How to Measure a Door for Replacement

To measure a door, take three width readings and three height readings from the opening itself rather than the existing door, work in millimetres, and use the smallest reading in each direction. Whether those measurements are taken inside the existing frame or brick to brick across the structural opening depends on one question: is the frame staying or going?

The stakes are higher with uPVC and composite doors than with timber. They are manufactured to the specified dimensions and arrive finished, so unlike a timber door they cannot be sanded or cut down on site. Measuring generously is an expensive error rather than a safe one.

This guide explains both measuring methods, what changes for French and sliding patio doors, and the five mistakes that most often lead to a door that does not fit. A professional survey is always carried out before a door is manufactured, so nothing here needs to be measured perfectly; the aim is to make quotations easier to compare and the surveyor’s visit easier to follow.

First, establish what is being replaced

There are two common scenarios, and the measurement points are different for each.

Replacing the door while keeping the frame. The existing frame stays in place and only the door leaf is replaced. Measurements are taken within the existing frame. Some external doors can be replaced this way where the frame is sound and modern enough to reuse; it is also the standard approach for internal doors.

Replacing the door and frame together. For most external door replacements, the door and frame are removed and replaced as a complete assembly, often called a doorset. Measurements are taken from the structural opening, referred to as the brickwork or brick-to-brick opening, rather than from the existing door or frame.

A new uPVC or composite external door is usually supplied as a complete door-and-frame assembly, but not always, so the quotation or specification should state clearly which approach applies. Establishing this first decides which of the two methods below to use.

What is needed

• A steel tape measure. Fabric tapes stretch and introduce errors. Measure in millimetres, the standard used for door manufacture and quotations.
• Pencil and paper. Each reading should be recorded as it is taken: three widths, three heights, the door thickness where the frame is staying, and the wall depth for a full doorset.
• A second person. Useful at height and across larger openings.
• Photographs of the opening. Clear photos from inside and outside pick up details the numbers miss, such as a sloping sill, a step, nearby pipework or other obstructions, and they make a remote quotation far more accurate.

How to measure a door when the frame is staying

Three measurements are needed, each taken more than once.

1. Width. Measure inside the frame, from the inside face of one side to the inside face of the other, at the top, in the middle and at the bottom.
2. Height. Measure from the floor or sill up to the underside of the frame head, on the left, in the centre and on the right.
3. Thickness. Measure the edge of the existing door. A replacement leaf has to sit in the same recess and hang on the same hinges.

Use the smallest of the three width readings and the smallest of the three height readings. The door has to pass through the tightest part of the opening, so the smallest measurement is the one that matters; sizing to the largest would leave a door that binds or fails to close at the narrowest point.

Readings that vary by more than a few millimetres suggest the frame is out of square or has moved over time. In that case the frame should be assessed before a replacement leaf is ordered, because the fault may need correcting at the frame rather than hidden in the door dimensions.

How to measure for a new PVC door and frame

When the door and frame are being replaced together, measurements are taken from the structural opening in the wall, the brick-to-brick opening, rather than from the existing frame. The old frame can stay in place while measuring, provided the tape reaches the brickwork at the sides and above the frame rather than stopping at the frame itself.

1. Width. Measure between the faces of the brickwork on each side of the opening, at the top, middle and bottom.
2. Height. Measure from the finished floor level or sill to the underside of the lintel, the structural support above the opening, on the left, in the centre and on the right.
3. Wall depth. Measure the depth of the reveal from the outer face of the wall to the internal finish. The frame depth and sill detail need to suit the wall construction, so this reading saves questions later.

Record all the readings and use the smallest in each direction, which allows for variation in the brickwork and sizes the new frame to the narrowest part of the opening.

Do not deduct a fitting allowance. The new frame needs an installation gap around it to be positioned, packed, levelled and sealed, but that allowance is applied when the final manufacturing size is calculated. Deducting it from the opening measurements means it comes off twice, and the door arrives smaller than it should be.

Do not add anything either. PVC exterior doors are made to the dimensions supplied and are not designed to be trimmed on site. Accurate opening measurements, with the manufacturer setting the tolerances, is the correct division of labour.

Measuring openings with sidelights or top lights

Where a door sits alongside glazed side panels, or beneath a glazed top light, the whole arrangement is made as one unit or as a coupled set. The measurement is therefore the full structural opening, not the door leaf. How that opening is divided between the door and the glazed panels is a design decision made at quotation stage, not a measuring one. Measuring only the door section of a wider opening is one of the more common and expensive mistakes with front doors.

How to measure by door type

The same principles apply across door types, but the points needing attention differ by configuration.

Consider the finished floor level

Floor height affects every door type. New flooring, tiles or underlay raise the finished floor level and reduce the available height, so where new flooring is planned, the height measurement should be taken to the expected finished level rather than the current surface. This matters most where the new door has to open inwards over a raised floor covering.

Five common door measurement mistakes

1. Measuring the existing door instead of the opening. The old door may have been trimmed at installation, altered over time or fitted to an opening that was never square, so its dimensions are unreliable. The opening is the truth.
2. Taking only one measurement. Openings vary, particularly in older properties. Three readings in each direction reveal the variation a single reading hides.
3. Using the largest measurement. The smallest reading identifies the tightest part of the opening, which is what the new frame has to fit through and into.
4. Adding or deducting a fitting allowance. The installation tolerance is applied when the manufacturing dimensions are calculated. Adjusting the raw measurements first means it is applied twice.
5. Ignoring the finished floor level. Carpet, laminate, tiles and underlay all raise the floor. The measurement should reflect the level after the planned work, not before it.

Why we carry out the final survey

Every door we supply is professionally surveyed before it is manufactured. The final manufacturing dimensions come from our surveyor’s measurements and the requirements of the installation, which is an additional check before the door is ordered and the point where the less obvious factors are caught: variation in the opening, floor levels, frame condition and access.

The measurements in this guide are therefore preparation, not a responsibility. They make an initial quotation easier to discuss and the survey easier to follow, and a free, no-obligation quote includes the survey itself. Door styles, colours and hardware can be compared in person at our Milton Keynes showroom.

For standard door dimensions rather than measurements of a specific opening, see our guide to standard door sizes.

Frequently asked questions

Do you measure a door from the inside or the outside?

From inside the property in both cases. Measure inside the frame if the frame is staying, or brick to brick across the structural opening if the frame is coming out with the door.

Should the largest or smallest measurement be used?

The smallest. Take three readings across the width and three down the height, and use the smallest of each, because the door has to fit the tightest part of the opening.

Can a uPVC or composite door be trimmed to fit?

No. Unlike a timber door, it is manufactured to the size supplied and arrives finished. A door made too large cannot be cut down, which is why measuring generously is an expensive error rather than a safe one.

What size is a standard front door?

There is no single standard, because uPVC and composite doors are made to measure. The most common size for a traditional timber front door leaf is 838mm by 1981mm (roughly 2ft 9in by 6ft 6in). Common opening sizes are covered in our size guide.

Do I need to allow for the fitting gap myself?

No. The manufacturer applies the allowance to the opening size. Deducting it first means it comes off twice, and the door arrives too small.

How do you measure a door with sidelights?

Measure the full structural opening, including the space the sidelights and any top light occupy. The split between the door and the glazed panels is decided at design stage, so the opening is measured as one.

Do I need to measure at all before getting a quote?

Yes. The manufacturer will need a rough measurement to prepare the quote, however, the surveyor takes the final measurements once an order is placed.