathon, are less reliable than predictions between similar distances.
- It does not model heat, elevation, course difficulty, target-distance training, official race records, course certifications, third-party results, scoring tables, or age-grading rules.
- The output is a mathematical estimate, not an official time, ranking, coaching plan, or medical advice.
Copy or Download the Result
Copy result copies the generated plain-text summary. Download result (.txt) downloads the same summary, including the formula, intermediate values, predicted values, and assumptions.
Privacy and Calculation Behavior
The calculator performs its arithmetic in the browser using the values entered into the tool. Its calculator script does not fetch external race records or results and does not submit tool inputs for the calculation. The page also contains separate analytics and subscription elements, so this statement applies to the calculator workflow only.
Frequently Asked Questions
What formula does this calculator use?
It uses Pete Riegel's exponential fatigue formula: T2 = T1 × (D2 ÷ D1) ^ k. T1 and D1 are the known time and distance, D2 is the target distance, and k defaults to 1.06. It is a mathematical model, not an official scoring standard.
How accurate is the prediction?
It is an estimate based on one known performance and a generic exponent. Accuracy depends on your preparation for the target distance, comparable terrain and conditions, and the distance gap. A 5K time used to predict a marathon is less reliable than a prediction between similar distances.
What is the fatigue exponent and can I change it?
The fatigue exponent k controls the distance-based adjustment in the formula. You can change it in Advanced: fatigue exponent. Values below roughly 1.0 predict pace holds steady or improves as distance increases, while values above roughly 1.15 predict a steeper slowdown. The calculator warns outside 1.00-1.15 but still calculates.
Does this calculator use official race records or results?
No. It performs a deterministic calculation on the distance and time you enter. It does not fetch official records, course certifications, or any third-party results, and it does not apply any specific race organization's scoring tables or age-grading rules.