Running pace calculator: pace, time, distance and a race-time estimate

Pace, time and distance are one relationship with three ways to read it. Know any two and you can work out the third. Enter what you know, choose what to work out, and the answer, the splits and a race-time estimate update as you type.

Work it out

The calculator needs JavaScript, which looks to be switched off. The numbers here are the worked example, a 25-minute 5K, and the formulas below work by hand.

How pace, time and distance fit together

Pace = time ÷ distance. A 25:00 5K is 1,500 seconds over 5 kilometres, which is 300 seconds a kilometre, or 5:00 per km. Rearranged, time = pace × distance, and distance = time ÷ pace. That’s all the calculator does for the top half of the page.

Kilometres and miles differ by a fixed factor. A mile is exactly 1.609344 km,¹ so a pace per kilometre becomes a pace per mile when you multiply it by 1.609344. The marathon is 42.195 km,² which makes the half marathon half of that, 21.0975 km.

The table is the conversion worked out at four paces, not a scale of what’s fast or slow.

What the split table is

The split table shows even splits: the same time for every kilometre or mile, with a shorter last row if the distance doesn’t divide evenly. It’s a practical way to check your watch against a target at each marker, not a prescription for how to pace a race. Plenty of runners pace by effort, and hills, wind and crowds change the split you can hold.

How the race-time estimate works

The estimate uses Riegel’s formula: T2 = T1 × (D2 ÷ D1)^1.06, where T1 is a time you ran over distance D1 and T2 is the estimated time over D2. Riegel described a power law linking record times and distance in a 1981 paper on athletic records,³ and 1.06 is the exponent a later paper fixes for it, describing the value as derived from world-record performances.⁴ The exponent is slightly above 1 because runners slow as the distance grows, which means the estimate for twice the distance is a little more than twice the time.⁵

A worked example: a 25:00 5K is 1,500 seconds. The distance ratio to 10K is 2, and 2 to the power 1.06 is about 2.085, so the estimate is about 3,127 seconds, or 52:07.

What the estimate can’t tell you

It is an estimate from two numbers. Here’s where it stops being reliable, from the studies that have looked at it:

• It's meant for endurance-length efforts. A 2021 review, citing Riegel's 1981 paper, puts endurance competitions at about 3 to 230 minutes.⁶ The calculator marks any time outside that range with †.
• The exponent isn’t one fixed number. One study reports that Riegel cited 1.08 for elite runners and 1.05 to 1.06 for male recreational runners aged 40 to 70, and chose 1.07 as an average for its own work.⁵ A 2024 paper on world running records describes the speed-and-distance power law from Riegel’s paper as having an exponent that varies between 0.054 and 0.083, depending on age and gender.⁷ That exponent is written for speed; in the time form used here it corresponds to roughly 1.054 to 1.083 (arithmetic: 1 plus the speed exponent). The calculator uses 1.06 for everyone.
• The marathon estimate was the roughest in one study. In a survey of 2,303 recreational runners, Riegel's formula was well-calibrated up to a half marathon. Its marathon predictions, using an exponent of 1.07, were 10 minutes or more too fast for about half of the runners in a 156-runner validation sample.⁵ Treat the marathon row as the roughest one.
• It knows two numbers and nothing else. Your training for the longer distance, the course, the weather and how you pace the day aren’t inputs.

So use it to see where a result sits against another distance, not to set a goal you’ll be held to. Racing hard is a big effort for a body that hasn’t been running. I’m a fitness coach, not a doctor, so if you have a medical question, or you haven’t been active in a while, check with your doctor before you train for a race.

Using the numbers

Pace and time are a way of keeping score. Getting faster comes from the training behind them. How to run faster covers what the trials measured for pace and for top speed, how to build endurance covers the easy running and the long run, and strength training for runners covers what lifting adds to a running week. If you want a fitness number from a run, the VO2 max calculator turns a 12-minute run into an estimate.

Common questions

How do I convert min/km to min/mile?

Multiply the pace per kilometre by 1.609344.¹ Pick “kilometres”, enter a distance and a time, and the line under the result shows both. Going the other way, divide a pace per mile by 1.609344.

How accurate is the race predictor?

It’s an estimate. In the one survey of recreational runners cited here it was well-calibrated up to the half marathon; for the marathon, with an exponent of 1.07, it was 10 minutes or more too fast for about half of a 156-runner validation sample.⁵

Does it work for a walk?

The pace, time and distance part is plain arithmetic, so yes. The studies cited here looked at runners, so leave the race-time estimate for runs. For walking calories, there’s the walking calorie calculator.

Is anything saved or sent?

No. The arithmetic runs in your browser, and the page keeps nothing once you close it.

If you’d rather have the week planned

Runs, lifting days and rest only help when they land in the right order, and that’s a scheduling problem as much as a training one. Coaching here is general strength and fitness coaching, not run coaching: your workouts are built around the days you actually have, and a weekly call looks at the week you just had and rebuilds the next one. How coaching works explains it, and apply is the first step. Otherwise: run the same route twice this month, note the time, and enter it above.