Take measurements in three places on each dimension

Measure your door frame in three spots — top, middle, and bottom for width, and left, center, and right for height — because frames settle and shift over time. Write down all six numbers. If the three measurements for width differ by more than a quarter inch, or the three for height differ by more than a quarter inch, your frame is out of square, and you will need to tell the door supplier or installer.

Use a steel tape measure, not a cloth one. Cloth measures stretch and give you false numbers. Measure from the inside of the frame — the opening where the door actually sits — not from the outside edge of the trim. The inside measurement is what determines whether a new door will fit.

Write measurements in inches and fractions of an inch. Most door suppliers work in quarter-inch increments, so round 2.6 inches up to 2.75 inches (two and three-quarters). If you measure 2.4 inches, round down to 2.25 inches (two and a quarter).

Key Takeaways

  • Measure the inside of the frame in three places for both width and height, because frames are often slightly out of square.
  • Use a steel tape measure and record all six measurements, even if they differ slightly from each other.
  • Measure the depth of the frame (how thick the wall is) because it determines what door slab thickness you need.
  • Note whether the door swings inward or outward and which side the hinges are on, because this affects which door you order.
  • If your frame is damaged, warped, or more than a quarter inch out of square, tell your supplier before ordering.

Measure the frame depth from front to back

The depth of your frame — how far it extends from the outside of the wall to the inside — determines the thickness of the door slab you need. Measure from the outer edge of the trim on one side to the outer edge of the trim on the other side. This is usually between 4.5 and 5.5 inches for interior doors and 1.75 to 2.25 inches for exterior doors, but measure yours anyway.

If your frame depth is unusual — very thick or very thin — mention it when you contact a supplier. Some doors come in standard thicknesses only, and a mismatch means the door will not close properly or will leave gaps.

Check which way the door swings and where the hinges are

Stand in the room where the door opens into. If the hinges are on your right side and the door swings away from you, that is a right-hand door. If the hinges are on your left side and the door swings away from you, that is a left-hand door. This matters because you cannot straightforward flip a door to the other side — the hinges, latch, and lock are built for one direction.

Some suppliers also ask whether the door is right-hand reverse or left-hand reverse, which describes doors that swing toward you instead of away. Take a photo of your door from the side where it opens into the room, with the hinges visible. Show this to your supplier if you are unsure.

Measure the distance from the frame to any obstacles

If there is a wall, cabinet, or other obstacle close to the door frame, measure how far it is from the edge of the frame. This affects whether the new door will open fully without hitting the obstacle. Measure from the outer edge of the frame trim to the nearest obstacle on the side where the door swings open.

Also check above the door. If there is a header, soffit, or light fixture very close to the top of the frame, measure the clearance. A new door with a thicker frame or different trim profile might not fit if there is less than an inch of clearance above.

Document the condition of the current frame

Look at the frame for rot, water damage, warping, or large gaps between the frame and the wall. If you see soft spots when you press on the wood, or if the frame is visibly bowed or twisted, write this down and tell your installer. A damaged frame may need to be replaced along with the door, which changes the cost and the work involved.

Check whether the frame is plumb — perfectly vertical — by holding a level against the sides. If the frame leans more than a quarter inch over its height, note that too. An out-of-plumb frame can make a new door stick or not close properly, and the installer needs to know before they order.

Create a straightforward sketch with all your numbers

Draw a rectangle representing your door frame. Write the three width measurements along the top, the three height measurements along the left side, and the depth measurement somewhere on the sketch. Add notes about the swing direction, any obstacles, and the condition of the frame. Take a photo of this sketch and your door from the side where it opens.

When you contact a door supplier or installer, send them the sketch, the photo, and a list of your measurements. This gives them everything they need to tell you whether a standard door will fit or whether you need a custom size. Many suppliers will not give you a price until they have these details, because the wrong door is more expensive than taking time to measure correctly.

Frequently Asked Questions

What if my three measurements for width are different from each other?

Differences of up to a quarter inch are normal and do not usually cause problems. Use the smallest measurement when you order, because the door needs to fit through the narrowest part of the opening. If the difference is more than a quarter inch, the frame is significantly out of square, and you should mention this to your installer before ordering.

Do I need to measure if I am replacing a door that is already in the frame?

Yes. The old door may have been cut down or shimmed to fit, so its size does not tell you the actual frame size. Measure the frame itself, not the door. This is especially important if the old door does not close smoothly or leaves large gaps.

What if the frame is damaged on one side?

Measure the undamaged side and note where the damage is. Your installer can assess whether the frame can be repaired or needs to be replaced. If the frame is replaced, the new frame will be square and plumb, and a standard door will fit properly.

Should I measure the door itself as well as the frame?

Measure the frame, not the door. The frame determines what size door you need. If you measure the door and it is warped, cut down, or shimmed, you will get the wrong size. The frame is the reference point.

Can I order a door before the installer measures?

You can get a rough idea of cost, but do not place a final order until the installer has measured or confirmed your measurements. Doors are expensive to return, and a door that does not fit the frame is a costly mistake. Most installers will measure before they order anyway.