How different is a 20K from a half marathon?

Barely. They are separated by less than 1.1 kilometers, so a 20K is essentially a half run a little short, and the predicted half is only a couple of minutes slower than your 20K.

Is this the most accurate prediction here?

Close to it. With less than two thirds of a mile between the distances, there is almost nothing to extrapolate, so the estimate is very precise.

How should I pace the half from a 20K?

Run it at the same effort as your 20K. The extra kilometer adds only a small slowdown, so holding your 20K pace puts you right on the predicted time.

Predict Your Half marathon Time From a 20K

Ran a 20K lately? Enter your time to see what it predicts for the Half marathon, plus the pace you would need to hold.

Your 20K time

min

sec

Pace in

Predicted Half marathon time

1:35:15

That is 7:16 per mi.

Worked example

A 20K of 1:30:00 predicts a Half marathon of about 1:35:15, a pace of 7:16 per mi. Change the time above to run the same math on your own result.

How a 20K predicts your Half marathon

This is the closest conversion on the entire site. A 20K and a half marathon are separated by less than 1.1 kilometers, about two thirds of a mile, so your 20K is very nearly a half marathon already. The prediction is correspondingly precise, your half time should be only a couple of minutes slower than your 20K.

If you have raced a 20K, you can treat it almost as a half marathon run a little short. The formula adds only a tiny slowdown for the extra distance, which makes this one of the most accurate predictions you will find. Pace the half exactly as you paced the 20K and you will be within a hair of the predicted finish.

Practically the same race

The half marathon is 21.0975 kilometers and a 20K is 20, leaving barely a kilometer between them. Riegel multiplies your 20K time by only about 1.058 to reach the half, so the predicted slowdown is minimal. There is almost no extrapolation here, which is why the estimate is so dependable.

The only real difference is that final kilometer, which arrives just as a hard 20K starts to bite. If you raced your 20K close to your limit, simply hold the same effort and pace through the extra distance. There is no new endurance demand to prepare for, just a few more minutes of the same work.

Want every distance at once? The race pace predictor shows your time for the whole range and prints a pace band, and the VDOT calculator turns this race into your training paces.

Related predictions

20K to Half marathon questions

How different is a 20K from a half marathon?: Barely. They are separated by less than 1.1 kilometers, so a 20K is essentially a half run a little short, and the predicted half is only a couple of minutes slower than your 20K.
Is this the most accurate prediction here?: Close to it. With less than two thirds of a mile between the distances, there is almost nothing to extrapolate, so the estimate is very precise.
How should I pace the half from a 20K?: Run it at the same effort as your 20K. The extra kilometer adds only a small slowdown, so holding your 20K pace puts you right on the predicted time.