How to Measure a Doorway for a Door?

Measuring a doorway before ordering a replacement door is the step that most people rush - and the one that causes the most expensive mistakes. This guide covers exactly how to measure for a replacement door leaf, a new door and frame set, and an opening that may not be square, with the specific numbers and tolerances you need to order the right size confidently.

Table of contents

1. Door Leaf vs Door Set: Which Are You Replacing?
2. What Tools You Need
3. How to Measure for a Replacement Door Leaf
4. How to Measure for a New Door Set
5. How to Check If a Doorway Is Square
6. Understanding Door Tolerances
7. How to Measure a Door That's Already in Place
8. Non-Standard Openings: What to Do
9. How to Find a Door Fitter
10. FAQs: Measuring a Doorway for a Door

This guide covers measuring for a door leaf and a full door set, checking for square, understanding tolerances, and what to do when the opening is non-standard, read through in order or jump to the scenario that applies to you.

Door Leaf vs Door Set: Which Are You Replacing?

Before taking any measurements, establish which of two scenarios applies - because the measurement method and what you're measuring are different for each.

Replacing a door leaf only (keeping the existing frame):

You're fitting a new door into an existing frame. Measure the old door leaf, not the frame opening. The new door needs to match the existing door's dimensions since it's going into the same frame - including the same thickness. This is the simpler scenario and applies when the existing frame is in good condition and you simply want a new door.

Replacing the complete door set (door and frame together):

You're removing the old frame and fitting a new frame and door together. Measure the structural opening, the masonry or timber reveal that the new frame will sit in. This is necessary when the existing frame is damaged, rotten, or the wrong size, or when you're fitting a door into a new opening.

Getting this distinction wrong, measuring the frame opening when you're only replacing the door leaf, for example, is the most common source of incorrect size orders. Establish which scenario applies before you pick up the tape measure.

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What Tools You Need

Accurate door measurement requires only a few basic tools - but the quality of those tools matters. A cheap tape that sags mid-measurement or a square with accumulated inaccuracy produce errors that propagate directly into an incorrectly ordered door.

What you need:

• Tape measure: a quality steel tape, minimum 5 metres. Ensure the hook at the end is secure and not bent, as a loose or damaged hook adds inaccuracy to every measurement from the start
• Pencil and paper or a phone: to record measurements immediately - never trust memory between the doorway and the supplier
• Spirit level: for checking whether the head and sill of the opening are level
• Steel square or combination square: for checking the squareness of corners
• Folding rule or rigid ruler: useful for measuring thicknesses accurately where a tape measure is awkward

How to Measure for a Replacement Door Leaf

When keeping the existing frame and fitting a new door leaf, measure the door leaf that's coming out, not the frame opening.

Step 1: Measure the door width.

Measure across the door leaf (from edge to edge) at three points: the top, the middle, and the bottom. Note all three measurements. If they differ (common in older properties where doors have swollen, warped, or been planed), use the smallest figure as your ordering dimension. The replacement door can be planed down to fit - you can remove material from a door, but you can't add it.

Step 2: Measure the door height.

Measure the height of the door leaf (from top edge to bottom edge) on the left side, the centre, and the right side. Again, note all three and use the smallest.

Step 3: Measure the door thickness.

Measure the door thickness from the face to the back. Standard UK internal door thickness is 35mm or 40mm; standard external is 44mm. This measurement must match, a 35mm door in a 40mm rebate doesn't close properly.

Note on clearances: A door leaf should have a consistent gap around all edges when fitted in the frame - typically 2-3mm at the top and sides, 5-8mm at the bottom (to clear the floor covering). When ordering a replacement based on existing door measurements, these clearances are already built in.

How to Measure for a New Door Set

When replacing the complete door and frame, measure the structural opening, the masonry or timber reveal, not the old frame.

Step 1: Remove any existing frame.

If a frame is in place, ideally remove it before measuring to get accurate structural opening dimensions. If this isn't practical before ordering, measure from the front face of the existing frame's outer edge in to the masonry or timber behind it and subtract to get the structural opening.

Step 2: Measure the structural opening width.

Measure across the structural opening at three heights - near the top, at mid-height, and near the bottom. Note all three measurements.

Step 3: Measure the structural opening height.

Measure the height on both sides and at the centre of the opening.

Step 4: Calculate the door set size.

The door set frame needs to fit inside the structural opening with a clearance of approximately 10mm on each side and at the top for packing and adjustment. Subtract 20mm from the structural opening width and 10mm from the structural opening height to get the recommended door set size. In practice, door set manufacturers specify their frame sizes, order the nearest standard size below the calculated dimension.

How to Check If a Doorway Is Square

A doorway that's significantly out of square causes problems regardless of how carefully you measure width and height, a square door in an out-of-square opening will either bind on one corner or leave a tapered gap on the opposite side.

Diagonal measurement check:

Measure the opening diagonally from the top-left corner to the bottom-right. Then measure from the top-right corner to the bottom-left. If both diagonals are equal (within 5mm for a standard internal doorway), the opening is sufficiently square for a standard door set.

