Running Pace Calculator

How pace, distance, and time relate

The three are linked by a simple formula: pace = time ÷ distance. The same relationship rearranged gives time = pace × distance, and distance = time ÷ pace. This calculator applies whichever version of the formula you need based on which two inputs you provide.

Pace is expressed in minutes and seconds per kilometre (or per mile), which is more intuitive for runners than speed in km/h. A pace of 5:00 per km means you cover one kilometre every five minutes. Speed and pace are the inverse of each other: 5:00 /km equals 12 km/h.

Pace per km vs pace per mile

One mile is 1.60934 km, so pace per mile is always longer than pace per km. A 5:00 /km runner is doing 8:03 /mile. Road races in the UK are measured in kilometres; in the US and some older UK races, miles are used. The calculator shows both so you can compare results regardless of the system used on race day.

Why a flat-pace prediction is rarely the whole story

The basic predictions in the table assume you maintain exactly the same pace for every kilometre. Shorter races, fine. A 10K is short enough that a well-paced effort is genuinely flat. The marathon is a different animal, and the half is somewhere in the middle. The optional "Race conditions and training" section adjusts the prediction for four real-world factors that experienced coaches will tell you matter more than runners realise.

Race size and GPS drift

If you have ever run London, Berlin, or New York, you know the feeling: you cross the line, glance at your watch, and it reads 26.7 miles. Not 26.2. The course is not measured wrong. The course is measured along the optimum racing line, which is the shortest legal route a single runner could take. Put 30,000 people on that course and you simply cannot hold that line. You are weaving round slower groups, drifting wide on bends, getting funnelled through narrow points. By the time you finish, your GPS has clocked an extra 500 to 800 metres. At marathon pace that is a chunk of time you did not plan for.

Small local races (under a thousand runners) almost never see this. Mid-sized regional races pick up a touch. Big-city majors are the ones that catch first-timers out, which is why the calculator quietly adds 0.3 to 1.5 percent to the target distance depending on how packed the start corral is going to be.

The sex difference at marathon distance

Pete Riegel's original formula uses an exponent of 1.06, treated as a universal constant. Decades of race data since then suggest it is not quite that universal. Women, on average, fade less from half-marathon to full-marathon than men do. There are physiological theories about fat metabolism, glycogen stores, and pacing discipline; pick whichever one sounds least handwavy to you. The pattern in the data is real either way. So this calculator uses 1.04 for women, 1.06 for men, and 1.05 if you would rather not say. The difference is small for a 10K, meaningful for a marathon: a 1:30 half-marathoner predicts a different finish time depending on which exponent applies.

Heat acclimation

If you trained through a cool British autumn for a marathon in Valencia or Houston in February, congratulations: you are about to find out what 18°C feels like when your body learned the race in 8°C. The rough rule used by coaches is 30 to 60 seconds per kilometre added per 5°C above the temperature you trained in, with a 5°C buffer before the penalty kicks in (because a few degrees warmer is fine and arguably helps). The penalty stacks. By the time race-day temperature is 15°C above what you have been training in, you are looking at well over a minute per km of slowdown, which obliterates a marathon target. The calculator caps the penalty at 180 s/km because past that point you are not racing, you are just trying to finish.

Acclimation works the other way too: spend two or three weeks training in heat before a hot race and the penalty shrinks. The calculator does not model that explicitly; the assumption is that the temperature you typed into "training temperature" is the one your body has actually adapted to.

Weekly mileage is the single biggest predictor

Of every input variable that goes into a marathon time prediction, weekly training volume is probably the strongest. Two runners with identical 1:30 half-marathons will produce wildly different marathon times if one is training 30 km/week and the other is training 80 km/week. The high-mileage runner has the aerobic base, glycogen storage, and structural durability to hold pace deeper into the race. The low-mileage runner is, as a coach friend puts it, racing on credit.

The calculator reflects this by adjusting the Riegel exponent up for low-mileage runners (predicts a slower marathon than the half would suggest) and down for high-mileage runners (predicts faster). It only kicks in for half-marathon and longer distances, because for a 5K or 10K it just does not move the needle in the same way. If you are training 16 km/week and predicting a marathon time, the calculator will be honest with you about what is likely to happen at 35 km.

Thanks to Sarah Elizabeth Jones, who runs marathon coaching at Lovely Marathon, for the input on race-size GPS drift, heat acclimation, and the sex-based fatigue difference. Running coaches see things in their athletes' data that no algorithm catches.

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Pace is the start. Here is the rest of the fitness maths.

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