if the port looks like an L (90degs)

how do i do it to get a 12" port

is it center line,inside line,out side line?

 

Measure it down the centre line.

If the port is using a wall of the enclosure, subtract 1/2 the port thickness (inside) from the overall length, or you'll find it tuned lower than you expected.

I usually knock off another 1/2" or so with a bent port to compensate for friction.

 

centre line. ignore any volume on the bend.

 

I agree in theory, but really, is anyone going to notice if their box is tuned to 43hz, instead 45?

 

Probably not.

I'm just saying that usually the construction factors (long thin slot ports, friction from extra glue and unfinished MDF, and the wall effect) usually means the box will be lower than predicted.

The width to height ratio of a port "should" be below 9:1, but in practice they often end up being 12:1 or more.

If you are going to be off, lean towards the high side.

It's also not too difficult to lower a port after it's built, by adding some material (making the port smaller), but almost impossible to raise.