Race Time Predictor
Predict race times from a recent result.
Reviewed by the Calculation Machine team · Updated 24 October 2026 · How we check our maths
Your values
ⓘ Live results
- Predicted marathon
- 3:33:55
- Predicted half marathon
- 1:42:36
- Predicted 10K
- 46:30
- Predicted 5K
- 22:18
- Predicted mile
- 6:42
- Current pace
- 4:39 per km
| Distance | Predicted time |
|---|---|
| 1 mile | 6:42 |
| 5K | 22:18 |
| 10K | 46:30 |
| Half marathon | 1:42:36 |
| Marathon | 3:33:55 |
Quick answer
A race time predictor estimates your time at a new distance from a recent race using Peter Riegel's formula: T₂ = T₁ × (D₂ ÷ D₁)^1.06. The 1.06 exponent reflects how pace slows as distance grows. A 46:30 10K predicts about 1:42:36 for a half marathon and 3:33:55 for a marathon, assuming you train properly for the longer race.
What the Race Time Predictor does
Enter the distance and finishing time of a recent all-out race and this calculator predicts your equivalent time for the mile, 5K, 10K, half marathon and marathon, plus your current pace per kilometer. Runners use it to choose a realistic goal for an upcoming race, set a starting pace, or pick the right corral.
Predictions are most reliable when the race you enter was run hard on a flat course in good weather, within the last six weeks or so, and when the target distance is not wildly longer — a half marathon predicts a marathon better than a 5K does. They assume you have done the training the longer distance needs; without long runs, most people run the marathon slower than predicted.
Once you have a target time, the running pace calculator converts it into the per-mile or per-km pace to run, and the marathon pace calculator gives splits for race day.
The formula
T₂ = T₁ × (D₂ ÷ D₁)^1.06
| Symbol | Meaning | Unit |
|---|---|---|
| T₁ | Time for the race you have run | h:mm:ss |
| D₁ | Distance of that race | km |
| D₂ | Distance you want a prediction for | km |
| T₂ | Predicted time at D₂ | h:mm:ss |
| 1.06 | Fatigue exponent — how much slower the average pace gets as distance doubles | — |
Peter Riegel, a research engineer and marathoner, published the endurance equation in American Scientist in 1981, fitting world records from about 3.5 minutes to 4 hours. Doubling the distance with a 1.06 exponent multiplies time by about 2.085, so pace is roughly 4% slower.
This page also covers searches for a McMillan running calculator or race pace predictor. McMillan-style calculators use their own tables built from runners' results and add training paces, so their figures differ slightly; Riegel is the simple, transparent formula many coaches and race organizers use.
Worked examples
From a 10K to a first marathon
A club runner ran 46:30 for 10K last month and wants a marathon goal.
Inputs
- Recent race distance10 km
- Hours0
- Minutes46
- Seconds30
Result
- Predicted marathon3:33:55
- Predicted half marathon1:42:36
- Predicted 10K46:30
- Predicted 5K22:18
- Predicted mile6:42
- Current pace4:39 per km
The prediction is 3:33:55, or about 5:04 per km (8:10 per mile). For a first marathon, starting 5–10 seconds per km slower than this is a safer plan.
5K parkrun to longer races
A newer runner finishes a 5K in 25:00.
Inputs
- Recent race distance5 km
- Hours0
- Minutes25
- Seconds0
Result
- Predicted marathon3:59:47
- Predicted half marathon1:55:00
- Predicted 10K52:07
- Predicted 5K25:00
- Predicted mile7:31
- Current pace5:00 per km
The formula gives 52:07 for 10K, 1:55:00 for a half and 3:59:47 for a marathon. The 10K and half predictions are realistic with some extra mileage; the sub-4 marathon needs months of long runs.
Half marathon as the best predictor
A runner finishes a half marathon (21.0975 km) in 1:45:00 and is eight weeks out from a marathon.
Inputs
- Recent race distance21.0975 km
- Hours1
- Minutes45
- Seconds0
Result
- Predicted marathon3:38:55
- Predicted half marathon1:45:00
- Predicted 10K47:35
- Predicted 5K22:50
- Predicted mile6:52
- Current pace4:59 per km
The marathon prediction is 3:38:55. Because the half is the closest distance to the marathon, this is the most trustworthy input of the three examples.
How the exponent changes a marathon prediction
The 1.06 exponent is an average. Highly trained endurance runners slow down less (closer to 1.05); runners with more speed than endurance, or with low weekly mileage, often slow down more (1.08 or higher over the marathon). This table shows a 46:30 10K converted with different exponents:
| Exponent | Runner profile | Predicted marathon |
|---|---|---|
| 1.05 | High-mileage endurance runner | 3:30:51 |
| 1.06 | Riegel standard (this calculator) | 3:33:55 |
| 1.07 | Moderate training | 3:37:01 |
| 1.08 | Speed-oriented or low mileage | 3:40:09 |
When predictions run too fast
- Big jumps in distance: predicting a marathon from a mile or 5K stretches the formula furthest. Use a 10K or half marathon if you have one.
- Not enough long runs: the marathon depends on fuel and muscular endurance that short races do not test. Runners averaging under about 30 miles (50 km) a week often miss the prediction.
- Course and conditions: hills, heat, humidity and wind all add time. A hot day can cost several minutes over a marathon.
- An off-day input: if your recent race was not an all-out effort, every prediction will be too slow; if it was downhill or short, they will be too fast.
- Ultras: Riegel's formula was fitted to races up to about four hours, so it becomes less reliable beyond the marathon.
Entering distances correctly
The calculator works in kilometers. Use these exact values for standard races:
- Mile: 1.609
- 5K: 5; 10K: 10; 15K: 15; 10 miles: 16.093
- Half marathon: 21.0975
- Marathon: 42.195
GPS watches often read 1–2% long in a race because of the line you run, so use the official course distance rather than your watch distance. To estimate your aerobic fitness from the same result, try the VO2 max calculator.
How to use the Race Time Predictor
- 1Enter the official distance of a recent race in kilometers (21.0975 for a half marathon, 1.609 for a mile).
- 2Enter your finishing time as hours, minutes and seconds — chip time if you have it.
- 3Read the predicted times for the mile, 5K, 10K, half and marathon, and your current pace per km.
- 4Treat longer-distance predictions as a best case, and adjust for hills, heat and your training volume.
Frequently asked questions
How accurate is the Riegel formula?
It is usually within a few percent for distances up to about four times the race you enter, provided you are trained for the target distance. Marathon predictions from short races are often several minutes optimistic for recreational runners.
What is a good half marathon time based on my 10K?
Multiply your 10K time by about 2.2 — the Riegel ratio for 10K to half marathon. A 50:00 10K suggests roughly 1:50 for the half.
Can I predict my marathon time from a 5K?
You can, but it is the least reliable conversion. A 25:00 5K predicts just under 4 hours, which only experienced, high-mileage runners tend to achieve.
Is this the same as the McMillan calculator?
No. McMillan uses its own performance tables and also gives training paces, so its predictions differ slightly. Both aim to show equivalent performances across distances.
Should I use a training run instead of a race?
Only if it was a genuine maximal effort, such as a time trial. Easy or tempo runs underestimate your ability and produce predictions that are too slow.
References
- Athletic Records and Human Endurance (Riegel, 1981) — American Scientist
- Daniels' Running Formula — Human Kinetics
About this calculator
- Built and reviewed by the Calculation Machine team · How we build and test calculators
- Last reviewed: October 24, 2026
- Report an error