Running Pace Calculator

What this calculator does

Three numbers describe every run: how far, how long, and how fast per unit of distance. This calculator takes whichever two you have and derives the third, then keeps going where most pace tools stop: a full split table at your choice of kilometre or mile marks, built even or as a deliberate negative or positive split; your speed in both unit systems alongside the pace; and projections to the other standard race distances using the Riegel formula, the endurance model published in 1977 and still the standard first estimate, with its exponent exposed so experienced runners can tune it to their own history. Swimming gets its own mode with per-100-metre and per-100-yard paces, because pool pace is its own language and this page does not pretend it converts into running.

How to use it

1. Pick what you want to find first. “My pace” wants a time and a distance. “My finish time” wants a pace and a distance. “My distance” wants a time and a pace. The form hides the field you are solving for, which prevents the classic error of entering all three from different runs and averaging your way to fiction.
2. Splits are a plan, not a description. Even splits are the reference: the same pace at every mark. A negative split plan, second half 2% faster, is how most distance records are set and how most coaches teach racing; the table rebuilds at the two half-paces so the total still lands on your target time. A positive split plan is included because honest planning sometimes expects a hilly or hot second half, and because seeing its numbers is how runners talk themselves out of starting too fast.
3. Read predictions as a range wearing a point estimate’s clothes. Riegel’s formula, T2 = T1 x (D2/D1) raised to the exponent, has predicted endurance performances for nearly fifty years. The default exponent 1.06 fits trained endurance runners; less endurance-trained runners fade more over distance and sit nearer 1.08 to 1.10, which is why the field is editable. A marathon prediction from a 5K is an extrapolation across a sixfold distance jump and should be read with the humility that deserves.
4. Pace and speed answer different questions. Pace (minutes per kilometre or mile) is how runners plan and race, because the marks on a course are distances. Speed (km/h or mph) is what treadmills in some countries display and what cyclists think in. Both are printed for every result so the translation is never the thing you are doing in your head at kilometre 30.
5. On a treadmill, add the grade the screen forgets. A treadmill at 0% gradient is slightly easier than the road at the same pace, because there is no air resistance; the common correction is about 1% grade to match outdoor effort. This calculator works in honest outdoor terms and leaves the 1% to you, mentioned here because it is the single most common treadmill misunderstanding.
6. Swimming pace is per 100, always. Enter the swim distance and time and the answer comes back per 100 metres or per 100 yards, matching the unit you swam in, plus speed. There is no conversion of swim pace into run pace anywhere on this page, because there is no defensible one.

How it is calculated

Pace = time / distance. Finish time = pace x distance. Distance = time / pace. Mile pace = kilometre pace x 1.609344. Speed = 3600 / pace in seconds per kilometre, in km/h; mph divides by 1.609344. Splits accumulate the chosen strategy’s pace over each split length: even uses the average pace throughout; negative sets the second-half pace at 0.98 of the first-half pace and solves so the total matches; positive mirrors it at 1.02. Predictions use Riegel: predicted time = known time x (target distance / known distance) raised to the exponent, default 1.06. Swimming pace = time / distance x 100 in the chosen unit.

Worked examples

The validation case: 5 km in 25:00 is a pace of exactly 5:00 per km, 8:03 per mile, at 12.00 km/h (7.46 mph). If a pace calculator ever tells you anything else for those inputs, close the tab.

A marathon at a set pace: 5:30 per km over 42.195 km gives 3:52:04. This is the arithmetic behind every “what do I need to run for a sub-4 marathon” conversation: the answer is a pace under about 5:41 per km, held 42 times in a row.

Distance from a lunchtime run: 40 minutes at 6:15 per km covers 6.40 km, 3.98 miles.

The negative split, priced: a 50:00 10K run evenly is 5:00 per km throughout. Run as a negative split, the halves become 5:03 and 4:57 per km, 25:15 out and 24:45 back: the same finish time, a calmer first half, and a second half that has to be found somewhere. The table shows every kilometre mark under both plans so the difference is visible, not rhetorical.

