Rough Opening Calculator

1. Pick the opening type (prehung door, bifold, pocket door or window).
2. Enter the unit width and unit height in inches.
3. Read the rough opening width, height and clearance breakdown.

Here is what each result line means:

For the header size that goes above this opening, see the header size calculator. For a full wall material list, see the framing calculator.

A rough opening is the framed hole in a wall where a door or window unit will be installed. It is always slightly larger than the unit itself to leave room for the jamb, shims, insulation and adjustment.

Getting the rough opening right is critical: too small and the unit will not fit, too large and you waste shimming material and compromise the seal.

Each opening type has a standard clearance:

• Prehung door: +2 in width (1 in each side), +2-1/4 in height.
• Bifold door: width equals the door width (track mounts inside), +3/4 in height.
• Pocket door: width = 2 x door width + 1 in (pocket cavity), +4-1/2 in height.
• Window: +1/2 in width (1/4 in each side), +1/2 in height.

1. Read the unit width and height in inches.
2. Add the clearance for the selected type.
3. Report the rough opening dimensions.

You are framing for a 36-inch prehung interior door, 80 inches tall.

Calculation: RO width = 36 + 2 = 38 inches. RO height = 80 + 2.25 = 82-1/4 inches. Frame the opening 38 inches wide by 82-1/4 inches tall.

Opening Type

Pocket doors need the widest rough opening (twice the door width plus 1 inch) because the door slides into the wall cavity.

Manufacturer Specs

Some window and door brands specify clearances different from the standard rules. Always check the installation guide for your unit.

Floor Finish

If the finished floor is not yet installed, add its thickness to the height clearance so the door clears the floor.

Exterior vs Interior

Exterior doors may need extra height clearance (up to 3-3/4 in for outswing units with a threshold) compared to interior doors.

Sill Pan and Flashing

Windows with a sill pan or flashing membrane may need an extra 1/4 to 1/2 inch in height.

The table below compares rough openings for a 36-inch-wide, 80-inch-tall unit across door types.

New Construction Framing

Frame every opening to the correct size before drywall.

Door or Window Replacement

Verify the existing rough opening fits the new unit, or plan how to resize it.

Adding an Opening to an Existing Wall

Cut the wall to the right rough opening the first time.

Pocket Door Conversions

Pocket doors need a much wider rough opening than swing doors. Check before committing.

1. Making the Opening Too Tight

Without clearance, the unit will not fit or will bind when shimmed. A too-tight opening means recutting studs.

2. Forgetting the Floor Finish

If the subfloor is down but the tile or hardwood is not, add the finish thickness to the height allowance.

3. Using Prehung Clearances for Pocket Doors

A pocket door needs twice the width. Using prehung rules leaves the cavity too short for the door to slide in.

4. Ignoring Manufacturer Specs

Some windows need more clearance for a nailing flange or integral J-channel. Check the spec sheet.

5. Measuring the Door Slab Instead of the Unit

A 30-inch slab comes in a 32-inch jamb assembly. Measure the assembled unit, not the slab alone.

The calculator uses standard industry clearances verified against Home Depot, Lowe’s and Johnson Hardware installation guides. Some manufacturers may specify different values.

For a standard prehung door: 38 inches wide by 82-1/4 inches tall (unit width + 2 in, unit height + 2-1/4 in).

32 inches wide by 82-1/4 inches tall for a prehung door.

For prehung doors, 2 inches wider (1 inch each side) and 2-1/4 inches taller.

Twice the door width plus 1 inch for the width, and the door height plus 4-1/2 inches for the height.

The width is the same as the bifold opening (the track mounts inside). The height is the door height plus 3/4 inch.

Typically 1/2 inch wider and 1/2 inch taller than the window unit (1/4 inch on each side). Check the manufacturer guide.

Measure the assembled prehung unit if possible. If you only have the slab width, add 2 inches for the jamb assembly, then add the rough opening clearance.

Shim the extra space. Up to about 1/2 inch extra on each side is normal. More than that may need filler strips.

Yes. If the finished floor is not installed yet, add its thickness to the height clearance so the door swings over the floor.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• Home Depot, Prehung Door Installation Guide (rough opening clearance rules for prehung entry doors).
• Bob Vila, Standard Door Sizes (common door and rough opening dimensions).
• Johnson Hardware, 1500 Series Pocket Door Installation (pocket door rough opening formula: 2x width + 1 in).
• Andersen Windows, Rough Opening Guidelines (window rough opening clearance practice).