Cutting the wrong rough opening is one of the most expensive framing mistakes you can make — it means either patching extra space or resizing the opening entirely. Understanding how door unit size, rough opening, and header size relate to each other will save you time, material, and failed inspections. This guide covers the key measurements and decisions for interior, exterior, and ADA-compliant door installations.
Door Size vs. Rough Opening
The door unit size printed on the package is the nominal size of the pre-hung assembly — door slab, jambs, and stops all together. The rough opening you frame must be larger than the door unit to allow for jamb thickness and shimming. Standard practice adds 2 inches to the width (for a 0.75-inch jamb leg on each side plus 0.25-inch shim space per side) and 2.5 inches to the height (for a threshold or door bottom plus shim space at the header).
Some framers prefer to add 2.5 inches to the width on exterior doors where thicker jambs or weatherstripping adjustments may be needed. The key is to match the rough opening to the manufacturer's installation instructions for your specific door unit. If you are framing before the door is purchased, use the standard +2 inches / +2.5 inches rule and verify before drywall goes up. Fixing a tight rough opening after drywall installation requires cutting into the wall and reframing — a costly and avoidable problem.
Header Size for Load-Bearing Walls
In a load-bearing wall, the header above the door carries all the structural load that the removed studs would have supported. Header size depends on the span (rough opening width), the wall type (single vs. two-story), and the load above (roof vs. floor). The IRC Table R602.7 provides minimum header sizes for common residential applications. For a 3-foot-wide door in a single-story exterior wall, a doubled 2×6 typically suffices. A 6-foot-wide opening usually requires a doubled 2×10 or an LVL beam.
Non-bearing walls — interior partition walls that do not carry floor or roof loads — can use a much simpler cripple header: a flat 2×4 spanning the opening with no jack studs required. This saves significant lumber, but you must confirm the wall is truly non-bearing before simplifying the framing. Oversizing the header in a non-bearing wall wastes material but does no structural harm. Undersizing in a bearing wall can cause deflection, sticking doors, and eventually structural damage.
ADA Requirements and Accessibility
ADA requires a minimum 32-inch clear passage width, measured from the door face to the opposite stop with the door open 90 degrees. A 32-inch door unit provides approximately 29.5 inches of clear width — close but technically below the ADA threshold. A 34-inch or 36-inch door reliably meets the 32-inch clear requirement. For accessible routes in commercial buildings, 36-inch doors are the standard minimum, with 42-inch doors used where wheelchairs need to pass through simultaneously with a companion.
Beyond width, ADA also addresses maneuvering clearance — the floor space adjacent to the door needed for a wheelchair user to open the door. Latch-side clearance is required: at least 18 inches on the pull side of a swinging door and 12 inches on the push side. If your door is at the end of a narrow corridor, the approach clearance may constrain your door size choice as much as the opening itself. This calculator flags doors that meet or miss the 32-inch clear threshold so you can address it during the planning phase rather than after framing.