How to use the rough opening calculator
Framing an opening too tight means planing jambs or reframing on site, and framing it too loose leaves big shim gaps that are hard to seal. This calculator turns a door or window size into the rough opening, then works out the header length that bears on the jack studs and, for windows, where the rough sill sits. It is meant for quick checks on drawings and framing lists.
- Choose units and the opening type. Switching type loads the usual allowances: 2 in by 2½ in for prehung doors and ½ in each way for windows.
- Enter the size. For a door use the slab size, such as 36 × 80 in. For a window use the unit's outside frame size.
- Check the allowances against the manufacturer's installation guide and change them if it gives a different rough opening.
- Set the jack studs and stud thickness. Use the number of jacks the header span table or engineer calls for.
- For windows, enter the head height above the subfloor to get the rough sill height.
Rough opening formulas
Rough opening width = unit width + width allowance
Rough opening height = unit height + height allowance
Header length = rough opening width + 2 × jacks per side × stud thickness
King-to-king distance = header length (kings sit outside the jacks)
Rough sill height = head height − rough opening height
The jack studs (also called trimmers) carry the header, and the king studs run full height beside them. Cripple studs fill the space above the header and below the rough sill on the usual layout spacing. The table shows common sizes using the default allowances.
| Unit | Rough opening | Header, one jack each side |
|---|---|---|
| 30 × 80 in door | 32 × 82½ in | 35 in |
| 32 × 80 in door | 34 × 82½ in | 37 in |
| 36 × 80 in door | 38 × 82½ in | 41 in |
| 35½ × 59½ in window | 36 × 60 in | 39 in |
| 47½ × 47½ in window | 48 × 48 in | 51 in |
Worked examples
A 36 inch entry door
A 36 × 80 in prehung door gets a rough opening of 38 × 82½ in. With one 1½ in jack each side the header is 38 + 3 = 41 in long, and the king studs sit 41 in apart. The jacks are cut to 82½ in from the subfloor to carry the header.
A bedroom window
A window unit is 35½ × 59½ in. With ½ in allowances the rough opening is 36 × 60 in and the header is 39 in. With the head lined up with the doors at 82½ in, the rough sill sits at 22½ in above the subfloor. If this is an egress window, check the sill height and clear opening against the code limit.
A wide patio door
A 72 × 80 in sliding door with two jack studs each side has a 74 × 82½ in rough opening and a header of 74 + 4 × 1½ = 80 in. Wide openings like this usually need an engineered or span-table header and should be checked for point loads above.
Who this calculator helps
- Architects and technologists dimensioning openings on framing plans and window and door schedules.
- Framers and carpenters cutting jacks, kings and headers for a wall.
- Contractors checking that ordered windows and doors match the framed openings before installation.
- Renovators resizing an opening for a replacement door or window.
Practical tips
- Check bedroom windows with the egress window calculator and glazing ratios with the window-to-wall ratio calculator.
- Size framing members for longer spans with the joist and rafter span calculator, and work out casing lengths with the baseboard and trim calculator.
- Allow for the finished floor build-up when framing door openings over thick tile, stone or underfloor heating.
- Measure the rough opening at the top, middle and bottom, and check the diagonals for square before ordering.
- Leave the shim space open for low-expansion foam or backer rod and sealant, and flash the sill before installing the unit.
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