🏃 Running Pace & Split Calculator
A running pace calculator for km and mile splits, treadmill speed, and race predictions that use Riegel's formula rather than flat 5K pace.
Predicted race times
Using Riegel’s endurance formula, not a straight multiplication of your pace. You cannot hold 5K pace for a marathon, and most pace calculators pretend you can.
What Running Pace & Split Calculator Does
Pace arithmetic is trivial — time divided by distance — and every calculator gets that part right. Where they separate is the question people actually care about: given what I ran today, what will I run over a longer distance? That is not division, and treating it as division produces answers that are wrong in a dangerous direction.
Run a 25:00 5K and multiply the pace out to 42.195 km and you get 3:30:59 for a marathon. The number is meaningless. Human endurance decays as distance grows, and Peter Riegel described that decay with a formula that has survived four decades of use: the time ratio equals the distance ratio raised to the power of 1.06. That gives 3:59:47 for the same runner — twenty-nine minutes slower, and far closer to reality.
This calculator uses Riegel, and it shows you what a flat-pace guess would have said so the size of the correction is visible. But it is worth going one step further than the competition and stating how good even the corrected number is, because someone is going to plan a race around it.
The honest answer: quite good up to the half marathon, and optimistic at the full. When researchers checked it against 2,303 recreational runners, Riegel was well calibrated through the half but produced marathon times at least ten minutes too fast for half the field. A marathon prediction is a ceiling on a good day, not a pacing plan.
How to Use Running Pace & Split Calculator
- Select a race distance preset (5K, 10K, Half Marathon, Marathon) or enter custom distance
- Input your target or achieved time in hours, minutes, and seconds
- View your exact pace per km, pace per mile, speed, and projected race split times
Formula Used by Running Pace & Split Calculator
Pace, speed and the conversions between them
pace (sec/km) = total seconds ÷ distance (km) · speed (km/h) = 3600 ÷ pace (sec/km)
- pace
- Time to cover one kilometer or one mile
- speed
- Distance per hour — the number a treadmill displays
- 1 km
- 0.621371 miles, so pace per mile = pace per km ÷ 0.621371
Worked example
5 km in 25 minutes
- 25 × 60 = 1,500 seconds
- 1,500 ÷ 5 = 300 sec/km = 5:00 per kilometer
- 3600 ÷ 300 = 12.0 km/h
- 300 ÷ 0.621371 = 482.8 sec/mile = 8:03 per mile
Result: 5:00/km · 12.0 km/h · 7.5 mph · 8:03 per mile
Riegel endurance formula — predicting a longer race
T₂ = T₁ × (D₂ ÷ D₁)^1.06
- T₁, D₁
- Time and distance of a race you have actually run
- T₂, D₂
- The predicted time over the new distance
- 1.06
- The fatigue exponent. At exactly 1.00 the formula collapses into a flat-pace guess
Worked example
A 25:00 5K, predicting the marathon
- Distance ratio: 42.195 ÷ 5 = 8.439
- 8.439^1.06 = 9.591
- 1,500 seconds × 9.591 = 14,387 seconds
- Flat multiplication would have given 1,500 × 8.439 = 12,659 seconds
Result: 3:59:47 by Riegel against 3:30:59 flat — a 28:48 difference
Equivalent race times from a 5K
Riegel predictions with the exponent at 1.06. Times through the half are well calibrated; the marathon column runs optimistic for most recreational runners.
| 5K | 10K | Half marathon | Marathon | 5K pace/km | Treadmill km/h |
|---|---|---|---|---|---|
| 20:00 | 41:42 | 1:32:00 | 3:11:49 | 4:00 | 15.0 |
| 22:30 | 46:55 | 1:43:30 | 3:35:48 | 4:30 | 13.3 |
| 25:00 | 52:07 | 1:55:00 | 3:59:47 | 5:00 | 12.0 |
| 27:30 | 57:20 | 2:06:30 | 4:23:45 | 5:30 | 10.9 |
| 30:00 | 1:02:33 | 2:18:00 | 4:47:44 | 6:00 | 10.0 |
| 35:00 | 1:12:58 | 2:41:00 | 5:35:41 | 7:00 | 8.6 |
Pace, speed and treadmill settings
Treadmills are set in km/h or mph rather than pace, which is why the conversion trips people up.
| Pace per km | Pace per mile | km/h | mph |
|---|---|---|---|
| 4:00 | 6:26 | 15.0 | 9.3 |
| 4:30 | 7:15 | 13.3 | 8.3 |
| 5:00 | 8:03 | 12.0 | 7.5 |
| 5:30 | 8:51 | 10.9 | 6.8 |
| 6:00 | 9:39 | 10.0 | 6.2 |
| 7:00 | 11:16 | 8.6 | 5.3 |
How much the fatigue exponent matters
Marathon prediction from a 25:00 5K at different exponents. Riegel proposed 1.06; some coaches argue for a higher figure for runners with low weekly mileage.
