About the Rough Opening Calculator
This rough opening calculator converts a door or window size into the framed opening you need to build, and gives the matching header length, jack (trimmer) stud length and, for windows, the rough sill height and cripple length. Enter the unit size, choose the type, and every measurement is shown in decimals and tape-measure fractions.
It is aimed at framers, remodelers and DIYers who are cutting a new opening or reframing an old one. The defaults follow common rules of thumb: a prehung door needs an opening 2 inches wider and 2-1/2 inches taller than the slab (a 32 × 80 door gets a 34 × 82-1/2 opening), a window opening is typically 1/2 inch larger than the unit in each direction, and a common pocket-door kit needs twice the door width plus 1 inch by the door height plus 4-1/2 inches.
Always confirm against the manufacturer’s published rough opening, which takes priority over any rule of thumb, especially for exterior doors, replacement windows and pocket-door kits. Use the custom allowances when the spec sheet gives different numbers.
How to use the rough opening calculator
- 1Choose the opening type: prehung door, window, pocket door or custom.
- 2Enter the door slab or window unit width and height in inches.
- 3For windows, enter the height of the top of the opening above the subfloor.
- 4Add the thickness of any flooring that is not installed yet.
- 5Read the rough opening, header and stud lengths and check them against the spec sheet.
Formula and method
The rough opening is the unit size plus an allowance for the frame and shimming space. For a prehung door the allowance is about 2 in in width (two 3/4 in jambs plus 1/4 in of shim space each side) and 2-1/2 in in height (head jamb, shim space and floor clearance). Windows are typically 1/2 in larger than the unit both ways. A common pocket-door kit needs twice the door width (m = 2) plus 1 in, and the door height plus 4-1/2 in for the track.
The header spans the opening and bears on a 1-1/2 in jack stud at each end, so it is 3 in longer than the rough width. Jack studs stand on the 1-1/2 in bottom plate. For windows, the rough sill height is the top of the opening minus the opening height, and cripples under a single flat sill are the sill height minus 3 in (plate plus sill).
- W, H
- Door slab or window unit width and height (in)
- a_w, a_h
- Width and height allowances (in)
- m
- Width multiplier (2 for pocket doors, else 1)
Worked examples
32 × 80 in interior prehung door
Adding 2 in to the width and 2-1/2 in to the height gives a 34 × 82-1/2 in rough opening. The header is 34 + 3 = 37 in long, and jack studs are 82-1/2 − 1-1/2 = 81 in.
36 × 48 in window with the header at 82-1/2 in
The window opening is 36-1/2 × 48-1/2 in. With its top at 82-1/2 in, the rough sill sits at 82.5 − 48.5 = 34 in, so cripples under a flat sill are 34 − 3 = 31 in. The header is 39-1/2 in.
30 × 80 in pocket door
The pocket must hold the full door, so the opening is 2 × 30 + 1 = 61 in wide and 80 + 4-1/2 = 84-1/2 in tall. The header is 64 in and jack studs 83 in.
Frequently asked questions
What is the rough opening for a 36 inch door?+
For a 36 × 80 in prehung door the usual rough opening is 38 in wide by 82-1/2 in tall. Some exterior door manufacturers specify 38-1/4 or 38-1/2 in, so check the spec sheet before framing an exterior door.
How much bigger should a window rough opening be?+
Most windows need an opening about 1/2 in wider and 1/2 in taller than the unit size, leaving roughly 1/4 in each side for shims and insulation. Manufacturers list the exact rough opening for each size.
Is a door listed by slab size or rough opening?+
Interior doors are usually sold by slab size, such as 30 × 80 (2-6 × 6-8 in trade terms). The rough opening is larger to fit the jambs and shims; the 2 in × 2-1/2 in rule converts slab size to opening size.
How long should a door header be?+
The header is the rough opening width plus 3 in, so it bears 1-1/2 in on a jack stud at each side. Openings wider than about 4 ft or in load-bearing walls may need double jacks and an engineered header size.
What is the rough opening for a pocket door?+
A common pocket door frame kit needs a rough opening of twice the door width plus 1 in, and the door height plus 4-1/2 in. A 30 × 80 in door needs about 61 × 84-1/2 in, but kits vary, so follow the kit instructions.