Correctly measuring the space for your new/replacement garage door is extremely important, as you need to consider the potential of needing a sub frame, whilst also taking into consideration the fact that manufacturers have different size references.
If you’re looking to purchase a new garage door, then you should measure four things;
1) Width and height of the brickwork opening, but without the frame and door.
2) If you’re leaving your timber sub frame in place, then you need to measure the width and height for the need door to fix to also.
3) Measure the space for internal headroom (space from the underneath of the lintel to the ceiling).
4) Measure the width of the internal return (the width of the faces which show either side, when you look outside from your garage).
Also make sure that you make a note of any obstructions in the garage.