Knowing how to measure windows correctly is the single most important step before ordering any replacement. Measure the rough opening width and height at three points each — top, middle, and bottom for width; left, centre, and right for height — then use the smallest reading in each direction. That gives you the true opening size so your new window fits without costly gaps or forced trimming.
Key takeaways:
- Always measure the rough opening (the framed hole in the wall), not the old window frame or sash.
- Take three width readings and three height readings; record the smallest in each direction.
- Measure in millimetres for precision — Canadian suppliers and installers work in metric.
- Check for square by measuring both diagonals; they should match within 6 mm.
- A professional window replacement crew will verify your measurements on-site before anything is ordered.
What You Need Before You Start
Gather a few basic tools first. Trying to measure with a cloth tape or a short ruler will cost you accuracy.
- Steel tape measure (at least 3 m)
- Pencil and notepad (or your phone)
- Step stool or ladder for higher windows
- Helper if the window is large
Write every reading down immediately. Memory is not reliable when you have six or eight windows to measure across a house.
Understanding the Rough Opening vs. the Window Frame
The rough opening is the framed hole in your wall — the space between the trimmer studs, the sill plate, and the header. The existing window frame sits inside that space, usually with a small gap filled by caulk, foam, or brickmould.
When ordering a replacement, installers need the rough opening size, not the size of the old sash or the visible glass area. If you measure the old frame from the outside and ignore the hidden gap, you will likely order a window that is 12–25 mm too small.
For insulated replacement frames, the rough opening is especially important because the new frame must seat flush and allow room for insulating foam and flashing tape around the perimeter.
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How to Measure Window Width
Open the interior trim or remove it if possible to expose the rough opening. If the trim is painted tight to the wall and you cannot remove it without damage, measure the inside of the existing frame and add approximately 12 mm per side (25 mm total) to estimate the rough opening. A site visit from an installer is a better approach in that case.
If you can access the rough opening directly:
- Measure across the top of the opening — from trimmer stud to trimmer stud.
- Measure across the middle.
- Measure across the bottom, just above the sill plate.
- Record all three numbers and use the smallest.
This smallest reading is your controlling width. It tells you the tightest point the window must pass through.
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How to Measure Window Height
Height works the same way, just rotated 90 degrees.
- Measure on the left side — from the top of the sill plate to the underside of the header.
- Measure in the centre.
- Measure on the right side.
- Record all three and use the smallest.
Sill plates can bow slightly over time, particularly in older Toronto bungalows and semi-detached homes where wood has been through decades of freeze-thaw cycling. Taking three readings catches any sag before it becomes a problem on installation day.
Checking for Square
A window opening that is out of square causes air leakage, operational problems, and in some cases structural stress on the frame. To check, measure from the top-left corner to the bottom-right corner, then from the top-right to the bottom-left.
| Reading | Acceptable Tolerance |
| Both diagonals equal | Opening is square — proceed normally |
| Difference up to 6 mm | Acceptable; shims during install will correct it |
| Difference 7–12 mm | Notify your installer before ordering |
| Difference over 12 mm | Structural check recommended before replacement |
If your diagonals are significantly off, your installer may need to adjust the rough opening before the new window goes in. This is not uncommon in homes built before the 1980s.
Standard Window Size Reference
Most Canadian window manufacturers, including TotalSeal, work to standard nominal sizes. Knowing these helps you confirm whether your measured opening aligns with a stock size or requires a custom order.
| Window Type | Common Rough Opening Width | Common Rough Opening Height |
| Casement (single) | 610–915 mm | 914–1,524 mm |
| Double hung | 610–1,219 mm | 914–1,524 mm |
| Sliding (2-lite) | 914–1,829 mm | 610–1,219 mm |
| Fixed / picture | 305–2,438 mm | 305–2,438 mm |
| Awning | 610–1,219 mm | 457–914 mm |
For specific guidance on common sizes, see our pages on double hung window sizes, casement window sizes, and sliding window sizes.
Measuring Single Hung Windows
Single hung windows have a fixed upper sash and a movable lower sash. Measuring them is straightforward but there is one common mistake: people measure the sash opening rather than the full frame.
For a single hung window replacement, measure the full rough opening from stud to stud and header to sill — not just the lower operable section. The replacement unit will include both sashes as one factory-built frame.
Our dedicated guide on how to measure single hung windows covers that window type in more detail if you have several in your home.
Measuring Bay and Bow Windows
Bay and bow windows need a different approach because they project outward from the wall plane. You need to record the rough opening width and height as above, but also:
- The projection depth (how far the unit extends beyond the exterior wall)
- The seat board height (the interior horizontal surface)
- The roof angle, if the bay has its own roof section
These measurements affect the framing, roofing, and interior finish work. If you are considering bay window replacement or curious about bay window sizes, it is strongly recommended to have a professional measure on-site rather than relying solely on DIY readings. Errors on a bay or bow are expensive.
