How to use the rough opening calculator
- Choose a door or a window, or pick a preset to fill in example allowances.
- Enter the width and height of the door slab, or of the window frame. For a pre-hung door, use the slab size, such as 30 by 80.
- Check the width and height allowances. They are example values, so replace them with the numbers the maker gives for the product.
- Read the rough opening, the header length and, for windows with a sill height, the height of the underside of the header.
Formula
Rough opening width = size + width allowance
Rough opening height = size + height allowance + finished floor thickness (doors)
Header length = rough opening width + 2 x stud thickness
Underside of header (windows) = sill height + rough opening height
The header rests on a jack stud at each side, so it is longer than the opening by two stud thicknesses. An actual 2x stud is 1.5 inches thick.
Worked example
A 30 by 80 inch pre-hung door slab with allowances of 2 inches in width and 2.5 inches in height needs a rough opening of 32 by 82.5 inches. The header is 32 + 3 = 35 inches (2 ft 11 in) long.
A 36 by 48 inch window with 0.5 inch allowances needs 36.5 by 48.5 inches. With a sill 36 inches above the floor, the underside of the header sits at 84.5 inches (7 ft 0.5 in). If you will lay 0.75 inch flooring after setting a door, the height becomes 83.25 inches.
What the allowances are for
A pre-hung door comes in its frame, which has side jambs and a head jamb. The rough opening is larger than the door slab so the frame can be set plumb and level with shims, and so the door clears the floor. Windows need a smaller gap, because the unit is built to fit the opening with shims on each side. The numbers here are example allowances, but manufacturers publish exact rough openings, and those always win.
Headers and framing
The header carries the load above the opening, and its size depends on the span, the load and your local code. This tool gives the length, not the size, so check your building code or ask an engineer. Studs on each side of the opening support the header, and you can estimate the studs for a wall with the stud calculator.
Finishing
Plan trim and drywall around the opening. See the drywall calculator and the baseboard trim calculator for materials, and the stair stringer calculator for other framing layouts.
Assumptions and limits
- The allowances are example values for pre-hung doors and windows, not manufacturer figures. The product maker's rough opening overrides them.
- Stud thickness is 1.5 inches, the actual thickness of nominal 2x lumber, and is editable.
- Header size and load are not calculated. Follow your local code.
- Measurements are in inches. Door swing, thresholds and out-of-plumb walls are not modeled.
Frequently asked questions
What size is the rough opening for a 30 inch door?
A common allowance gives 32 inches wide and 82.5 inches tall for a 30 by 80 pre-hung door slab. Check the maker's instructions for the exact figure.
Why is the rough opening larger than the door?
It leaves room for the frame, shims and adjustment so the door can be set plumb, level and square.
How long should the header be?
It rests on a jack stud at each side, so it is the rough opening width plus the thickness of two studs, 3 inches for standard 2x lumber.
Does this work for exterior doors and windows?
It gives the arithmetic, but exterior units often have different allowances and flashing details. Use the maker's rough opening and the code.