How to Measure for a New Door

Quick answer

Measure width and height in three places (top, middle, bottom for width; both sides for height). For a slab, measure the door itself. For prehung, measure rough opening jamb to jamb and floor to header. Note thickness, swing, and handing. Use our tape measure guide; pair with weatherstripping if drafts persist after install.

Ordering the wrong door size delays projects and creates return hassles. Measure carefully before you buy—standard sizes exist, but older homes and remodels often do not match catalog labels.

Slab vs prehung: what to measure

If you are only replacing a damaged slab and keeping the hinge locations, match the old slab dimensions and hinge spacing.

Measure door width (slab)

1. Remove trim only if needed for clarity—usually measure the door face.
2. Measure top, middle, and bottom width.
3. Record the smallest width if they differ slightly.

Standard nominal widths (actual may be slightly under): 24", 28", 30", 32", 36".

Measure door height (slab)

Measure left and right from top of door to bottom edge. Use the shorter reading if unequal.

Common height: 80" (6'-8"). Some entries use 84" or 96" heights.

Measure thickness

Typical interior slabs: 1-3/8" or 1-3/4". Exterior doors are often 1-3/4". Match for hinge mortises and latch alignment.

Rough opening for prehung doors

Measure inside the frame where the old unit sat:

• Width: jamb to jamb in three places
• Height: floor (or subfloor) to head jamb in two places
• Depth: jamb width must fit wall thickness (4-9/16", 6-9/16", etc.)

Manufacturers specify required rough opening—usually slightly larger than the unit. Compare your readings to the spec sheet, not guess.

Swing and handing

Stand on the hinge side when the door opens toward you:

• Hinges on left = left-hand
• Hinges on right = right-hand

Exterior security doors and some sliders use different conventions—confirm on the order form.

Account for flooring

If new flooring will be higher than current, the door may drag. Plan undercut or order height accordingly before finishing floor install—coordinate with flooring measurement.

After install: seal gaps

Even a well-sized door needs weatherstripping at exterior entries. See how to weatherstrip a door for jamb seals and sweeps.

DIY vs professional install

Homeowner-friendly: swapping a slab on existing hinges when sizes match; installing prehung units in standard openings with good shims and level.

Hire a carpenter for:

• Non-standard masonry openings
• Structural header changes
• Severely out-of-square frames
• Entry doors with sidelights and multipoint hardware

Tools

A sturdy tape measure from your beginner DIY tool kit is enough. A level helps check head jamb straightness before ordering custom sizes.