Measure the door frame opening, not the door itself

The opening is what matters. Measure the space where the door sits now, not the existing door. Most replacement doors come in standard sizes — 32, 36, or 42 inches wide for interior doors, and 36 or 42 inches for exterior doors — and the frame opening determines which size you need. If your opening is 35.5 inches wide, you need a 36-inch door. If it is 35 inches, you need a 32-inch door.

Take three measurements of the width: one at the top of the opening, one in the middle, and one at the bottom. Write down all three. If they differ by more than a quarter inch, your frame is out of square, and you will need to know that before you buy. The smallest of the three is the one you use to pick your door size.

Do the same for height. Measure on the left side, in the middle, and on the right side of the opening. Again, use the smallest number. Standard door heights are 80 inches for interior doors and 80 or 84 inches for exterior doors.

Key Takeaways

  • Measure the frame opening, not the existing door, at three points for both width and height, and use the smallest measurement for each.
  • Standard interior door widths are 32 or 36 inches; standard exterior widths are 36 or 42 inches; standard heights are 80 inches for interior and 80 or 84 inches for exterior.
  • If your three measurements differ by more than a quarter inch, the frame is out of square and you should tell the installer or retailer before ordering.
  • Measure the depth of the frame jamb (the side pieces) to know whether you need a standard or thick-wall door frame.
  • Check for swing direction by standing outside the door and noting which side the hinges are on, or by standing inside and noting which way the door opens.

Measure the frame jamb depth

The jamb is the wood or metal frame piece that runs along the sides and top of the opening. Its thickness tells you what depth of replacement door frame you need. Measure from the inside edge of the jamb to the outside edge. Standard jambs are 4.625 inches deep, but older homes and thick-wall construction may be 5.25 inches or deeper.

If your jamb is thicker than standard, you have two options: buy a door frame kit with an adjustable jamb extension, or have the installer shim and adjust the frame to fit. Knowing the depth before you order saves time and money.

Determine the swing direction

The swing direction tells you which way the door opens and which side the hinges go on. Stand outside the door (or on the side the door swings away from) and look at the hinges. If the hinges are on your right, it is a right-hand door. If they are on your left, it is a left-hand door.

Another way: stand inside the room the door opens into. If the hinges are on your right, it is a left-hand door. If they are on your left, it is a right-hand door. The two methods give opposite answers because you are standing on opposite sides — pick one method and stick with it to avoid confusion.

Write down the swing direction clearly. Many retailers ask "left or right" without specifying which side you should be standing on, so include a note like "right-hand, hinges on right when viewed from outside" to be certain.

Check for level and plumb

A frame that is not level or plumb will cause the new door to stick or not close properly. Use a 2-foot or 4-foot level. Place it vertically against the left jamb and check whether the bubble is centered. Do the same on the right jamb. Then place it horizontally across the top of the opening.

If any of these is off by more than a quarter inch over the height or width of the door, the frame is out of plumb or level. Tell the installer or retailer. Some doors can be shimmed to fit; others may need the frame to be straightened first. Knowing this before you order prevents surprises during installation.

Measure the existing door thickness

Interior doors are typically 1.375 inches thick. Exterior doors are usually 1.75 inches thick. Measure the edge of your existing door with a tape measure to confirm. If you are replacing an interior door with an exterior door (or vice versa), the new door may not fit the existing hinges and strike plate without adjustment.

If the thickness is unusual — thicker or thinner than standard — note it. Some older homes have doors that are 1.25 inches or even 2 inches thick. The replacement door should match the existing thickness, or the hardware will need to be adjusted or replaced.

Note the hinge and strike plate locations

Measure the distance from the top of the frame to the center of the top hinge. Do the same for the middle and bottom hinges. Standard spacing is 10 inches from the top, 10 inches from the bottom, and one hinge in the middle. If your spacing is different, note it.

Also measure the height of the strike plate (the metal piece on the jamb that the latch enters). Standard height is 38 to 40 inches from the floor. If your strike plate is at a different height, the new door may need adjustment or the strike plate may need to be moved.

Record all measurements on a sketch

Draw a straightforward rectangle representing the door opening. Write the width measurements (top, middle, bottom) along the top and sides. Write the height measurements (left, middle, right) along the left and right sides. Add the jamb depth, the swing direction, and notes about level and plumb. Take a photo of the sketch and the opening with your phone.

When you go to the store or call a retailer, you will have everything they need to confirm the right size and style. If the measurements are unclear or the frame is out of square, bring the photo — a picture of the actual opening is worth more than a written description.

Frequently Asked Questions

What if my opening is between two standard sizes?

Order the smaller size. A 35-inch opening gets a 32-inch door, not a 36-inch door. The installer will shim and adjust the frame to close the gap. Ordering a door that is too large will not fit without cutting the frame.

Do I need to remove the old door before measuring?

No. You can measure around the existing door. Measure the opening from jamb to jamb, not from the outside of the frame. If the existing door is warped or swollen, it may be sitting tight in the opening — in that case, remove it first to get an accurate opening measurement.

What if the frame is out of square by half an inch or more?

Tell the installer or retailer before you order. A frame that is significantly out of square may need to be straightened, shimmed heavily, or have the opening adjusted. Some installers can handle this; others will not. Knowing this in advance prevents delays.

Can I use the old door's measurements instead of measuring the opening?

No. The old door may have been cut down, warped, or installed incorrectly. Always measure the opening itself. The opening size is what determines the replacement door size.

Should I measure in inches or centimeters?

Use inches. All standard door sizes in North America are listed in inches. Retailers and installers expect measurements in inches. If you measure in centimeters, convert before you order.