WindowMD
Measurement Guide
How to Measure Your Windows
Accurate measurements lead to accurate quotes. Here's how to get them right.
Before you start
Understanding what you're measuring
There are two key measurement sets for every window: the rough opening (framed space inside the wall) and the masonry / exterior opening (the opening visible from outside). Recording both helps your contractor order the right size with confidence.
Rough Opening (from inside)
The rough opening is the framed space between the studs where the window sits. Measure between the inside faces of the studs (or the jamb edges if drywall is covering them). Measure width at three points (top, middle, bottom) and height at three points (left, center, right). Record the smallest of each.
- Shows the actual framed space the window fits into
- Used by manufacturers to determine window unit size
- Always measure at 3 points — openings are rarely perfectly square
Masonry / Exterior Opening
The masonry opening is the space visible from the outside of the house. Measure from the inside edge of the brick mold (exterior casing) on one side to the inside edge on the other. This confirms the exterior dimension and helps catch discrepancies with the interior measurement.
- Confirms the true opening size from the outside
- Not affected by interior drywall, paint, or trim changes
- Especially important for brick, stucco, or stone exteriors
Why record both? The rough opening is what the window manufacturer needs, but the exterior measurement catches cases where the framing has shifted or where interior finishing makes the opening appear smaller than it really is. Recording both gives your contractor the full picture.
Step by step
Five checks for every window
For each window: check squareness, measure the rough opening (inside), the masonry opening (outside), the jamb depth, and note the sill angle. Always measure at three points and record the smallest.
Check for Squareness
Measure diagonally from corner to corner on both sides of the opening. The two diagonal measurements should not differ by more than ¼".
If the opening is out of square by more than ¼", note it — the replacement window may need modification or shimming to install correctly.
Tip: This is the most commonly skipped step, but catching an out-of-square opening early saves time and money during installation.
Measure the Rough Opening
Width (A): Measure the distance between the inside faces of the studs at the top, middle, and bottom of the opening. Record the smallest measurement.
Height (B): Measure from the sill plate up to the header at the left, center, and right. Record the smallest measurement.
Tip: Remove any trim or obstructions that may interfere with accurate measurements. The rough opening is the framed space your window fits into.
Measure the Exterior Opening
Width (D): From outside, measure from the inside edge of the brick mold on one side to the inside edge on the other. Measure at top, middle, and bottom. Record the smallest.
Height (E): Measure from the inside edge of the top casing to the inside edge of the sill casing. Measure at left, center, and right. Record the smallest.
Tip: If stucco or render covers the edges, carefully expose enough to see the true masonry dimension.
Measure the Jamb Depth
Depth (C): Measure from the interior stop straight through to the exterior stop. This is the total depth your new window frame needs to fit within.
Ensure there's at least 3¼" of depth to accommodate most window frames. If the depth is shallower, jamb extensions will be needed — WindowMD calculates this automatically.
Tip: Standard 2×4 walls are about 4¾" deep. 2×6 walls are about 6¾".
Check the Sill Angle
The window sill is usually angled to shed water. Determine whether it's a flat sill (0°) or a sloped sill (typically 7–14°).
Place a level on the sill. If the bubble isn't centered, the sill is sloped. You can also place a straight edge across the sill and measure the gap at the high end with a ruler.
Replacement windows are manufactured with specific sill angles. Ordering the wrong angle means the window won't sit flush — water can pool or leak.
Tip: Most residential double-hung windows have a sloped sill between 7° and 14°. Casements and awnings are typically 0–14°.
Contractor Tool
Mulled Assembly Calculator
Building a transom above a window? Each manufacturer uses different joining material dimensions. Enter frame size heights from the catalog — this tool computes the combined rough opening for you.
Mulled Assembly Calculator
Enter the frame size heights of the lower window and transom unit. This tool adds the manufacturer's joining material and clearance to compute the combined rough opening height.
Frame size height from catalog
Frame size height from catalog
If provided, calculates the combined RO width too
Tips for Accurate Measurements
- •Use a steel tape measure — cloth tapes stretch and lose accuracy.
- •Always take the smallest of three measurements for width and height.
- •Deduct about ¼" from each side for proper fitting and shimming space.
- •Take a photo of each window while measuring — it helps when configuring products.
- •Note the floor level and any access issues (steep roof, bushes, tight spaces).
- •Double-check all measurements and label them clearly to avoid confusion.
Common Mistakes to Avoid
- ✕Measuring only the visible glass instead of the full frame opening.
- ✕Skipping the diagonal check — uneven openings cause fitting issues.
- ✕Measuring from only one side (interior or exterior) — record both.
- ✕Relying on the old frame's dimensions — they may be warped or inaccurate.
Ready to Measure?
Create a project, enter your measurements, and generate a professional report you can share with any contractor.