A rough opening is the framed hole in a wall where a door or window will sit, measured before any jamb, trim, or finish goes in. Get it wrong and the unit won't fit or won't seal. As a general rule, a pre-hung door needs a rough opening about 2 inches wider and 2 to 2.5 inches taller than the door slab, but always confirm against the manufacturer's spec sheet before you order.
What Is a Rough Opening?
The rough opening is the space between the studs, the header above, and the sill or floor below. It is always larger than the door or window itself, because the unit needs room for shims, leveling, and insulation around the frame. Builders measure it at the framing stage. Remodelers measure it after the old trim or finish materials come off.
Door Size vs. Frame Size vs. Rough Opening
Most ordering mistakes come from mixing up three different measurements.
| Term | What it means |
| Door (slab) size | The size of the door panel itself, like 3'0" x 6'8" |
| Unit (frame) size | The door plus its frame, slightly larger than the slab |
| Rough opening | The framed hole in the wall, larger than the unit |
When you shop, check whether the size listed is the door or the full unit. This matters most with sidelites. A door listed as 3'0" x 8'0" with two sidelites can be a 64" x 96" unit overall, and the rough opening has to fit the whole unit, not just the door.
How to Measure a Rough Opening for a Prehung Door
- Measure the width - between the studs at three points: top, middle, and bottom. Use the smallest number.
- Measure the height - from the floor to the underside of the header at three points: left, center, and right. Use the smallest number.
- Account for flooring - If finished flooring isn't in yet, measure to where the finished floor surface will be, since flooring thickness reduces the available height.
- Check for square - by measuring both diagonals. If they differ by more than a quarter inch or so, tell your installer before you order.
- Compare against the spec sheet -Prehung doors typically need about 2 inches of extra width and 2 to 2.5 inches of extra height over the door slab.
| Door size (width x height) | Approximate rough opening |
| 2'8" x 6'8" | About 34" x 82" |
| 3'0" x 6'8" | About 38" x 82" |
| 3'0" x 8'0" | About 38" x 98" |
How to Measure for a New Construction Window
New construction windows have a nailing fin and install into a framed opening before siding goes on. Measure the framed opening's width between the studs and its height from the sill plate to the header, at several points, and note the smallest. These windows are typically sized a half inch or so smaller than the rough opening, which leaves room to shim and level. Confirm the exact allowance for the window you're ordering. Not sure which window type your project needs? See our guide to new construction vs. replacement windows.
How to Measure for a Replacement Window
Replacement windows fit inside an existing frame, so you measure the existing opening, not the wall framing.
- Measure the width inside the frame at the top, middle, and bottom. Use the smallest.
- Measure the height at the left, center, and right. Use the smallest.
- Check the frame depth to confirm the new unit will fit.
- Ask your supplier whether the listed size is the actual unit size or the opening size, since replacement windows are often made slightly smaller than the opening to allow for shimming.
Common Measuring Mistakes
- Measuring from the trim instead of the framing or the inside of the frame
- Using one measurement instead of three, then ordering to the largest
- Forgetting that flooring changes the finished height
- Treating the door size as the unit size on a sidelite door
- Ordering the wrong handing. Stand outside facing the door: hinges on your left mean left-hand, hinges on your right mean right-hand. Also confirm inswing or outswing
Frequently Asked Questions
Q. What is a rough opening for a door?
A. It's the framed opening in the wall, measured between the studs and from the floor to the header, before any jamb or trim is installed. It's always larger than the door unit to leave room for shimming and leveling.
Q. How much bigger than the door should the rough opening be?
A. For most prehung doors, about 2 inches wider and 2 to 2.5 inches taller than the door slab. Exact allowances vary by manufacturer, so check the spec sheet.
Q. Is the rough opening the same as the window size?
A. No. The rough opening is the framed hole, and the window is the unit that goes inside it. New construction windows are typically a half inch or so smaller than the rough opening.
Q. How do I measure for a replacement window?
A. Measure the inside of the existing frame at three points for width and three for height, use the smallest of each, and check the frame depth. Ask your supplier how their listed sizes relate to the opening.
Q. Do sidelites change the rough opening?
A. Yes. The rough opening has to fit the entire unit, including the sidelites, so use the overall dimensions listed for the unit, not just the door size.
Ready to order? Have your smallest width, smallest height, handing, and finished floor height on hand. Shop our windows, exterior doors, and interior doors, or contact our DFW team with your measurements and sizing questions.