Measuring for Fixed and Picture Windows
Fixed windows have no moving parts, which makes them simpler structurally but unforgiving dimensionally — there is no operable hardware to adjust after installation. For fixed replacement windows and picture units, confirm your rough opening readings twice before submitting any order.
See also our comparison of fixed window sizes if you are planning a large glazed area in a living room or stairwell.
Expert tip — Marcus Tremblay, RHI, OAHI Member, Toronto: “One mistake I see constantly is homeowners measuring the window from the outside, glass face to glass face. That gives you the daylight opening, which is completely useless for ordering a replacement unit. You need the rough opening or, at minimum, the outside frame dimension. When in doubt, remove a piece of interior casing on one side — it takes five minutes and saves a very expensive re-order.”
The Deduction Rule: How Much Smaller Should Your Window Be?
Replacement windows are manufactured slightly smaller than the rough opening to allow for shimming, levelling, and perimeter insulation. A common rule of thumb in Ontario:
- Subtract 12 mm from each dimension (width and height) for the nominal window unit size.
- This leaves a 6 mm gap on each side for shimming and foam backer rod.
So if your rough opening is 914 mm wide × 1,219 mm tall, you would typically order a 902 mm × 1,207 mm unit. Your installer may use a slightly different deduction based on their installation method and the frame system — always confirm with them before submitting the order.
Common Measuring Mistakes to Avoid
Getting the measurements wrong means delays, extra costs, or windows that simply will not fit. The most common errors:
- Measuring the old sash instead of the rough opening
- Taking only one reading per dimension (missing bows or tilts)
- Measuring in inches when the supplier works in metric
- Forgetting to check for square
- Not accounting for the deduction needed for shimming
- Assuming all windows in the same house are the same size
Older homes in Mississauga, Etobicoke, and Scarborough often have mixed window sizes from renovation work done across different decades. Measure each opening individually, even if they look identical.
Expert tip — Sandra Ouellette, BCIN Registered Designer, Building Science Consultant, Hamilton, ON: “A lot of homeowners skip the diagonal check and then wonder why their new window won’t close flush. Out-of-square openings are incredibly common in wood-frame construction, especially in homes that have settled. I always tell clients: if the diagonals differ by more than 6 mm, flag it before you order anything. Correcting the opening first costs far less than returning a custom window unit.”
When to Call a Professional Measurer
DIY measuring works well for simple single or double hung windows in a standard wood-frame wall. But you should bring in a professional for:
- Bay, bow, or garden windows
- Openings where trim cannot be removed
- Masonry or brick construction
- Any window over 2 m in width or height
- Basement hopper window replacement where the sill may be partially below grade
At TotalSeal, every project includes a professional on-site measurement visit before production begins. Our in-house crews — not subcontractors — verify every opening across Toronto and the GTA. If you are wondering how often windows should be replaced or want to check whether your current units qualify for ENERGY STAR™ certified replacements, our team can assess that during the same visit.
Our windows meet ENERGY STAR™ certification requirements for Climate Zone 2 (Southern Ontario), so when your measurements confirm a good fit, you can move forward knowing the product will also meet NRCan thermal performance standards.
Use our window cost estimator to get a ballpark figure once you have your rough opening dimensions in hand.
Quick Measuring Checklist
Before you submit any measurements to a supplier or installer, confirm you have:
- Three width readings per opening (top, middle, bottom)
- Three height readings per opening (left, centre, right)
- Recorded the smallest in each direction
- Checked both diagonals for square
- Noted the wall thickness (for frame depth selection)
- Identified the window type (hung, sliding, casement, fixed, etc.)
- Applied the standard 12 mm deduction (or confirmed deduction with installer)
- Measured each window opening individually, even if they look the same
FAQ
Does window size refer to the rough opening or the window unit?
Window unit size (also called the frame size) is always smaller than the rough opening. Manufacturers use the rough opening measurement to determine which unit to build, then subtract for installation clearance. Always confirm which dimension your supplier is quoting.
Can I measure from the outside?
You can measure the exterior frame as a rough check, but exterior measurements include brickmould and casing that the replacement unit will not replicate exactly. For ordering purposes, interior rough opening measurements are far more reliable.
What if I cannot access the rough opening because of painted trim?
Measure the inside of the existing window frame (frame-to-frame) and add 25 mm to each dimension as an estimate. Then ask your installer to verify on-site — they carry pry tools and can check the actual rough opening without damaging your interior finish.
Do I need to measure wall thickness too?
Yes. Wall thickness determines the frame depth your replacement window needs. Standard Canadian wood-frame walls are typically 89 mm (2×4) or 140 mm (2×6) with drywall and sheathing. A frame depth mismatch means the window will not sit flush with interior or exterior finishes.
How accurate do my measurements need to be?
Within 3 mm is ideal for width and height. For the diagonal square check, within 6 mm is acceptable. Beyond that, flag the issue with your installer. Precision matters most for fixed and picture windows, where there is no hardware adjustment available after installation.
