Door Jamb Width Calculator

1. Pick the stud depth (2×4, 2×6 or metal stud).
2. Pick the finish on side A (interior drywall thickness).
3. Pick the finish on side B (exterior or hallway drywall thickness, or none).
4. Read the jamb width, nearest standard size and whether you need an extension jamb.

Here is what each result line means:

A door jamb width calculator works out the jamb size you need from the stud depth and the finish thickness on each side of the wall. The jamb must be wide enough to cover the full wall thickness so the casing (trim) sits flat on both faces.

A jamb that is too narrow exposes the rough opening behind the trim. A jamb that is too wide sticks out past the drywall and must be planed or shimmed.

d is the stud depth in inches, a is the finish thickness on side A and b is the finish thickness on side B.

1. Add the stud depth and both finish thicknesses.
2. Compare the total to the three standard jamb sizes: 4-9/16, 5-1/2 and 6-9/16 inches.
3. If the wall is thicker than 4-9/16 inches, an extension jamb may be needed.

You are installing a prehung door in a 2x4 wall (3.5 in stud depth) with 1/2-inch drywall on both sides.

Calculation: wall thickness = 3.5 + 0.5 + 0.5 = 4.5 inches. The nearest standard jamb is 4-9/16 in (4.5625 in), which is 0.0625 in (1/16 in) wider than the wall. That 1/16-inch clearance is the standard tolerance.

Stud Size

A 2×4 stud is 3.5 inches deep. A 2×6 is 5.5 inches. Metal studs come in 2-1/2, 3-5/8 and 6-inch depths. The stud depth is the biggest factor in the jamb width.

Drywall Thickness

Standard interior drywall is 1/2 inch. Fire-rated drywall is 5/8 inch. Older homes may have 3/8-inch drywall. Each 1/8-inch change adds or subtracts 1/8 inch from the jamb width (1/4 inch if both sides change).

Exterior Sheathing and Siding

If side B has sheathing (1/2 in) and siding (3/4 in) instead of drywall, the wall is much thicker. You may need a custom jamb or an extension.

Tile or Wainscoting

Tile backerboard (1/4 to 1/2 in) plus tile (1/4 to 3/8 in) adds to the wall finish thickness. Measure the actual finished surface before ordering the door.

The three standard prehung door jamb widths and the wall types they fit.

These sizes include about 1/16-inch tolerance over the wall thickness.

Ordering a Prehung Door

The jamb width is a required dimension when you order a prehung interior or exterior door.

Replacing a Door in an Older Home

Older walls may have non-standard drywall or plaster thickness. Measure and calculate instead of assuming.

Installing in a 2×6 Wall

A standard 4-9/16 jamb does not fit a 2×6 wall. You need either a 6-9/16 jamb or an extension.

Mixed Finishes

When one side of the wall has tile and the other has drywall, the calculator shows the exact width needed.

1. Assuming 4-9/16 Fits Every Wall

The 4-9/16-inch jamb fits a standard 2×4 wall with 1/2-inch drywall. Fire-rated drywall (5/8 in), plaster, or 2×6 walls need a wider jamb.

2. Measuring the Old Jamb Instead of the Wall

The old jamb may have been trimmed or shimmed. Measure the actual wall thickness at the rough opening.

3. Forgetting the Exterior Sheathing

An exterior door jamb must cover the sheathing and sometimes the siding, not just the stud and drywall.

4. Ordering a Jamb That Is Too Narrow

A narrow jamb exposes the rough framing behind the casing. The casing cannot sit flat and the joint opens up.

5. Ignoring the 1/16-inch Tolerance

Standard jambs are 1/16 inch wider than the nominal wall thickness. This tolerance prevents a tight fit that makes the casing bow.

The calculator adds the stud depth and two finish thicknesses. It does not account for shims, adhesive, out-of-plumb studs or uneven drywall. Always verify the wall thickness with a tape measure at the rough opening.

4-9/16 inches, assuming 1/2-inch drywall on both sides. That is 3.5 + 0.5 + 0.5 = 4.5 inches of wall, plus 1/16-inch tolerance.

6-9/16 inches, assuming 1/2-inch drywall on both sides. That is 5.5 + 0.5 + 0.5 = 6.5 inches of wall, plus 1/16-inch tolerance.

A strip of wood added to the edge of a standard jamb to make it wider. You need one when the wall is thicker than the standard jamb size, such as a 2×6 wall with a 4-9/16-inch jamb.

Measure the wall thickness at the rough opening, from the face of the finish on one side to the face of the finish on the other side. That measurement is the jamb width you need.

No. The jamb is the frame inside the rough opening. The casing (trim) is nailed to the face of the wall around the jamb. They are separate pieces.

Order a custom jamb from the door manufacturer, or buy a standard jamb and add extension strips to make up the difference.

The standard jamb is 1/16 inch wider than the nominal wall thickness so the casing can sit flat without the jamb being flush or recessed. This small gap is hidden by the casing.

Yes. A jamb that is slightly too wide can be planed or ripped down. But ordering the correct width saves time and gives a cleaner fit.

Fire-rated walls use 5/8-inch drywall instead of 1/2-inch. A 2×4 wall with 5/8-inch drywall on both sides is 4.75 inches thick and needs a 4-13/16-inch or wider jamb, not a standard 4-9/16.

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

• Doornmore, how to measure jamb width (standard jamb widths for 2×4 and 2×6 walls with drywall).
• Bob Vila, standard size for doors (standard door and jamb dimensions for residential construction).
• Door Design Lab, how to measure jamb width (wall thickness measurement and jamb selection guide).
• Fine Homebuilding, door jamb sizing (arithmetic check: 3.5 + 0.5 + 0.5 + 1/16 tolerance = 4-9/16 in).