How to measure for a new door slab 

It's important to get the measurements for a new door correct before you order it. Ordering the wrong size door can be a costly mistake. You should also determine the swing or handing of the door you want to replace, if it's hinged, so you can order the correct door type.

For more information see: How to determine the swing of a door

This guide looks at taking measurements for a new slab door, or door leaf without a frame. If you need to replace the entire assembly you might want to order a doorset, pre-hung in a frame. External doors, in particular, need to be accurately fitted, so they close tightly enough for the weather seals to prevent any leaks or drafts. They also need to be properly installed for security. If an external door is not fitted properly it can invalidate your home insurance. Internal doors will usually be more forgiving.

There are two different ways to measure for a new door slab. If the old door is the correct size and fits squarely within its frame, you can measure the old door leaf itself. If the door seems to have warped or no longer fits properly, or if you're installing a door into an empty frame, you can get the correct measurements from the existing frame.  

Before you begin 

Check that the door frame is suitable for a new door to be fitted. If there is any significant damage or wear you might need to get a full door set or assembly which includes a door leaf and frame. You may need to remove the old door so you can inspect the frame properly. 

If you need to replace the entire door set, see: How to measure for a new doorset

You also need to check that your old frame is straight, level, and plumb. The easiest way to do this is to measure from corner to corner, diagonally, in both directions. The measurements should be within 5mm of each other, otherwise, you may need to adjust the frame, or purchase a new one. 

Slab doors usually come without any hardware fitted. If the old door hardware is in good condition you might be able to reuse it. Otherwise, be aware that you'll need to purchase new hardware separately and install it onto the new door before it is fitted. You will also need to measure where the old hardware was so it can be installed in the same positions on the new door to fit correctly to the old frame, as hardware placement is not usually standardised.

Measuring the door leaf 

Step 1 - Measure basic dimensions

If you've removed the old door, make sure it's lying on a flat surface and measure its thickness. Then measure the width and height. It's a good idea to take each measurement two or three times at different points on the door.

If the measurements don't match, use the largest measurements taken for each dimension. Many doors can be trimmed slightly to get a perfect fit. But remember, you can't add anything to a door. Make a note of each measurement.  

Step 2 - Measure for hardware

The next measurements you'll probably need will help you to fit the new door with hardware in the correct places to line up with the old frame. To measure for the hinges measure from the top of the door to the top of each hinge. There will usually be at least three hinges. Ensure the tape is flat against the face of the door and is running parallel from the edge of the door.

You should also measure the hinge reveal. This is the distance between the edge of the door and the edge of the hinge. This measurement tells how far into the side of the door the hinges sit. Measure the sizes of the hinges so you can also match these.

Measure for the position of the lockset from the top and side of the door. The lockset is the locking or latching mechanism in between the handles. Measure from the top of the door to the centre of the lockset. If there is other hardware on the door you want to match you can measure their positions as well.   

Measuring the door frame

If you are going to measure the door frame to determine which door size is best for you, make sure it is only the frame you're measuring. There will usually be architrave or some other trim fixed to the walls, and door stops fixed to the door's jambs and head piece.

Step 1 - Measure basic dimensions

Measure the height and width of the frame. Take two or three measurements for each dimension from different positions on the frame. Always use the smallest of these measurements and deduct 10 mm from the height and width measurements for door clearance. Make sure you measure from the highest point of flooring, or if the flooring is not installed yet, allow for the thickness of the planned flooring.

When measuring the thickness of the frame, make sure you just measure from the edge of the jamb to the door stop, and don't include any architrave, or trim. Measure from the front edge of the jamb, on the door side, to the front edge of the door stop.

Choosing the right size of door

Once you have all the measurements, you should be able to find a standard door which is a close match. Check how much individual doors can be trimmed, because most doors need to have few millimetres taken off for the best fit, but not all doors can be trimmed. 

For more information on popular door sizes, see: What internal door sizes are available? and What external door sizes are available?

Alternatively, many companies produce bespoke (made-to-measure) doors to fit in non-standard door frames. If you are unsure of which door size you should choose you should discuss this with the manufacturer or seller.