Section 01Understanding Door Dimensions

Before measuring, you need to understand the three different size references used in door installations. Confusing these measurements is one of the most common causes of ordering errors.

1. Rough Opening (R.O.)

The rough opening is the structural framed opening in the wall, measured from stud-to-stud horizontally and from subfloor-to-header vertically. This is the measurement you'll use when ordering a pre-hung door. The rough opening provides space for the door frame plus shimming room for leveling and plumbing the installation.

2. Frame/Jamb Size

The frame size (also called unit size) includes the door jamb that fits within the rough opening. Frame size is typically ½″ to 1″ smaller than the rough opening on each side to allow for shimming. When you order a pre-hung door by door size, the manufacturer calculates the appropriate frame size.

3. Door Slab Size

The door slab is the door itself, without the frame. It's smaller than the frame opening to allow for a ⅛″ reveal (gap) around all sides for proper operation. When replacing just the door slab in an existing frame, measure the existing door precisely.

Section 02Tools & Equipment Checklist

Having the right tools ensures accurate measurements. Gather these before you begin.

Essential Tools

• 25-foot retractable tape measure with a stiff metal blade
• 6-foot level for checking plumb (vertical alignment)
• 2-foot level or torpedo level for horizontal surfaces
• Pencil and notepad for recording measurements immediately
• Utility knife for removing caulk if needed
• Pry bar for carefully removing interior trim/casing
• Wide putty knife to protect wall surfaces during trim removal

Recommended Extras

• Laser distance measurer for tall openings
• Framing square to verify 90° corners
• Flashlight to illuminate shadowed areas
• Camera or smartphone to document existing conditions
• A helper to hold tape measures and verify readings

Section 03Step-by-Step Measurement Instructions

Step 1: Prepare the Opening

For replacement doors, you must remove the interior trim (casing) to access the true rough opening. Measuring an installed door or frame will not give you accurate rough opening dimensions.

1. Use a utility knife to score along the caulk line between the trim and wall to prevent paint tearing.
2. Insert a wide putty knife behind the trim to protect the wall surface.
3. Carefully pry off the trim using the pry bar, working from one end to the other.
4. Remove any insulation or debris from the gap between the frame and rough opening.
5. Inspect the rough opening for rot, damage, or previous modifications.

Step 2: Measure the Width

Take three width measurements to account for any irregularity in the opening:

Record the smallest measurement. This ensures your door will fit at the narrowest point.

Step 3: Measure the Height

Take two height measurements:

Record the smallest measurement. If you're installing over existing flooring, also note the finished floor height for threshold planning.

Step 4: Measure the Wall Thickness (Jamb Depth)

Wall thickness determines the jamb depth you need. Measure from the exterior wall surface to the interior wall surface at six points — top, middle, and bottom on both sides.

Record the largest measurement. Unlike width and height, you want the jamb to cover the thickest part of the wall. If your measurement falls between standard sizes, order the next size up.

Step 5: Check for Square and Plumb

An out-of-square or out-of-plumb opening can cause significant installation problems.

Checking for Square (Diagonal Method)

Measure diagonally from the top-left corner to the bottom-right corner, then from the top-right to bottom-left. If the two measurements match within ¼″, the opening is square. Larger variances may require frame modification.

Checking for Plumb (Vertical)

Place a 6-foot level against each side jamb stud. Maximum acceptable deviation is ⅛″ to ¼″ over the height of the opening.

Checking for Level (Horizontal)

Place a level across the threshold/sill area and across the header. Note any deviation from level.

Section 04Standard US Door Sizes Reference

These are industry-standard dimensions. Always verify with your specific door manufacturer before ordering.

Single Entry Door Sizes

Double Door Configurations

Door with Sidelights

Standard Door Thickness

Now that you know your rough opening dimensions and the standard door size that fits, you can start shopping. Browse our full collection of exterior entry doors — including fiberglass, steel, and solid mahogany options — or use the Door Configurator to build a complete door system matched to your exact measurements.

