Tool

Pace calculator: goal time to pace, splits and halfway

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How this works

There is no model here and no estimation — just division, done carefully, in both units.

Pace per kilometre is your goal time in seconds divided by the distance in kilometres. Pace per mile is that number multiplied by 1.609344. Splits are the per-unit pace accumulated, with the final row carrying the odd remainder: a marathon is 42.195 km, so the last row is 195 m, not a full kilometre. That detail matters more than it looks, because a pace band that quietly rounds the marathon to 42 km will put you 66 seconds off at 4-hour pace.

The halfway figure is the goal time divided by two. The negative-split option multiplies that by 1.02, which puts the first half 2% slower than even pace and the second half 2% faster — a modest, achievable version of the strategy rather than the aggressive one.

Why the watch pace is different from the goal pace

Races are measured along the shortest legal route, with a short-course prevention factor added so a certified course is never accidentally short. You will not run that shortest route. You will run the racing line badly, weave round people at drink stations, and start from the side of the road.

On top of that, a GPS trace adds distance through positional noise: every sample sits slightly off the true position, and summing the errors inflates the total. Both effects push the same way, and together they typically produce 1 to 1.5% more distance on the watch than the course you were scored on.

The practical consequence is a pace target, not a complaint. To finish a marathon in 4:00:00 with a watch that reads 1.5% long, the screen has to show about 5:36/km (9:01/mi), not 5:41/km. Runners who pace off the watch's average without adjusting for this finish a minute or two late and cannot work out why.

What this calculator will not tell you

Whether the time is realistic for you. That is a different question with different inputs — a recent race, your weekly volume, your longest run. The race predictor is the tool for it, and the sub-4 reality check is the long answer. A pace calculator is arithmetic. It will happily tell you the pace for a 2:30 marathon, and it has no opinion about whether you should attempt one.

It also assumes a flat course. On a hilly one, even effort and even pace are different things, and the pace you should hold varies by grade.

A worked example

Take a 2:00:00 half marathon. That is 7,200 seconds over 21.0975 km, so 341.3 s/km — 5:41/km, or 9:09/mi. The same pace as a 4:00:00 marathon, which is a useful thing to know: if a 2-hour half feels like a hard day out, a 4-hour marathon is not the next race.

Halfway is 1:00:00 at 10.55 km. On a 2% negative split you would come through in 1:01:12 and run the second half in 58:48. And with a watch reading 1.5% long, the screen pace to hold is about 5:36/km (9:01/mi).