To measure single hung windows, take three width readings across the inside of the frame — top, middle, and bottom — and three height readings down each side. Use the smallest number from each set. That smallest dimension is your working size, and it’s the number any window supplier or installer needs before ordering a replacement unit.
Getting this wrong is expensive. An undersized window leaks air and rattles in high wind. An oversized one simply won’t fit. Whether you’re pricing a single hung window replacement or ordering direct, this guide covers every measurement you need, in plain language.
Tools to Gather Before You Start
You don’t need much. Before touching the window, set these out:
- Steel tape measure — not a fabric one, which stretches over distance
- Pencil and a notepad, or your phone’s notes app
- A step stool if the window is above shoulder height
- A small level to check whether the frame is plumb and square
- A helper is useful for tall windows but not essential
Skip laser distance finders for this job. They bounce off glass at odd angles and produce unreliable readings on window openings.
The Three Types of Window Measurements
Before you measure anything, it pays to know what each number means. Mixing these up is the single most common reason a replacement window comes back the wrong size.
| Measurement Type | What It Refers To | When You Need It |
| Rough opening (RO) | The framed hole in the stud wall | New construction or full-frame replacement |
| Frame size | Outer edge of the window unit | Ordering a direct insert replacement |
| Glass (daylight) size | Visible glazed area | Comparing views, checking glazing specs |
For most replacement jobs in existing homes, frame size is what you’re after. The rough opening is slightly larger — it’s the hole in the wall before the window goes in. The glass size is always smaller than the frame; measuring it by mistake is one of the most common errors homeowners make.
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How to Measure Width
Open the lower sash and measure across the inside of the frame. Take readings at three points: near the top of the frame, across the middle, and near the sill at the bottom.
Write down all three numbers. Your working width is the smallest of the three.
For a pocket or insert replacement — where the new window slides into the existing frame — subtract 6 mm (¼ inch) from that smallest reading. This clearance lets the unit seat properly without forcing it into place.
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How to Measure Height
Measure from the inside of the sill (the flat horizontal surface at the very base of the frame) up to the inside of the head jamb at the top. Take three readings: along the left side, through the centre, and along the right side.
Record all three. Use the smallest.
“Most measurement errors happen at the sill. Homeowners measure to the stool — that’s the decorative trim piece sitting on top of the sill — instead of the actual sill itself. Missing that gap by even 10 mm throws the whole order off. Always get your tape down to bare wood or the frame surface, not the trim.” — Mark Hennessey, Senior Window Installer, Ottawa ON, 22 years in residential fenestration
A single hung window has a fixed upper sash and a moveable lower sash that slides vertically. The lower sash takes the most wear over time. If it sticks, has broken weatherstripping, or lets in cold air along the sides, that’s usually the sign a replacement is overdue.
Step-by-Step Measurement Checklist
Work through this in order and you won’t miss anything:
- Open the lower sash fully to give yourself clean access to the frame interior.
- Measure width at the top, middle, and bottom of the opening — write down all three.
- Measure height at the left edge, centre, and right edge — write down all three.
- Note the frame depth (front to back) if ordering a full-frame replacement unit.
- Photograph the original window’s factory label — it’s usually on the frame or glass edge and shows the original size code.
- Check the frame for racking: place a level on the sill and against the side jambs. More than 6 mm out of square and a pocket replacement may not work cleanly.
Rough Opening vs. Frame Size: Quick Reference
| Scenario | What to Measure | Typical Size Allowance |
| Pocket / insert replacement | Inside frame width × height | Deduct 6–10 mm from both dimensions |
| Full-frame replacement | Rough opening width × height | Window unit is RO minus 12–18 mm per side |
| New construction | Rough opening only | Framer sets the RO; window is ordered to match |
Homes in older parts of Scarborough and Mississauga often have brick or block construction where rough openings have shifted over decades. In those cases, taking multiple readings isn’t a suggestion — it’s the only way to know what you’re working with before ordering.
Signs Your Frame May Need Full Replacement, Not Just an Insert
Sometimes the frame itself is past saving. Watch for these:
- Visible rot or soft wood when you press on the sill or jamb
- Gaps between the frame and the surrounding masonry or cladding
- Condensation forming inside the frame cavity, not just on the glass
- The frame rocks or flexes when you push on it
- Paint or caulk are the only things holding air out
If any of these are present, an insert replacement won’t solve the problem. You’ll need a full-frame swap, which means a slightly different measuring approach — rough opening dimensions rather than frame size. An installer can assess this quickly during a site visit.
Comparing Single Hung to Other Window Styles
If you’re on the fence about whether to replace like-for-like or switch to a different style, it’s worth reading about single vs. double hung windows before you order. The rough opening dimensions are typically the same for both, but double hung units can be heavier, which affects what the sill and frame need to support.
Double hung window replacement follows the same measuring principles. So does ordering to standard double hung window sizes. Three widths, three heights, smallest wins — that rule applies across styles.
If you’re also comparing against fixed options for a non-venting wall, fixed replacement windows are measured exactly the same way.
Common Measuring Mistakes to Avoid
Most errors are simple and easy to prevent once you know what to watch for:
- Taking only one measurement and assuming the frame is perfectly square
- Measuring the glass area instead of the inside of the frame
- Forgetting the 6 mm deduction on pocket replacements
- Measuring to the interior trim rather than the actual frame edge
- Not checking whether the frame is plumb before ordering”A lot of DIYers measure the glass instead of the frame and then wonder why the new unit won’t seat properly. The glass is always smaller than the frame — anywhere from 25 to 50 mm on each side depending on the profile. You measure the frame. Always.” — Sandra Kowalski, Certified Window Consultant, Toronto ON, member of the Fenestration Association of Canada
When to Hand It Over to a Professional
Some windows are awkward to measure safely on your own. If yours is above a stairwell, in a basement hopper configuration, or part of a bay window assembly, professional measurement is the sensible call. Installers know how to read wall conditions, account for insulation depth, and check drainage slopes — details that affect both fit and long-term performance.
At TotalSeal, we include free in-home consultations with every quote. Our team measures each opening before a single profile is cut at our 60,000 sq ft manufacturing facility, so the window we build fits on the first visit. If you’re in Vaughan, Brampton, or anywhere across the GTA, contact us and we’ll arrange a site visit at no cost.
Every TotalSeal window is ENERGY STAR™ certified and manufactured to CAN/CSA-A440 standards. Accurate measurements don’t just mean a good fit — they mean a window that performs exactly as designed, sealing out cold air and holding its efficiency rating year after year.
FAQ
How many measurements do I need to take for a single hung window?
Take three width readings (top, middle, bottom) and three height readings (left, centre, right). That’s six total. Use the smallest figure from each set as your working dimension when ordering a replacement unit.
What is the difference between frame size and rough opening on a single hung window?
The rough opening is the framed hole in the wall before any window is installed. The frame size is the outer dimension of the window unit itself. For most replacement jobs, you measure the frame size and subtract a small clearance allowance.
Do I measure from inside or outside the house?
Measure from inside wherever possible. You get direct access to the actual frame edges without exterior cladding, brick moulding, or flashing getting in the way, which can give misleading numbers.
How much smaller should a replacement single hung window be than the opening?
For a pocket or insert replacement, order the window 6–10 mm smaller in both width and height than the inside frame measurement. For a full-frame replacement, the unit is typically 12–18 mm smaller than the rough opening per side.
What if my single hung window frame is out of square?
If the frame is racked by more than 6 mm, a standard pocket replacement may not seat correctly. You will likely need a full-frame replacement. An installer can confirm this on-site and advise whether the existing frame is worth retaining or needs to come out entirely.
