Quick answer
The Riegel formula predicts T₂ = T₁ × (D₂ ÷ D₁)^1.06. A 25:00 5 km run predicts a 10 km time of 52:07, a pace of 5:13 per km (8:23 per mile).
Enter a known race distance and time and a target distance. The calculator returns the predicted time and pace. Use this race time prediction calculator to check your own figures.
Related tools: Marathon Pace Calculator: Pace needed to finish a race distance in a target time. Running Training Pace Calculator: Estimate VO₂max (VDOT) from a race result and get easy, threshold and interval training paces. Pace Distance Calculator: Distance covered at a steady pace for a given time.
How to use the Race Time Prediction Calculator
- Enter known race distance (above 0).
- Choose unit: Kilometers and Miles.
- Optionally, enter hours (0 or more).
- Optionally, enter minutes (0 or more).
- Optionally, enter seconds (0 or more).
- Enter target distance (above 0).
- Choose unit: Kilometers and Miles.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Known race distance | A number | > 0 |
| Unit | Choose one: Kilometers; Miles | Starts at Kilometers |
| Hours (optional) | A number | ≥ 0 |
| Minutes (optional) | A number | ≥ 0 |
| Seconds (optional) | A number | ≥ 0 |
| Target distance | A number | > 0 |
| Unit | Choose one: Kilometers; Miles | Starts at Kilometers |
Formula and method
Formula
T₂ = T₁ × (D₂ ÷ D₁)^1.06
IEEE-754 double precision; shown to the decimals indicated.
Riegel's endurance model; it is least reliable for distances far from the known race and for very short or very long events. Informational estimate, not medical advice. Talk to a qualified health professional about your own situation.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Known race distance = 5; Unit = Kilometers; Hours = 0; Minutes = 25; Seconds = 0; Target distance = 10; Unit = Kilometers.
| Result | Value |
|---|---|
| Predicted time | 52:07 |
| Predicted pace per km | 5:13 |
| Predicted pace per mile | 8:23 |
Example 2
Inputs: Known race distance = 5; Unit = Kilometers; Hours = 0; Minutes = 20; Seconds = 0; Target distance = 21.0975; Unit = Kilometers.
| Result | Value |
|---|---|
| Predicted time | 1:32:00 |
| Predicted pace per km | 4:22 |
| Predicted pace per mile | 7:01 |
Example 3
Inputs: Known race distance = 10; Unit = Miles; Hours = 1; Minutes = 20; Seconds = 0; Target distance = 26.2; Unit = Miles.
| Result | Value |
|---|---|
| Predicted time | 3:42:04 |
| Predicted pace per km | 5:16 |
| Predicted pace per mile | 8:29 |
The first line is the headline result (predicted time). Below it the race Time Prediction Calculator lists predicted pace per km and predicted pace per mile, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
The exponent
Doubling the distance multiplies time by 2^1.06 = 2.085, a bit more than double because fatigue slows pace. For 25 minutes that is 52.1 minutes. Marathon predictions from short races tend to be optimistic, since the exponent underestimates the fade.
Assumptions
Predictions assume similar training for the target distance. A 5 km runner who only runs short distances will not match a 10 km prediction. Use a recent, hard effort as the input.
How the result changes with known race distance
Holding every other input at the first worked example, this table changes only known race distance. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Known race distance | Predicted time | Predicted pace per km | Predicted pace per mile |
|---|---|---|---|
| 2.5 | 1:48:40 | 10:52 | 17:29 |
| 5 | 52:07 | 5:13 | 8:23 |
| 10 | 25:00 | 2:30 | 4:01 |
| 20 | 11:59 | 1:12 | 1:56 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Known race distance: entering "0" shows "Known race distance must be greater than 0." beside that field and no result.
- Minutes: entering "0" shows "Enter a time greater than zero." beside that field and no result.
Choosing between the Race Time Prediction Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Marathon Pace Calculator](/fitness/marathon-pace-calculator/) | race | Pace per km |
| [Running Training Pace Calculator](/fitness/training-pace-calculator/) | race distance, hours and minutes | VDOT (estimated VO₂max) |
| [Pace Distance Calculator](/fitness/pace-distance-calculator/) | pace minutes and pace per | Distance |
| [Pace Calculator](/fitness/pace-calculator/) | distance and distance unit | Pace per km |
| [Calories per Mile Calculator](/fitness/calories-per-mile-calculator/) | body weight and pace | Calories per mile |
Background: fitness estimates
Fitness formulas are estimates whose accuracy depends on the input. One-rep-max formulas such as Epley and Brzycki give different answers, and predictions from hard sets of a few repetitions are better than from sets of ten or more. Age-based maximum heart rate formulas can be off by 10 to 30 beats a minute for an individual; Tanaka's 208 − 0.7 × age is generally preferred to 220 − age but is still an estimate.
Pace models built from a race result assume a maximal effort and work best for distances from about 5 km to the marathon. Use the output as a guide, not a prescription.
Assumptions and limitations
- Predictions assume similar training for the target distance
- Not a diagnosis
Estimates from formulas describe averages. They are not a training or medical prescription; check with a qualified professional before changing your exercise or diet.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for known race distance and minutes to confirm this.
Frequently asked questions
How accurate is Riegel?
Reasonable between 5 km and the half marathon.
What if I train for the longer distance?
The prediction is more accurate.
Can I predict from a mile?
Less reliably: a mile is mostly aerobic speed work, so predictions for longer races tend to be optimistic.
What does the Race Time Prediction Calculator calculate?
Predict a finish time at another distance with Riegel's formula. It returns Predicted time, Predicted pace per km and Predicted pace per mile.
What formula does the Race Time Prediction Calculator use?
T₂ = T₁ × (D₂ ÷ D₁)^1.06. Riegel's endurance model; it is least reliable for distances far from the known race and for very short or very long events.
What do I need to enter in the race Time Prediction Calculator?
You enter Known race distance, Unit, Target distance and Unit. Optional: Hours, Minutes and Seconds.
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Predictions assume similar training for the target distance.
What are the limits of this calculator?
Predictions assume similar training for the target distance. Not a diagnosis.
What does the “Unit” option change?
It sets how the race Time Prediction is worked out. The choices are Kilometers and Miles.
What is an example of a race Time Prediction result?
With known race distance 5, unit Kilometers, hours 0, minutes 25 and seconds 0, the predicted time is 52:07.
Where does the method come from?
The method follows: Riegel PS, Athletic Records and Human Endurance, American Scientist 69 (1981) 285–290.
Can I rely on this calculator for a decision?
Estimates from formulas describe averages. They are not a training or medical prescription; check with a qualified professional before changing your exercise or diet. For this calculator in particular: Predictions assume similar training for the target distance.
Related calculators
- Marathon Pace Calculator — Pace needed to finish a race distance in a target time.
- Running Training Pace Calculator — Estimate VO₂max (VDOT) from a race result and get easy, threshold and interval training paces.
- Pace Distance Calculator — Distance covered at a steady pace for a given time.
- Pace Calculator — Pace and speed from a distance and a time.
- Calories per Mile Calculator — Energy cost of running a mile from body weight and pace (ACSM equation).
- Time Converter — Convert seconds, minutes, hours, days, weeks, months and years.
Sources
- Riegel PS, Athletic Records and Human Endurance, American Scientist 69 (1981) 285–290
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.