Measurement Guide
How to Measure Your
Windows & Doors
Get it right the first time. Our step-by-step illustrated guide walks you through everything — including the deductions that ensure a perfect fit.
Measure each window individually. Even windows in the same room that look identical can have different rough openings. Never assume two windows are the same size — houses settle over time and frames shift.
Before You Start
What You'll Need
📏
Steel Tape Measure
Minimum 25-ft, reads in 1/16" increments
🖊️
Pencil & Notepad
Record all 6 measurements for each window
🔦
Good Lighting
Check inside the frame clearly
👤
A Helper
Keeps the tape straight across wide openings
Step-by-Step
4 Steps to Accurate Measurements
Follow these steps in order. Each one builds on the last.
1
Measure the Width — 3 Times
Measure the inside width of the frame (jamb to jamb) at three positions: top, middle, and bottom.
- Measure at the very top of the opening
- Measure at the exact middle
- Measure at the bottom, just above the sill
- Record the smallest of the three — this is your working width
2
Measure the Height — 3 Times
Measure inside height (head jamb to sill surface) at three positions: left side, center, and right side.
- Left side: top of sill to underside of head jamb
- Center of the opening
- Right side: same as left
- Record the smallest of the three as your working height
- Do NOT measure to interior trim or stops
3
Check for Square
An out-of-square frame can cause the new window to bind, not seal properly, or need excessive shimming.
- Measure diagonally: top-left corner to bottom-right corner
- Measure diagonally: top-right corner to bottom-left corner
- If the difference is less than ¼" — you're in good shape
- If more than ¼" — note this for our installer; extra shimming may be needed
4
Measure the Frame Depth
For replacement installs, check that you have enough frame depth to seat the new window unit properly.
- Open existing window and measure from interior stop to exterior stop
- Minimum depth required is typically 3¼ inches
- Shallower frames may require a custom sub-sill or slimline frame profile
- Note any obstructions (radiators, blinds, curtain hardware)
Size Deductions
How Much Smaller Should You Order?
Windows are never ordered at the same size as the rough opening. A small deduction ensures there's room for shimming, levelling, and expansion — so the window operates perfectly for decades.
Standard Deduction Formula
½″ per side
= 1″ total deduction from width AND height
🪟 Window Width
Order Width = RO Width − 1"
Example: 36" RO → Order 35" window
Example: 36" RO → Order 35" window
½" gap on each side gives room to shim and ensure plumb, level installation.
📏 Window Height
Order Height = RO Height − 1"
Example: 48" RO → Order 47" window
Example: 48" RO → Order 47" window
½" at top and bottom for shimming, prevents the window from being a load-bearing element.
🚪 Door Width
Frame Width = RO Width − ½"
Example: 36" RO → 35½" door frame
Example: 36" RO → 35½" door frame
Doors typically need a tighter fit. Most pre-hung door units include their own frame.
🚪 Door Height
Frame Height = RO Height − ¾"
Example: 82" RO → 81¼" door frame
Example: 82" RO → 81¼" door frame
Allows for threshold installation and adjustment of the door sweep at the bottom.
In our quote wizard, you tell us the Rough Opening size. We apply the correct deductions automatically when preparing your order — so just measure the opening itself.
Quick Calculator
What Size Should I Order?
Enter your rough opening dimensions and we'll calculate the window size you should order.
Order Width
—
inches
Order Height
—
inches
Best Practices
Do's & Don'ts
✅ DO
Measure every window separately, even if they look identical
Measure at 3 points for width AND 3 points for height
Always use the smallest measurement
Measure from jamb to jamb (not from trim to trim)
Double-check your measurements before submitting
Note any obstructions (pipes, wires, cabinetry)
Take photos of the opening from multiple angles
❌ DON'T
Measure from the outside of the casing or trim
Assume all windows in a row are the same size
Round measurements — use the exact 1/8" or 1/16"
Measure the glass area only (measure the full frame)
Order at rough opening size — always apply deductions
Measure from the sash (the moving part) — use the frame
Guess — if unsure, call us before ordering
Terminology
Key Terms Explained
Rough Opening (RO)
The framed hole in the wall before any window or door is installed. Always larger than the finished window size.
Frame / Unit Size
The actual dimensions of the window or door unit that gets ordered — after deductions are applied to the rough opening.
Jamb
The vertical sides of the window or door frame. Width is measured from jamb to jamb on the inside.
Head Jamb / Head
The horizontal top piece of the window frame. Height is measured from the sill up to the underside of the head.
Sill
The horizontal bottom of the window opening inside the frame. Height is measured to the top surface of the sill.
Shimming
Thin wedges inserted between the window frame and rough opening to level, plumb, and square the window during installation.
Interior Stop
A small strip inside the window frame that holds the lower sash in position. Do not measure to the stop — measure to the jamb.
Retrofit / Insert
Installing a new window into an existing frame without removing the surrounding trim or frame — most common replacement method.
Ready to use your measurements?
Enter your rough opening sizes directly in our quote wizard — we'll handle the rest.