Running Race Predictor Calculator
Predict your time at any race distance from a recent result using Riegel's formula, with the pace you'd need and a full table of equivalent times.
Running Race Predictor Calculator: with the default inputs, predicted time is 3:28:30.
A half marathon is 21.0975 km, a marathon 42.195 km, a 10K exactly 10 km.
- Predicted time in minutes
- 208.49
- Predicted pace per km
- 4:56 /km
- Predicted pace per mile
- 7:57 /mi
- Average speed
- 12.14 km/h
- Pace of the race you ran, per km
- 4:44 /km
- Distance ratio
- 2Target ÷ known. Riegel is reliable up to about 2× and degrades sharply beyond it.
Assumptions
- A single power law with Riegel's exponent of 1.06 unless you change it.
- Assumes equivalent training for both distances, similar conditions and accurate courses.
- Fitted on races of roughly 3.5 minutes to 3.5 hours; predictions outside that range, and beyond about a doubling of distance, degrade quickly.
- Takes no account of terrain, altitude, heat, fuelling or long-run volume.
| Race | km | Predicted time | Pace /km | Pace /mi |
|---|---|---|---|---|
| 1500 m | 1.5 | 6:04 | 4:03 | 6:31 |
| 1 mile | 1.609 | 6:32 | 4:04 | 6:32 |
| 3 km | 3 | 12:39 | 4:13 | 6:47 |
| 5K | 5 | 21:44 | 4:21 | 7:00 |
| 8K | 8 | 35:47 | 4:28 | 7:12 |
| 10K | 10 | 45:19 | 4:32 | 7:18 |
| 15K | 15 | 1:09:39 | 4:39 | 7:28 |
| 10 miles | 16.093 | 1:15:03 | 4:40 | 7:30 |
| Half marathon | 21.098 | 1:40:00 | 4:44 | 7:38 |
| Marathon | 42.195 | 3:28:30 | 4:56 | 7:57 |
| 50K | 50 | 4:09:35 | 5:00 | 8:02 |
All derived from the same reference race. Trust the rows closest to the distance you actually ran.
How this is worked out
The formula
Riegel (1981): T₂ = T₁ × (D₂ ÷ D₁)^1.06 T₁ = time for the race you have run D₁ = its distance D₂ = the distance you want predicted 1.06 = the fatigue exponent — doubling the distance takes 2^1.06 ≈ 2.085× as long, not twice as long
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Distance you've raced
- A half marathon is 21.0975 km, a marathon 42.195 km, a 10K exactly 10 km.in km, mi, m · from 0.05 to 500 · defaults to 21.0975
- Time — hours
- A number.from 0 to 99 · whole numbers only · defaults to 1
- Minutes
- A number.from 0 to 59 · whole numbers only · defaults to 40
- Seconds
- A number.from 0 to 59 · whole numbers only · defaults to 0
- Distance you want to predict
- A number.in km, mi, m · from 0.05 to 500 · defaults to 42.195
- Fatigue exponent(under More options)
- Riegel's 1.06 is the published value. Below 1.06 assumes you hold pace unusually well; above it assumes you fade.from 1 to 1.2 · defaults to 1.06
What you get back
- Predicted timemain answer
- Predicted time in minutes
- Predicted pace per km
- Predicted pace per mile
- Average speed
- Pace of the race you ran, per km
- Distance ratio
- Target ÷ known. Riegel is reliable up to about 2× and degrades sharply beyond it.
What this assumes
- A single power law with Riegel's exponent of 1.06 unless you change it.
- Assumes equivalent training for both distances, similar conditions and accurate courses.
- Fitted on races of roughly 3.5 minutes to 3.5 hours; predictions outside that range, and beyond about a doubling of distance, degrade quickly.
- Takes no account of terrain, altitude, heat, fuelling or long-run volume.
About this calculator
Pete Riegel, an engineer and course measurer, fitted a single power law to world-record and club race times in 1981 and published it in Runner's World: time scales with distance raised to the power 1.06. That small exponent above 1 is the whole idea. If running were purely a matter of holding pace, doubling the distance would double the time. It does not — it takes about 8.5% longer than that, because pace has to drop as distance grows.
Feed it one honest race result and it will predict every other distance.
How to use it
Enter a race you have actually run — recently, rested, on an accurate course — along with its distance and your time, then the distance you want predicted. The table gives every standard distance at once, which is usually more useful than a single answer.
The fatigue exponent under More options defaults to Riegel's 1.06. Runners with a strong endurance base and high mileage often behave like 1.04–1.05; speed-oriented runners with low mileage fade closer to 1.07–1.08. If you have two real results at different distances, you can solve for your own exponent: b = ln(T₂/T₁) ÷ ln(D₂/D₁).
Where it works, and where it stops working
Riegel fitted the exponent on races roughly between 3.5 minutes and 3.5 hours, and it is at its best when the two distances are within a factor of two of each other. Predicting a 10K from a 5K is reliable. Predicting a half marathon from a 10K is reasonable. Predicting a marathon from a 5K is not — the formula assumes your endurance scales with your speed, and for most runners it does not.
The marathon is where the model fails hardest, and always in the optimistic direction. Beyond roughly two hours of running, performance is limited by things the formula cannot see: glycogen depletion, muscle damage from repeated eccentric loading, core temperature, gut tolerance of fuel, and whether you have ever run 30 km before. A runner who has raced only 10Ks will be handed a marathon prediction they cannot hold past halfway. Treat a Riegel marathon time as the ceiling of what your current speed permits, achievable only with the long-run volume to back it up.
Reading the results
The distance ratio is the honesty check: at or below 2 the prediction is worth planning around; at 4 or more it is entertainment. The pace columns are the practical output — knowing you need 4:45/km for 42.195 km tells you immediately whether the goal is realistic, because you can go and run a few kilometres at that pace and see how it feels.
Other equivalency models exist and behave differently at the extremes: Daniels' VDOT tables come from measured oxygen cost and are gentler on long distances, and Cameron's formula is tuned for the marathon end. When Riegel and your gut disagree about a marathon, your gut is probably right.
Frequently asked questions
▸What is Riegel's formula?
T₂ = T₁ × (D₂ ÷ D₁)^1.06. Published by Pete Riegel in 1981, it says that doubling the race distance takes about 2.085 times as long, not twice as long.
▸Can I predict my marathon time from a half marathon?
Yes, and this is the ratio the formula handles best. A 1:40 half predicts roughly a 3:28:30 marathon — but only if you have done the long-run training. Without it, add 10–20 minutes.
▸How accurate is a race time predictor?
Within a couple of percent when the two distances are close and you are trained for both. Errors grow quickly past a doubling of distance, and the formula is always optimistic about the marathon.
▸Why is 1.06 the exponent?
It is the empirical best fit Riegel found across world records and club results. It encodes the observation that pace falls by a small, remarkably consistent amount each time distance doubles.
▸What if my own fade is different?
Solve for it from two of your own races: b = ln(T₂/T₁) ÷ ln(D₂/D₁). High-mileage endurance runners often land near 1.04; runners with a strong 5K and low mileage nearer 1.08.
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