Metric Door Frame - How many bricks high?

Hello - I hope someone can shed some light on how many bricks high is a metric door frame.

I went on a brick laying course to be able to build a brick shed - And for a 826mm wide door it would be 4 bricks wide. However I'm a little confused about the height of the door opening. External Door frames are quoted as being anything from 2118mm to 2133mm in height (ext dimension).

My question: Does the opening for the door frame need to be 28 or 29 bricks? If it is 28 that would equal 2110mm (28 bricks + 29 joints) but that would be a bit tight for a 2118mm door frame. If it were 29 it would be 2185mm - which seems far to big. I understand that you would be some blocks under the cill and on top of the DPC - but still it seems not quite right - or is it?

Thanks for your help in advance.

A typical door opening size (height) is 2100mm, or 28 metric courses.

Yep, as noseall said.