Predictions from a 50:00 10K: Riegel at 1.06 projects 23:59 for 5K (4:48 per km), 1:50:19 for the half marathon (5:14 per km) and 3:50:01 for the marathon (5:27 per km). Notice the pace decay: the model assumes you slow as distance grows, which is not pessimism, it is physiology, and it is why a marathon at your 10K pace is not a plan.

The pool: 1,500 metres in 30:00 is 2:00 per 100 metres at 3.00 km/h, the freestyle benchmark club swimmers measure themselves against.

Reading your result

Your split table is the deliverable: take it to the race. Check it against your watch at each mark and adjust in the cheap kilometres, not the expensive ones; a 10-second error at kilometre 3 costs seconds to fix, the same error discovered at kilometre 38 costs minutes. When the prediction table disagrees with your ambition, believe the disagreement is information: a marathon prediction far slower than your goal usually means the endurance base is not there yet, and the fix is months of volume, not a faster first half on the day. And when two of your known results predict each other badly through the formula, the exponent field is where your personal physiology lives; fit it to your history and the predictions sharpen noticeably.

Limitations

• Flat, calm, sea-level maths. The calculator knows nothing of hills, trails, heat, altitude, wind or a course that measures long. All of them cost time, some of them cost a great deal, and no honest pace tool can price a course it has never seen.
• Predictions assume comparable training. Riegel extrapolates the endurance you have demonstrated, at the distance you demonstrated it over. A 5K prediction of marathon time assumes marathon training happens; without the long runs, the real fade is far worse than any exponent offered here.
• GPS distances are approximate. Watches measure short on twisty courses and long on some tracks; a pace computed from a watch’s distance inherits its error. Course-measured distances, the painted marks, are the ground truth the split table assumes.
• This is arithmetic, not coaching or medicine. Nothing here prescribes training loads or clears anyone to race. Chest pain, dizziness and unusual breathlessness are stop signals, not data, whatever the table says.

Frequently asked questions

What pace do I need for a sub-4-hour marathon?

Four hours over 42.195 km is 5:41 per km (about 9:09 per mile), and 3:52:04 at 5:30 per km in the worked example shows the shape of the buffer most runners aim for, because nobody runs a perfect even race on a real course. Work backwards from your goal with the finish-time mode: set the distance to the marathon shortcut, try paces, and read the finish times until one matches your target with something in hand.

What is a negative split, and why do coaches love it?

Running the second half faster than the first: here, 2% faster, which turned a 50:00 10K into 25:15 out and 24:45 back. It works because the cost of early speed is paid late with interest: glycogen, heat and muscle damage all compound, so holding a little back early buys more than it costs. Almost every distance world record has been set this way, and the split table on this page exists so club runners can rehearse the same shape at their own scale.

How accurate is the Riegel prediction?

For runners racing at their demonstrated fitness over distances within about a doubling of the known result, typically within a few percent, which at marathon scale is several minutes either way. Accuracy decays with the distance jump and with any difference in course, weather or training focus between the two performances. Treat the prediction as the centre of a band, use your own exponent once you have two comparable results to fit it with, and never let a formula talk you into a first half your training has not earned.

Why does my treadmill pace feel different from the road?

Three reasons: no air resistance (worth roughly a 1% grade correction at typical training paces), a belt that assists leg turnover slightly, and speed calibration that varies by machine, sometimes by several percent. The calculator’s paces are road paces; if your treadmill disagrees with your watch outdoors, the watch on measured ground is usually the honest witness.

Related calculators

• All Sports and Travel Calculators
• Speed Distance Time Calculator
• Steps to Distance Calculator
• Heart Rate Zone Calculator

Method: pace, time and distance from the defining identities; predictions by the Riegel formula (Pete Riegel, 1977) with an editable exponent defaulting to 1.06; split strategies as defined on this page. Last reviewed: 13 October 2026.