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Impact Window Measuring Tool
Record each opening the way a working, licensed Florida building contractor measures flange-frame impact windows in the field: width first, then height, in inches — from outside, concrete to concrete, right at the window — and get the ordering size for every opening, including the extra inch when you had to measure from inside over drywall. Your measurements never leave this page.
Openings
1 openingWidth first, then height, in inches. Fractions as decimals (e.g. 52.5). Pick where you measured.
Live measuring sheet
| Opening | Where measured | Measured W × H (in) | Ordering size W × H (in) |
|---|---|---|---|
| Opening 1 | Outside — concrete to concrete | — | — |
A measuring guide for planning and estimating — final measurements are the installing contractor's responsibility. Your window must match the opening type and carry the required Florida product approvals; in Broward and Miami-Dade (the High-Velocity Hurricane Zone) that means a Miami-Dade NOA.
Fin-frame (nail-fin) windows are measured differently
The method above is for flange-frame windows, which suit masonry and block openings — the standard construction in South Florida. Nail-fin windows are a different construction type: they are made for wood-framed walls and anchor to the framing through a perimeter fin around the window.
For a fin window you measure the wood rough opening instead: jack stud to jack stud for the width, and sill to header for the height — taken at three points each, using the smallest of the three. Manufacturers typically size the window about 1/2 inch smaller than the rough opening, so always order from the manufacturer's sizing chart, and follow their installation instructions and the product's Florida Product Approval.
Sources: Pella nail-fin installation guide (PDF) · Marvin on nailing fins
Frequently asked questions
What is the difference between a flange-frame and a fin-frame window?
A flange-frame window is made for masonry and block openings — the standard construction in South Florida — and fastens into the concrete around the opening. A fin-frame (nail-fin) window is made for wood-framed walls and anchors through a perimeter fin fastened to the wood framing. They are measured differently: this tool covers the flange-frame method, while fin windows are sized from the wood rough opening using the manufacturer's sizing chart.
Why measure from outside, concrete to concrete?
Because that is the actual masonry opening the flange-frame window sets into. Measuring from outside, right at the window, concrete to concrete — the inside dimension of the opening only — gives you the true opening size with no interior finishes in the way.
Why add an inch when measuring from inside over drywall?
That is the field rule this tool is built on: measuring from inside over drywall, the drywall and the window sill hide about an inch of the true opening. Adding 1 inch to both width and height gets you back to the same numbers you would get measuring the concrete directly. On an interior concrete wall nothing is hidden, so the numbers match the outside measurement as-is.
Do impact windows need a Miami-Dade NOA?
In the High-Velocity Hurricane Zone — Broward and Miami-Dade counties — yes: products need a Miami-Dade Notice of Acceptance (NOA). Everywhere in Florida, the window must match the opening type and carry the required Florida product approvals for the application.