How to Measure a Window for Replacement: A Step-by-Step Guide

Whether your old window is rotting, draughty, or just plain ugly, replacing it is a great way to boost your home's comfort and kerb appeal. But before you head to the hardware store or call a tradie, you need to get your measurements right. An incorrectly measured window can mean costly returns, delays, and a gap in your wall — not ideal, especially with Sydney's summer storms rolling in.

Here's how to measure a window for replacement like a pro.

What You'll Need

• Steel tape measure (not a fabric one — they stretch)
• Pencil and notepad
• Step stool or ladder if needed
• Level (optional but helpful)

Step 1: Understand What You're Measuring

There are two key measurements to know:

• Rough opening – the framed hole in the wall before any window is installed
• Window unit size – the actual finished dimensions of the window you're buying

For a replacement window, you'll usually be measuring the existing window frame opening, not the glass itself.

Step 2: Measure the Width

Measure the width of the window opening in three places: top, middle, and bottom. Take the smallest of the three measurements — this is your working width. Windows and walls aren't always perfectly square, especially in older Sydney homes, so this accounts for any variation.

Write this number down.

Step 3: Measure the Height

Now measure the height in three places: left side, centre, and right side. Again, take the smallest measurement. Measure from the very bottom of the frame (or sill) to the top of the opening.

Note this down alongside your width.

Step 4: Check for Square

Measure diagonally from corner to corner — both ways. If both diagonal measurements are the same, your opening is square. If they differ by more than 6mm, you've got an out-of-square opening, which is common in older homes. You'll need to mention this when ordering your window or consult a professional, as extra work may be required to bring it into square before installation.

Step 5: Account for Clearance

When ordering a replacement window, subtract about 10–12mm from both width and height to allow for a small gap on each side. This gap gets filled with expanding foam or packing and covered by trims or reveals. It gives you room to square and level the window during installation.

So if your opening measures 900mm wide × 1200mm high, you'd order a window approximately 888mm × 1188mm (or ask your supplier what clearance they recommend).

Step 6: Note the Window Type and Frame Material

Before ordering, also confirm:

• Window style – sliding, awning, casement, double-hung?
• Frame material – aluminium, timber, uPVC?
• Glazing – single, double, or laminated?
• Any special requirements – bushfire zones, cyclone ratings, or heritage overlays

In Sydney, many older suburbs like Bondi, Newtown, or Paddington have heritage considerations that may limit what you can install, so it's worth checking with your local council.

Common Mistakes to Avoid

• Measuring the glass, not the frame — always measure the opening, not just the pane
• Only measuring once — always measure at least three times in each direction
• Ignoring out-of-square openings — this can cause the window to jam or leak
• Forgetting about reveals and trims — these need to be re-fitted or replaced after installation

DIY vs Professional Cost (AUD)

If you're handy and the opening is standard and square, a DIY window replacement is achievable — but it does involve working with walls, possibly dealing with flashing, and ensuring a weathertight seal.

• DIY cost: $200–$800+ for the window unit (depending on size and type), plus tools and fixings
• Professional installation: $300–$700 in labour on top of the window cost, depending on complexity

For a straightforward single window swap, expect to pay $600–$1,500 all up when using a professional handyman.

When to Call a Professional

Call in a handyman or window installer if:

• The opening is out of square or the surrounding frame is damaged
• You're working above ground floor
• The window is large or unusually shaped
• You're dealing with a heritage property
• You're unsure about waterproofing, flashing, or re-sealing
• You need to modify the rough opening size

A poorly installed window can let in water, cause mould, or fail to meet safety standards — it's not a job worth cutting corners on.

Need a window measured and installed properly? Get a free quote from Handyman Bondi today — we service Bondi and across Sydney's eastern suburbs with quality work and honest pricing.