Running Pace Calculator

Pace tables are not hard to calculate once; they are hard to adjust while training. Ten seconds faster means every split needs recalculating, and a negative-split plan usually needs scratch paper. This tool links distance, total time and pace: change any one and the other two follow, with split tables and checkpoint times redrawn immediately:

• Three linked fields: the default loads 42.195 km in 3:30:00, which reads as 4:59/km, 12.06 km/h, 8:01/mile, 3.35 m/s and a 1:59 400 m lap; set 4:30 pace for 10 km and the total snaps back to 45:00
• Four quick distances: 5 km, 10 km, half marathon and marathon, using the official 21.0975 and 42.195 distances
• Split plans: every 1, 2 or 5 units, or any decimal length; progression from -30 to +30 seconds per segment, positive meaning start slower and finish faster
• Progression keeps the finish time: segment paces are weighted by length and spread symmetrically, so a five-segment plan lands on the same finish line as even pacing. Only segments pushed to the 0:30 per unit floor increase the total; these are marked * and the difference is explained
• Milestone times: the marathon view lists 12 checkpoints (1, 2, 3, 5, 10, 15, 21.0975, 25, 30, 35, 40 km and the finish), each with even-pace cumulative time, plan cumulative time, the pace needed for that segment and the difference, ready for a pacer to read aloud
• Race prediction: the Riegel power law extrapolates from a current result to other distances, with the exponent defaulting to 1.06 and adjustable from 0.90 to 1.20
• Target suggestions: seven targets from 18:00 to 35:00 for 5 km, seven from 35:00 to 1:10:00 for 10 km, eight each for the half and full marathon, and six bands at plus or minus 5%, 10% and 15% for non-standard distances
• Kilometres or miles: one mile is 1.609344 km; switching only changes the display, while speed, pace per mile, metres per second and 400 m lap readings stay the same
• Copy as plain text: the split table with intervals, segment times and cumulative distance is formatted for a notes app or watch note

When to use it

• Breaking a goal time into 5 km checkpoints before a race and handing it to a pacer
• Writing a negative-split session and knowing how much slower or faster each 2 km should be
• Setting a treadmill that only accepts speed, so pace must become km/h first
• Having raced only 5 km and wanting a realistic half marathon target
• Running fixed laps and needing 400 m lap pace rather than only per-kilometre pace
• Converting an imperial 8:00/mile to metric to see whether you can hold it

Common result comparisons

A 1:45:00 half and a 3:30:00 marathon sit at the same pace, which is where the idea that the marathon is exactly twice the half comes from. In practice a marathon needs reserve beyond that pace, which is why the prediction exponent is adjustable.

What a negative split looks like

With the example loaded (half marathon in 1:45:00, a segment every 5 km, 10 seconds progression), the five paces are:

The total is exactly 1:45:00, no different from even pacing; only the effort distribution moves. The matching checkpoints: 10 km in 51:39 (even pace would be 49:46, so 1:53 slower), 15 km in 1:16:14, a first half of 54:21 and a second half of 50:39.

FAQ

• How do pace and speed convert? Seconds per kilometre is total seconds divided by kilometres, and speed is 3600 divided by seconds per kilometre. 4:59/km is about 298.6 seconds, hence 12.06 km/h; a treadmill at 12 km/h is 5:00/km
• How much should I trust the prediction? The Riegel law is reasonable when training is comparable and distances are within range. A 20:00 5 km at exponent 1.06 predicts 41:42 for 10 km, 1:32:00 for a half and 3:11:49 for a marathon. Setting the exponent to 1.00 reduces it to even-pace extrapolation (40:00, 1:24:23, 2:48:47), while 0.90 gives a 2:16:22 marathon. The spread shows that endurance, not speed, drives the long distances
• What exponent should I use? With two or more real results, predict the longer from the shorter, then adjust the exponent until prediction and reality agree; that number belongs to you more than the default 1.06
• Why are some segments marked *? When progression is large, later segments fall below the practical floor of 0:30 per unit. The tool lifts them to the floor, marks them and lets the total grow. For a 3:30:00 marathon with +30 seconds progression, 13 of 43 segments hit the floor and the finish is 36:54 over target, showing that setting is not viable over 42 km
• Can segment length be a decimal? Yes. 1.5 over 3 km makes two segments at 5:33/km and 8:20 each; a length longer than the total produces one segment and the tool says so
• What happens if I enter 70 seconds? It carries over and the total is explained, so 1 hour 70 minutes becomes 2:10:00; 5 minutes 60 seconds in the pace field is normalised to 6:00 so the interface and the result agree
• Does the mile view change the numbers? Only the display. The same 3:30:00 marathon reads as 26.2188 miles at 8:01/mile, while 12.06 km/h, 7.49 mph, 3.35 m/s and 1:59 per 400 m stay the same
• Is anything uploaded? No. Distance, time, plans and predictions are calculated in the browser, with no database writes and no requests