How to Measure for Replacement Windows – DIY Guide
How to Measure for Replacement Windows
Get accurate measurements first time and avoid costly ordering mistakes.
To measure for replacement windows, measure the width and height of the existing frame opening in three places each — top, middle, and bottom for width; left, centre, and right for height. Record the smallest dimension in each direction and use those figures when ordering.
- Steel tape measure
- Spirit level
- Pencil
- Notepad or phone for recording measurements
- Step stool or small ladder
- Torch or work light
- Measurement record sheet (paper or digital)
- Masking tape (for marking reference points)
- Silicone sealant (for gap-filling after fitting)
- Window frame packers or shims
- Expanding foam filler
- Low-tack tape (to label window locations)
Step-by-Step Guide
Identify your window type and frame material
Before you take a single measurement, establish what type of window you have — casement, sash, tilt-and-turn, or fixed — and whether the frame is timber, uPVC, or aluminium. This determines whether you are measuring for a direct replacement (same frame type and size) or a full new installation, which requires different measurements. Knowing your frame material also helps when specifying like-for-like to a supplier. If your existing frame shows signs of rot or damage, check our guide to fixing a rotting window sill before proceeding, as a compromised sill will affect how your replacement sits.
Measure the width of the frame opening
Using a steel tape measure, measure the internal width of the existing window frame opening — this is the reveal, not the outer frame face. Take three measurements: one across the top, one across the middle, and one across the bottom of the opening. Record all three figures precisely to the nearest millimetre. The smallest of the three is your ordering width. Windows are rarely perfectly square, and using the largest figure will produce a unit that does not fit.
Measure the height of the frame opening
Now measure the internal height of the opening in three places: down the left side, down the centre, and down the right side. Again, record all three and use the smallest as your ordering height. For sash windows, measure from the bottom of the top frame to the top of the bottom sill, not from the very top to the very bottom of the outer box frame — your supplier will specify which datum points they require, so always confirm before ordering. If you are also dealing with condensation in your double glazing, note it down as it may affect the glass specification you need.
Check the frame for square using diagonal measurements
Measure diagonally from corner to corner in both directions across the opening — top-left to bottom-right, then top-right to bottom-left. If both diagonals are within 5 mm of each other, the opening is acceptably square. A larger difference means the opening is out of square and you will need to inform your supplier so the unit can be packed and sealed correctly on installation. Ignoring this step is one of the most common reasons replacement windows fit poorly and let in draughts.
Record and label every window individually
Measure every window you intend to replace separately — even windows in the same room or that appear identical in size. Manufacturing tolerances and building movement mean openings that look the same can differ by several millimetres. Label each measurement set clearly: for example, ‘Living Room Left’ or ‘Bedroom Front’. Use low-tack tape on the frame to mark each window’s reference code so you can match the delivery to the correct opening. This matters especially if you are replacing window seals as part of the same project.
Confirm measurements with your supplier before ordering
Contact your window supplier with your recorded figures before placing any order. Most reputable suppliers will ask whether your dimensions are the frame opening size or the overall unit size — these are different things, and mixing them up is an expensive mistake. Some suppliers deduct a standard installation tolerance (typically 5–10 mm per side) from your opening size to give the unit size; others require you to supply the exact unit size you need. Confirm the allowance policy, the glass specification, and any lead times in writing. Also check whether your windows require building regulations approval — according to GOV.UK planning guidance, replacement windows in England must meet energy efficiency standards and be notified to building control unless installed by a FENSA- or Certass-registered company.
Frequently Asked Questions
Do I measure the inside or outside of the window frame?
Measure the internal reveal — the opening between the inner edges of the existing frame. This is the dimension your replacement unit needs to fit into. Measuring the outside face of the frame will give you the overall frame size, which is not what the supplier needs unless they specifically ask for it.
What is the standard installation tolerance for replacement windows?
Most suppliers work to a 5–10 mm tolerance on each side, meaning the unit they manufacture will be slightly smaller than your opening to allow for fitting and sealing. Always confirm the exact allowance with your specific supplier, as policies vary — never assume and deduct the tolerance yourself unless instructed to.
Do replacement windows need building regulations approval?
Yes, in most cases in England. Replacement windows must meet minimum thermal performance standards under Part L of the Building Regulations. The easiest route is to use a FENSA- or Certass-registered installer, who can self-certify the work. If you fit the windows yourself, you must notify your local authority building control department. See the Doors & Windows Guide for a broader overview of compliance requirements.
Can I measure for replacement windows in a listed building or conservation area?
Listed buildings and properties in designated conservation areas usually require planning permission for replacement windows, particularly where the style, material, or glazing type would change. Contact your local planning authority before ordering anything. Permitted development rights do not apply in these circumstances.
What if my window opening is badly out of square?
If the diagonal measurements differ by more than 5 mm, inform your supplier before ordering. The opening may need to be built out or packed on installation to create a square seat for the new unit. A badly out-of-square opening that is ignored will cause the window to bind, not seal correctly, or distort over time. You may also want to inspect for underlying structural movement — if you spot cracking around the frame, see our guide to why walls crack before proceeding.
Always measure on a dry day when timber frames are at their baseline dimension — timber swells noticeably in wet weather, and measuring during or after rain can give you a width that is 3–5 mm larger than the true opening. For uPVC frames, also check that the existing frame has not bowed inward at the centre; a bowed frame will give a falsely narrow mid-point measurement that causes a correctly sized unit to appear too wide on delivery.
Sources
- GOV.UK — Approved Document L: Conservation of Fuel and Power — gov.uk
- FENSA — Why use a FENSA registered company — fensa.org.uk
- Which? — Buying new windows: everything you need to know — which.co.uk
This guide is for general information only. Always work safely and follow manufacturer instructions. DIYnut accepts no liability for injury or damage arising from DIY work.
