Window Measurement Guide

How to Measure Windows for Replacement: A Complete Guide

Accurate window measurements are essential for ordering replacement windows that fit correctly. Whether you are replacing a single window or fitting out an entire property, taking precise measurements will save time, reduce costs, and ensure a professional finish. This guide covers everything you need to know about measuring windows in UK homes.

For a detailed walkthrough with photos, see our blog post: A Step-by-Step Guide to Measuring Windows for Replacement.

Tools You Will Need

• Steel tape measure -- at least 5 metres long. Avoid cloth or plastic tapes as they can stretch and give inaccurate readings.
• Pencil and notepad -- or use your smartphone to record each measurement immediately.
• Step ladder -- needed for upper-floor windows or tall openings.
• Spirit level (optional) -- useful for checking whether sills and lintels are level.

Measuring the Width

Measure the width of the window opening from the inside face of the brickwork on one side to the inside face on the other. Take this measurement at three points: the top of the opening, the middle, and the bottom. Use the smallest of the three readings. Openings are rarely perfectly consistent, especially in older properties, so taking multiple readings is essential.

Measuring the Height

Measure from the top of the internal sill (or the top of the sub-sill if one is present) to the underside of the lintel. Measure at three points: left side, centre, and right side. Again, use the smallest measurement. This ensures the new window will fit comfortably in the opening even at its narrowest point.

Checking Squareness

After measuring width and height, check that the opening is square by measuring both diagonals (corner to corner). In a perfectly square opening, both diagonals will be identical. In practice, a difference of up to 5mm is considered acceptable. If the difference exceeds 10mm, the opening is significantly out of square and a professional survey is strongly recommended before ordering.

Common Measurement Mistakes

The most frequent errors homeowners make when measuring windows include:

• Measuring the existing frame instead of the brick-to-brick opening. The new window needs to fit inside the brickwork, not inside the old frame.
• Taking only one measurement for width or height. Openings can vary by 10mm or more from top to bottom.
• Using a cloth tape measure that can sag and stretch, leading to readings that are several millimetres too long.
• Forgetting fitting tolerances. A standard 10mm gap on each side is needed for foam and sealant. Without it the window will not fit.
• Confusing millimetres and centimetres. Always work in millimetres for window orders.

Standard Window Sizes in the UK

Standard UK window sizes are more cost-effective and typically have shorter lead times than custom sizes. If your opening closely matches a standard size, you could save 20--30% compared to a bespoke window. The table below lists the most common standard sizes available from UK manufacturers.

When to Get a Professional Survey

While DIY measuring works well for straightforward rectangular openings, certain situations call for a professional survey:

• The opening is significantly out of square (diagonal difference exceeding 10mm).
• You have non-standard shaped windows such as arched, circular, or bay windows.
• There are signs of structural damage around the opening -- cracks in the lintel, crumbling brickwork, or damp staining.
• The property is listed or in a conservation area, where specific window styles and materials may be required.
• You are ordering timber sash windows or other high-value products where precision is critical.

At Harrington Windows we offer free, no-obligation surveys across London, Hertfordshire, Essex, and Cambridgeshire. Our surveyors use laser measuring equipment and can identify potential structural issues before you commit to an order.

Measurement Considerations by Window Type

Different window types may have specific measurement requirements:

• Casement windows -- Standard rectangular measurement. Ensure you note whether the opening is for a single or double casement.
• Sash windows -- Measure the full opening height. Note the sash horn positions and glazing bar layout if matching existing windows.
• Bay windows -- Each section must be measured individually, plus the overall projection depth and the angle between sections.
• Tilt and turn windows -- Standard measurement, but ensure sufficient internal clearance for the window to open fully inward.
• Flush sash windows -- Same as standard sash measurements, noting any decorative features to be replicated.

How Material Choice Affects Sizing

The frame material influences the visible glass area for a given opening size:

• uPVC frames are typically 60--70mm wide, providing a good balance of strength and glass area.
• Aluminium frames can be as slim as 35--45mm, maximising the glass area and natural light.
• Timber frames are usually 55--68mm wide, offering excellent thermal performance with a natural appearance.

To compare the energy performance of different glazing options for your window size, try our Window U-Value Calculator. For pricing across materials and styles, use our Window Price Calculator.