As i stated earlier you will want the finished opening to be the same size or slightly smaller than the door.
Should openig be same size as garage door.
From a structural standpoint it is fairly easy to reduce the size of the opening in a garage thus creating the need for a smaller garage door.
Measure the width and height of your garage door with a tape measure so you know what you have.
The size of the finished opening should be the exact same size as the sectional garage door or down to 1 smaller on each side and or the top.
This way the door will have a nice tight seal once the weatherseal or door stop molding is applied.
Doors that are 10 feet wide by 8 feet tall are single car garage doors.
Entry doors are usually 36 inches wide.
The width of the rough opening should be 3 inches greater than the width of the garage door you are purchasing.
You get the picture.
For instance if the width of the garage door is 8ft the rough opening should be 8ft 3 inches wide.
Once again it should be obvious that if you have an arched opening the height would be the same as the highest point of the arch.
While a door width of 36 inches is oversized for an interior door it s the standard size for front doors the standard height remains the same however.
Single garage door openers generally do fine with smaller openers while double doors may need a little more horsepower.
While technically the door could overlap the opening as much as the space will allow the design on the outside of the door disappearing behind the jamb will tend to distract from the overall outside appearance.
If it s 9ft the opening must be 9ft 3 inches and so on.