How to use the Race Equivalent Calculator
Start with a completed race or hard effort. Enter Known distance and Known race time, choose Target distance, and select Calculate. The result shows a predicted time, pace, speed, and the values used in the formula.
- Choose a preset in Known distance, or select Custom distance... and enter a positive number.
- Enter Known race time as hours:minutes:seconds or minutes:seconds, such as
00:25:00or25:00. - Choose a preset in Target distance, or select Custom distance... and enter a positive number.
- For a custom distance, choose km or mi in Known distance unit or Target distance unit.
- Open Advanced: fatigue exponent to change the value, then select Calculate.
- After the result appears, select Copy result or Download result (.txt). Reset clears the numeric inputs, restores 5K and 10K, hides the result, and clears the exponent.
How the Riegel formula uses your inputs
The calculator applies Pete Riegel's formula, T2 = T1 × (D2 ÷ D1) ^ k. T1 and D1 are the known time and distance. D2 is the target distance, and k is the fatigue exponent. Leave Fatigue exponent (k) - default 1.06 blank to use 1.06.
Known distance: 5K
Known race time: 25:00
Target distance: 10K
Fatigue exponent: 1.06
Predicted target time: 0:52:07The input accepts values between 0.5 and 2.0. The calculator shows a warning below 1.00 or above 1.15, then continues the calculation.
Custom distances and valid time formats
Known distance and Target distance include 5K, 10K, 15K, 10 Mile, 20K, Half Marathon (21.0975 km), 30K, Marathon (42.195 km), 50K, 50 Mile, 100K, and 100 Mile. Choose Custom distance... for another positive distance. The two custom fields can use different units, and the calculator converts both distances to kilometers before applying the formula.
Known race time must be greater than zero. Seconds must stay below 60. If you enter hours, the minute part must also stay below 60. A valid result shows the predicted time, pace, and speed, followed by each value used in the calculation, including the distance ratio and the ratio raised to the exponent.
What the result includes
The result shows the predicted target time, pace, and speed. It also lists the known and target distances, the known time, both paces and speeds, the distance ratio, the fatigue exponent, and the ratio raised to the exponent.
When a custom Target distance uses mi, the displayed distance, pace, and speed use mi. Copy result puts the summary on the clipboard. Download result (.txt) saves the same details as a text file.
Choose another sports calculator
Use Sports Calculators for another sports calculation. Choose Race Finish Time Calculator when you have a target pace or speed and need a finish-time estimate. Choose Cycling Calculator for cycling speed, distance, time, pace, route splits, or energy use. Choose Heart Rate Zone Calculator for a training zone based on maximum heart rate, heart-rate reserve, or lactate threshold.
The estimate uses one known performance and one selected exponent. The calculation runs in the browser. It does not submit the inputs or fetch external race results, and it does not use official race records, course certifications, third-party results, scoring tables, age-grading rules, heat, elevation, or course difficulty. The output is a mathematical estimate, not an official time or coaching plan.
Frequently asked questions
How do I use a race equivalent calculator?
Choose Known distance, enter Known race time, choose Target distance, and select Calculate. The result includes the predicted time, pace, speed, and calculation details.
Can I predict a 10K time from a 5K time?
Choose 5K in Known distance and 10K in Target distance, then enter the completed 5K time. The calculator applies the selected fatigue exponent to estimate the 10K result.
What does the Riegel fatigue exponent change?
The fatigue exponent controls the distance-based adjustment in the formula. Leave it blank for 1.06, or enter a value between 0.5 and 2.0. Values outside 1.00-1.15 show a warning while the calculation continues.
Can I enter miles and a custom race distance?
Select Custom distance... and choose mi in Known distance unit or Target distance unit. The calculator converts the entered distances to kilometers for the calculation and formats the result using the target distance unit.