| Exponent | Predicted marathon | Versus Riegel |
|---|---|---|
| 1.00 (flat pace — wrong) | 3:30:59 | 28:48 faster |
| 1.04 | 3:49:46 | 10:01 faster |
| 1.06 (Riegel) | 3:59:47 | — |
| 1.08 | 4:10:14 | 10:27 slower |
| 1.10 | 4:21:08 | 21:21 slower |
How to Read Your Result
Why the exponent is greater than one
If humans could hold one speed indefinitely the exponent would be exactly 1.00 and pace would be a constant. We cannot, because the energy systems that supply a 5K and a marathon are not the same. As duration grows, the sustainable fraction of your maximum aerobic capacity falls, glycogen becomes limiting and, past roughly two hours, so does thermoregulation. The exponent 1.06 is an empirical summary of all of that in a single number.
Where the prediction breaks down
The formula knows exactly one thing about you: one race result. It does not know your weekly mileage, whether you have ever run beyond 20 km, or whether you fuel properly. That is why marathon predictions skew fast — a runner with a sharp 5K and 30 km a week will not run the predicted marathon, while the same 5K on 80 km a week might beat it. Predictions from a half marathon are considerably more trustworthy than predictions from a 5K, simply because the extrapolation is shorter.
What "at least 10 minutes too fast for half of runners" means
It means the median error at the marathon was ten minutes in the optimistic direction, so half of runners were off by more than that. The same study found that models built on one or two recent race results beat Riegel substantially — mean squared error of 228 and 208 against Riegel's 381. If you have run two recent races at different distances, the pattern between them tells you more about your endurance than any single formula can.
Reading the pace chart
Two things fall out of the arithmetic that runners find useful. First, 5:00/km is exactly 12.0 km/h, one of the rare round conversions, which makes it a convenient treadmill anchor. Second, pace and speed are reciprocals, so equal pace improvements get harder as you get faster: going from 6:00 to 5:30 per kilometer adds 0.9 km/h, while 4:30 to 4:00 adds 1.7 km/h for the same thirty seconds.
Even splits versus the negative split
The predictions here assume a steady effort throughout. In practice most personal bests come from running the second half marginally faster than the first, and most disasters come from the reverse. Starting three to five seconds per kilometer slower than target for the first few kilometers costs very little and protects against the far more expensive error of going out too hard.
Limitations & Accuracy Notes
- Riegel assumes appropriate training for the target distance. It is an extrapolation of endurance, not a prediction of what an untrained runner will do over an unfamiliar distance.
- Marathon predictions are optimistic. In a study of 2,303 recreational runners the formula gave times at least ten minutes too fast for half the field at the full distance, while remaining well calibrated up to the half.
- The exponent 1.06 is a population average. Runners with high weekly mileage tend to beat it over long distances and those with low mileage tend to fall short, which is why the exponent-sensitivity table is included.
- Nothing here accounts for course profile, heat, humidity, wind or altitude. A hilly marathon on a hot day can cost far more than the difference between any two exponents in that table.
- Treadmill speeds are not directly comparable to road running. Belt assistance and the absence of air resistance make a given speed slightly easier indoors; adding a 1% gradient is the usual rough correction.
Frequently Asked Questions
How do you calculate running pace?
Why is my predicted marathon time slower than my pace times 42?
How accurate are race time predictions?
Does the prediction assume I have trained for the distance?
What is a good 5K pace for a beginner?
What is the difference between pace and speed?
Can it predict a race time from a shorter one?
What are negative splits?
Why is my treadmill pace easier than outdoors?
Does pace convert directly between kilometers and miles?
Is my data stored?
References & Further Reading
- Vickers AJ, Vertosick EA — An empirical study of race times in recreational endurance runners — BMC Sports Sci Med Rehabil 2016;8:26 — 2,303 runners; found Riegel well calibrated to the half marathon but at least 10 minutes too fast at the full for half of runners
- Blythe DA, Király FJ — Prediction and Quantification of Individual Athletic Performance of Runners — PLoS One 2016;11(6):e0157257 — 164,746 runners and 1,417,432 performances; the systematic comparison of race-time predictors
- Peter Riegel — Athletic Records and Human Endurance — American Scientist 1981;69(3):285-290 — the origin of the T₂ = T₁ × (D₂/D₁)^1.06 formula used throughout this page