ReckonBoxHow the pace calculator works
Running pace is time divided by distance, expressed as minutes per kilometre or per mile. A 20 minute 5K is 4:00 per kilometre, which is 6:26 per mile, which is 15 km/h. Enter any two of pace, time and distance and the tool solves the third.
Underneath that it does something the ranked calculators mostly avoid: it predicts your time at other race distances two different ways, and shows you both.
Why your marathon prediction is probably too fast
Race times do not scale linearly with distance, because endurance decays as the race gets longer. Peter Riegel published the relationship in 1977, and it's still the standard: T2 = T1 x (D2/D1)^1.06.
That exponent is the whole story. Doubling the distance multiplies your time by 2^1.06, which is about 2.08 rather than exactly 2. Small on a 5K to 10K jump. Enormous by the marathon.
Take the 20 minute 5K. Hold the pace flat and multiply, and you get a marathon of 2:48:47. Run the same input through Riegel and it returns 3:11:49. Those two answers are 23 minutes apart, and one of them will ruin your race.
| Prediction from a 20 minute 5K | Linear | Riegel |
|---|---|---|
| 10K | 40:00 | 41:42 |
| Half marathon | 1:24:23 | 1:32:00 |
| Marathon | 2:48:47 | 3:11:49 |
Strava's calculator describes its method as multiplying pace by distance, and states plainly that it doesn't account for fatigue, course conditions or hills. calculator.net offers a finish-time tool without naming any prediction method. Omni says its race time is adjusted because faster paces can't be held as long, then declines to say how. Runhive advertises a marathon time predictor with no methodology at all.
How much should you trust any of this?
Less than the tidy output suggests. RunnersConnect puts traditional prediction formulas at roughly 80% accuracy, which means about one runner in five significantly misses the predicted time.
The models don't even agree with each other. For an 18 minute 5K runner, Riegel returns a 2:56:05 marathon, McMillan 2:55:23 and Jack Daniels' VDOT 2:52:45. Three minutes of spread, which is roughly 8 seconds per mile, and enough to change how you go out.
RunnersConnect also names what none of the formulas can see: fast-twitch dominant runners tend to underperform their marathon predictions, runners below the recommended weekly mileage miss by 3% to 10%, and track times run about 30 to 45 seconds faster per 5K equivalent than road times.
Pace, speed, and the conversion people get backwards
Pace and speed are inverses of each other, and mixing them up is the most common arithmetic error in running. Pace tells you the time to cover one unit of distance; speed tells you the distance covered in one unit of time.
To convert, divide 60 by the other one. A 4:00 per kilometre pace is 60 divided by 4, so 15 km/h. A 10 km/h runner is 60 divided by 10, so 6:00 per kilometre.
The unit trap is separate and just as common. 5:00 per kilometre and 5:00 per mile are not close: the second is roughly 38 seconds per kilometre slower, because a mile is 1.609 km. This calculator keeps the pace unit and the distance unit as separate fields for exactly that reason, so you can enter miles per hour against a distance in kilometres without silently corrupting the answer.
What this calculator leaves out
VDOT and McMillan predictions are absent. Both are named above because RunnersConnect published their outputs, but neither publishes a reproducible equation, and guessing at a proprietary table would be worse than omitting it.
Environmental adjustment is out too. Wind, heat and elevation all move race times, and no source fetched for this page supplies usable coefficients, so this tool doesn't pretend to model them. Same for Cameron's formula, which surfaced in the search without any page stating it in full.
A prediction is an estimate of what your current fitness supports on a good day, not a training plan. To see the effort side of the same run, the target heart rate calculator covers training zones, and the calories burned calculator handles the energy cost.
Frequently asked questions
How do you calculate running pace? Pace is time divided by distance. A 20 minute 5K is 20 divided by 5, which is 4 minutes per kilometre, or 6:26 per mile. Speed inverts it: 60 divided by the pace in minutes per kilometre gives 15 km/h.
What is the Riegel formula? The Riegel formula predicts a race time at a new distance as T2 = T1 x (D2/D1) raised to the power 1.06, published by Peter Riegel in 1977. The exponent sits above 1 because endurance decays as distance grows, so doubling the distance multiplies your time by about 2.08 instead of exactly 2.
Why do pace calculators overpredict marathon times? Because several of them hold your pace constant and multiply. Strava describes its method as multiplying pace by distance and states that it does not account for fatigue, course conditions or hills. Run a 20 minute 5K through that model and you get a 2:48:47 marathon; run it through Riegel and you get 3:11:49, a difference of 23 minutes.
How accurate are race time predictors? RunnersConnect puts traditional prediction formulas at roughly 80% accuracy, meaning about one runner in five significantly misses the predicted time. It also shows how far the models disagree: for an 18 minute 5K, Riegel gives a 2:56:05 marathon, McMillan 2:55:23 and Jack Daniels VDOT 2:52:45.
What does a race predictor not account for? Weekly training volume, muscle fibre composition, and the weather. RunnersConnect notes that fast-twitch dominant runners tend to underperform marathon predictions and that runners below the recommended weekly mileage miss by 3% to 10%. No calculator here adjusts for wind, heat or elevation.
How do I convert pace to speed? Divide 60 by your pace in minutes per kilometre to get km/h. A 4:00 per kilometre pace is 60 divided by 4, so 15 km/h. For miles, divide 60 by the pace in minutes per mile.