The SevenDay guide on how to measure for a new window or door

Getting the measurements right is the foundation of every successful window job. An error of a few millimetres can mean a frame that won’t fit, gaps that compromise weather performance, or a costly remake. This guide walks through how to measure accurately and, just as importantly, the specification points you need to confirm before placing an order.

It is often assumed that measuring for your windows and doors is a simple job, however there are many risks and complications that may occur along the way. Therefore, always seek expert advice or ask your SevenDay team for advice if you looking to engage in some DIY at your premise and are in any way unsure.

The following is a guide only and measurement requirements can differ depending upon the property type, age and the method of original installation.

Read this first

Measuring a window is well within the reach of a competent DIYer or tradesperson, but a few things genuinely matter for safety and for the rest of the job going smoothly:

• Replacement glazing in England and Wales is covered by Building Regulations. The work must either be signed off by Building Control or carried out by an installer registered with a competent person scheme such as FENSA or CERTASS. If you remove and replace yourself, you are responsible for compliance and the certificate.
• Older windows (pre-2000) and their surrounding paint or sealant can contain asbestos or lead paint. If you suspect either, stop and get it tested before disturbing it.
• Glass and old frames are heavy and can shatter. Work with a second person wherever possible, especially above ground-floor level.
• Never lean out of or put weight on an opening you have started to dismantle, and never work at height off makeshift platforms — use proper access equipment.
• If the window is structural, unusually large, a fire-escape window, or you are at all unsure about the lintel above it, get a professional to assess it before you start.

Tools you will need

A good quality steel tape measure (avoid cloth or fabric tapes), a notepad or measuring sheet, a spirit level, and ideally a second person for larger openings. Always record measurements in millimetres for consistency.

Measuring Step by Step

Step 1: Measure the width

Measure the width of the existing opening (the brick-to-brick or structural opening, not the old frame) in three places: across the top, the middle, and the bottom. Openings are rarely perfectly square, so take the smallest of the three measurements as your working width. From this, deduct your fitting tolerance — typically 10mm off the brick dimension to allow for packing and expansion. Confirm the exact deduction with your local SevenDay depot, as it can vary depending on the installation.

Step 2: Measure the height

Measure the height in three places too: down the left, the centre, and the right. Again, take the smallest reading and apply the appropriate fitting tolerance. Measure from the underside of the lintel down to the window board or sill, and be clear about whether the existing cill is being retained or replaced, as this affects the overall height of the frame.

Step 3: Check for square and plumb

Use your spirit level to check whether the opening is level and plumb, and measure both diagonals. If the diagonal measurements differ significantly, the opening is out of square and the frame may need adjustment or additional packing on fitting. Note any bowing in the reveals.

Step 4: Check the internal dimensions

Don’t just measure the external opening alone. Always check the internal dimensions too, because plaster depth, plasterboard, dot-and-dab, or tiling around the reveal can reduce the effective opening on the inside. The internal aperture will be smaller than the brick opening, and if you measure outside only, the frame can foul the plaster or tiles on fitting. Where the internal dimension is significantly smaller due to thick plaster lines or tiles, the frame may need to be made slightly smaller to suit — with add-ons or frame extenders then fitted to pack out to the brickwork and give a clean, finished line both inside and out. Measure both faces, work to the smallest controlling dimension, and note where extenders or add-ons will be needed. Speak to your local SevenDay depot to understand the specification and options of the add-ons / frame extenders – note these just clip on to the edge of the frames into the Euro Grooves.

Step 5: Note the configuration

Record how the existing window opens — the number of opening sashes, their positions, and the direction they open. Photograph the window from inside and out. This helps confirm the configuration and is invaluable when specifying the replacement. Also be clear when ordering if you are recording the openings from internal or external perspective – get it wrong and the opening may be a different side than what was desired!

Critical Specification Points

Measuring correctly is only half the job. Before ordering, work through these points carefully — they affect compliance, safety, and the end user’s experience. Ensure you make it clear as to which dimension is the width and which is the height [tip] industry norm/expectation is to specify the width as the first dimension and then the height.

Fire escape (egress) windows

Building Regulations require certain habitable rooms — particularly first-floor bedrooms and some ground-floor rooms — to have an openable window suitable for emergency escape. An egress window generally needs an unobstructed openable area of at least 0.33m² with a minimum opening width and height of 450mm, and the bottom of the openable area no more than 1100mm above floor level – this is strictly mandated on new build installations, however on replacement windows the directive is to make the fire escape situation no worse. But for safety if fire escape provision could be made better then it should be incorporated into your design. To achieve the clear opening, you’ll usually need to specify egress (fire escape) hinges, which allow the sash to open wider and shift sideways to free up the aperture. Always check which rooms require egress provision before finalising the configuration.

Hinge specification: fire escape vs. easy-clean

This is a common point of confusion, so it’s worth being deliberate. Standard friction hinges suit most openers, but two specialist types come up regularly:

• Egress (fire escape) hinges open the sash to a wider angle and de-restrict it to maximise the clear opening for escape — specified where Building Regs demand it. However as the window opens close to the frame on the hinge side there is no room to clean the window from the inside. 
• Easy-clean hinges open the sash to slide across when opened, so the outer face of the glass can be cleaned from inside the building — useful for upper-floor windows where external access is difficult.
• Hinges can be added that open to the fire egress (escape) and then with a press of the button can be slid to allow easy clean functionality, or vice versa. This can be a great compromise, but be clear when specifying on your order which is the standard opening preference.

Note, fire escape opening windows are generally side hung windows, the hinges are normally not “man enough” to hold open a top hung window to the size needed for it to be classed as a fire egress (escape) open.

The two opening options perform different jobs, so be clear on which is required for each opener and never assume one covers the other as standard.

Safety glass

Building Regulations (Approved Document K) require toughened or laminated safety glass in “critical locations” on all installations— broadly, glazing in or next to doors (within 300mm of the door edge) and any glazing below 800mm from floor level. Low-level windows, full-height glazing, and side panels almost always need safety glass. When in doubt, specify safety glass; it’s a small cost uplift against a significant compliance and safety risk.

Transom heights

The transom is the horizontal bar dividing a window, often separating a fixed light below from an opening light above (or vice versa). Transom height affects both appearance and function. Get it wrong and the window can look unbalanced against neighbouring windows or fail to match the property’s existing style. Where a window is also an egress route, the transom position directly affects whether the openable section meets the clear-opening requirement, so the two must be considered together. Measure and confirm transom heights against the existing windows or the customer’s specification, and check consistency across multiple windows on the same elevation.

Other points worth confirming

• Glazing specification — the U-value or energy rating required, and whether obscure or privacy glass is needed for bathrooms.
• Trickle vents — now required on most replacement windows under current Building Regs for background ventilation in habitable rooms.
• Frame colour and finish — including whether it’s the same inside and out or a dual-colour specification.
• Cill type and projection.
• Threshold or upstand requirements where the window sits above a door or in an unusual position.

A Final Word

Measure twice, order once. If anything about an opening looks unusual — out of square, structural concerns, or an awkward egress situation — speak to your depot before ordering. Our local SevenDay counter teams have years of hands-on experience and would far rather talk a measurement through with you up front than see a frame come back as a remake.

This guide covers a standard domestic window. Bay windows, structural openings, and anything load-bearing or unusually large are a different job and are best handled by, or checked with, a professional installer.