If the diagonals differ by 10-20mm, the opening is out of square. For a door leaf replacement into an existing frame, the frame itself may compensate - but for a new door set, this needs addressing. The frame can be packed out during installation to compensate for moderate out-of-square, but significant distortion requires either cutting the frame to match the opening profile or remedying the structural issue.

Level checks:

Check the head (top of the opening) with a spirit level. A head that's significantly off level will make the door gap appear tapered at the top. Check the floor at the threshold with a level too, a sloped floor affects the door's resting position and gap at the bottom.

Understanding Door Tolerances

Door sizes aren't exact - they're manufactured to within specific tolerances, and fitting requires small clearances to be left at the frame. Understanding these numbers prevents ordering errors.

Door leaf to frame clearances:

• Top of door to head of frame: 2-3mm
• Sides of door to frame rebate: 2-3mm each side
• Bottom of door to finished floor: 5-8mm (to clear floor covering without dragging)

Frame to structural opening clearances:

• Frame to masonry sides: 10-12mm each side (for packing and plumb adjustment)
• Frame head to structural lintel: 10-12mm (for packing)

Door leaf thickness tolerance: Door frames are rebated (have a step cut into the frame face that the door closes against) - the rebate depth must match the door thickness. A 35mm door in a 40mm rebate sits proud and doesn't seal; a 40mm door in a 35mm rebate doesn't close.

How to Measure a Door That's Already in Place

When the door to be replaced is still in its frame and you can't remove it before ordering, measurement is still accurate but requires a slightly different approach.

Width: Measure the door leaf width by opening the door to 90 degrees and measuring from the hinge edge to the latch edge. For most purposes, this gives the door leaf width directly. Alternatively, measure the frame rebate width (the distance between the frame stop on one side and the frame stop on the other) and subtract the clearances: the door leaf width equals the rebate width minus approximately 4mm (2mm per side).

Height: Measure from the finished floor to the underside of the door head, then subtract the clearances: the door leaf height equals the head-to-floor measurement minus approximately 7–10mm.

Thickness: Open the door and measure its thickness directly with a tape or folding rule. See the MyBuilder new front door cost guide, internal doors cost guide, and bifold door cost guide for typical supply and fitting costs across different door types.

Non-Standard Openings: What to Do

When accurate measurement reveals the opening doesn't correspond to any standard door size, the options are:

• Small difference (under 20mm): A standard door can be trimmed to width by a joiner or the door supplier before delivery. Width reduction of up to 10mm per side is possible on most hollow-core internal doors without compromising the door's structural integrity. Height reduction of up to 20mm from the bottom is also possible.

• Larger difference: A made-to-measure door is the most practical solution - available from specialist door manufacturers with a lead time of 2–6 weeks. For external doors, the door set (door and frame) is typically ordered made-to-measure together.

• Non-standard due to out-of-square: An out-of-square opening can be compensated for during installation by packing the frame selectively - packing one side more than the other to bring the frame plumb even within an out-of-plumb opening. A skilled door fitter can accommodate moderate out-of-square; severe structural distortion needs addressing before door fitting.

How to Find a Door Fitter

You can find and compare available door fitters near you on MyBuilder. Post your job with details of the door type (internal or external), whether you're replacing the leaf only or the full set, and any information about the property age or opening dimensions - fitters in your area will respond with their assessment and quotes.

All tradespeople on MyBuilder undergo checks at registration, such as ID documents, company details, certifications for regulated jobs and skill assessments, allowing you to hire with confidence.

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FAQs: Measuring a Doorway for a Door

Should I Measure the Door or the Frame?

It depends on what you're replacing. For a door leaf only (keeping the existing frame), measure the existing door leaf - not the frame opening. For a complete door set (new door and frame), measure the structural opening. Getting this distinction wrong is the most common source of incorrect size orders.

What Clearance Should There Be Around a Door?

For a standard fitted internal door: 2–3mm at the top and 2-3mm each side between door leaf and frame; 5-8mm at the bottom to clear the floor covering. External doors require tighter clearances at the sides (2mm) but a similar bottom gap depending on the threshold detail. You can browse door fitting projects on MyBuilder to see what professional results look like in different property types.

How Do I Know If My Door Opening Is Square?

Measure diagonally across the opening in both directions (top-left to bottom-right, and top-right to bottom-left). If the two measurements are equal within 5mm, the opening is effectively square. A difference of more than 10-15mm indicates significant distortion that needs to be accounted for during frame installation or may require remedial work.

Can I Order a Door Online Without Someone Measuring It?

Yes, if you measure carefully using the steps in this guide and are ordering a standard-size door for a replacement door leaf scenario. For a complete door set in an older property or any non-standard scenario, having a fitter survey the opening first significantly reduces the risk of ordering the wrong size.

What Happens If the Door Is Slightly Too Wide?

A door that's a few millimetres too wide can be planed down to fit, this is standard practice and any competent door fitter can do it on site.

A door that's more than approximately 10-12mm too wide requires a joiner to cut it down with a circular saw and re-lipping the edge, which is more involved. A door that's too narrow (where you've over-measured) cannot be made wider, return and reorder is the only option.