Section 05Special Situations & House Types

New Construction vs. Replacement

Older & Historic Homes

Homes built before 1950 often have non-standard door sizes. Common challenges include doors as narrow as 30″–32″ (vs. today's 36″ standard), heights of 78″ instead of 80″, thinner 1-3/8″ slabs, significant settling, and balloon framing with different settling patterns.

Solution: Take extra-detailed measurements and consider custom door sizing. Many manufacturers offer extensive size options that can accommodate historic dimensions. Document everything carefully — custom orders cannot be returned.

Architectural Styles

Regional & Climate Considerations

Coastal / Hurricane Zones

Doors in Florida's High Velocity Hurricane Zones (Miami-Dade and Broward Counties) must be rated for 170–200 mph winds with large missile impact testing. Wind-Borne Debris Regions within one mile of the coast where wind speeds exceed 130 mph require impact-resistant glass. These specialized doors may have different sizing requirements.

Cold Climates

Northern climates prioritize thermal performance with R-5 or higher insulation values. Homes in these areas often have 2×6 walls requiring 6-9/16″ jamb depth rather than the standard 4-9/16″. Account for this when measuring jamb depth.

Flood Zones

FEMA regulations require elevated thresholds in flood zones. In Zone AE, living space must be at minimum 1 foot above Base Flood Elevation. Measure carefully and verify local requirements.

Section 06Pre-Hung vs. Slab Doors

What Do Entry Doors Actually Cost?

If you're replacing just the door slab and your existing frame is in good condition, individual slabs at Pease Doors start at $407 for a steel 6-panel door. Fiberglass slabs — which offer better insulation and don't dent — range from $509 for a classic 6-panel to $611 for a Craftsman 3-panel. For a premium solid wood look, our hand-crafted Brazilian mahogany slabs start at $999, with glass options like the V-Groove half lite and ¾ glass 6-lite at $1,599.

However, buying a slab means you still need to source framing, weatherstripping, a threshold, hardware, and hinges separately — which adds up quickly and introduces the risk of compatibility issues.

Section 07Measuring for Sidelights & Transoms

Adding sidelights or transoms significantly expands the rough opening requirements and may introduce structural considerations that require professional evaluation.

Sidelight Measurements

Standard sidelight widths are 12″ and 14″, built in 2-inch increments. To calculate the rough opening width for a door with sidelights:

1. Start with your door width (e.g., 36″)
2. Add sidelight width(s) — e.g., 12″ each side = 24″
3. Add approximately 3″ for trim and framing between units
4. Add 2″ for rough opening shimming space

Example: A 36″ door with two 12″ sidelights requires a rough opening of approximately 64-3/8″ wide.

Transom Measurements

Standard transom height is 12″ with 13½″ total unit height including framing. Custom transoms can reach 24″ but nothing larger. For balanced proportions, the transom should be ⅓ to ¼ of the door height. Account for approximately 1½″ of transom frame above the door frame, and verify structural capacity for wider headers.

Section 08ADA Accessibility Requirements

The Americans with Disabilities Act establishes specific door requirements for commercial buildings, public facilities, and certain residential buildings. Even if not legally required, these standards provide excellent guidance for universal design.

Clear Width

Minimum clear width is 32″ (measured from face of door to door stop when open 90°), with a 48″ maximum. For openings deeper than 24″, minimum clear width increases to 36″. A standard 36″ door provides approximately 32″ of clear opening.

Threshold

Maximum threshold height is ½″ for new construction. Thresholds above ¼″ must have a beveled edge. Exterior sliding doors or altered thresholds allow up to ¾″.

Hardware

Hardware must be mounted 34″ to 48″ above the floor and must be operable with one hand without tight grasping, pinching, or twisting. Round doorknobs are not compliant — lever handles are required. Interior hinged doors require a maximum of 5 pounds of force to open.

Where ADA Applies

• All commercial and public facilities built after January 26, 1992
• State and local government facilities
• Multi-family housing with 4+ units first occupied after March 13, 1991
• Elevator buildings: all units must meet accessibility requirements
• Buildings without elevators: all ground-floor units must comply

Section 09Common Measurement Mistakes to Avoid

Section 10Measurement Worksheet

Use this interactive worksheet to record your measurements. The form automatically identifies the correct value to use for ordering.