Licensed Impact Window Installer · CGC1540149
Mon–Fri 8–6 · Sat 9–2
(689) 219-5507
Measure an opening, pick window or door, and get a clear estimate of the recommended unit size, rough opening (RO), and masonry opening (MO) — built on standard industry clearances. Free to use, no email required. Serving homeowners across Orlando and Orange County, FL.
Choose your project type, pick window or door, and type the opening you measured. Results update as you type.
Enter a width and height to see your recommended unit size, rough opening, and masonry opening.
Estimates only. These figures use standard industry clearances and are not an order. Real rough-opening and masonry-opening sizes vary by manufacturer (e.g., Andersen, Pella), product line, and wall construction. Always confirm against the manufacturer's spec sheet and a free in-home professional measure — final sizing is the installer's responsibility.
Three numbers describe every window or door job. Knowing the difference is what keeps an order from coming back wrong.
The nominal (call) size of the actual window or door you order — the frame dimensions of the unit itself. It is always a little smaller than the hole it goes into, so there is room to set it square. This is the number that ends up on your order.
The framed opening in a wood- or metal-framed wall — stud to stud, header to sill. As a general industry rule it runs about a half inch larger than the unit in both directions, leaving roughly a quarter-inch shim gap on each side to plumb and level the unit.
The opening in a concrete-block or masonry wall — common in Florida homes. It is cut larger because a wood "buck" is installed inside it first, and the unit then anchors to that buck. That extra clearance is why a block opening is bigger than a framed rough opening.
Older Florida homes settle, so almost no opening is perfectly square. Measuring in three places — and trusting the smallest — is how the pros avoid a unit that won't fit.
Measure the opening width at the top, the middle, and the bottom. Write down all three. The smallest width is the one that matters — the unit has to clear the tightest spot.
Measure the height on the left, the center, and the right. Again, keep all three and use the smallest height. Measure to the structural opening, not to old trim or stops.
Measure both diagonals — if they match, the opening is square. Note the wall depth too, since block and frame walls take different units. When in doubt, a free professional measure settles it.
Getting a window or door to fit is not about one measurement — it is about three numbers that depend on each other. The nominal unit size is the product you order. The rough opening is the framed hole it sets into. The masonry opening is the block hole in a CMU wall that holds the wood buck the unit anchors to. Mix those up, and a unit either won't go in or leaves gaps too wide to seal and flash correctly.
In new construction, you start from the unit. You pick a nominal size, then frame the rough opening about a half inch larger in both width and height so there is a shim gap to set the unit plumb and level. The same logic drives the masonry opening on a block wall — it is cut larger still to accept the buck. Because you control the framing, the sizes work out cleanly from the unit up.
A replacement (retrofit) works the other way around. The opening already exists, so you measure it and order a unit slightly smaller — commonly about a half to three-quarter inch less in width and height — so the new unit slides into the existing frame and can be sealed. That is why our calculator deducts from your measured opening on a replacement, but adds to your unit size on new construction. It is the same standard clearance, applied in the direction your project runs.
On an ordinary window, a sizing mistake is an inconvenience. On an impact (hurricane-rated) window or door, the opening is part of the safety system. These units have to be anchored to the structure so they can resist Florida's wind loads and pressure differentials in a storm. That depends on a correctly sized, structurally sound rough opening, the right fasteners into solid framing or a properly installed buck, and an install that meets the Florida Building Code. Too large an opening, a weak buck, or the wrong anchor pattern can compromise the rating — which is exactly why exact sizing and proper anchoring are non-negotiable on impact projects.
Standard rough-opening guidance is published by the manufacturers — companies like Andersen and Pella list exact rough-opening dimensions for every product they make. Those published numbers always win over any general estimate, because clearances shift by product line, frame material, and fin or flange type. Treat this calculator as a way to understand your project and sanity-check a quote — not as a substitute for the spec sheet.
When you are ready for real numbers, a free in-home measure is the next step. A licensed installer measures every opening, checks for square and wall type, confirms the manufacturer's rough-opening spec for the impact product you choose, and takes responsibility for the final sizing and the permitted, code-compliant install. As a division of American Impact & Energy LLC — a Florida-licensed General Contractor (CGC1540149) with a BBB A- Rating — that is what we do across Orlando and Orange County. Call (689) 219-5507 or request your free quote to get exact sizes for your home.
A rough opening (RO) is the framed hole in the wall — the space between the studs, header, and sill — that a window or door unit sets into. It is intentionally larger than the unit itself so the installer has room to plumb, level, and shim it. As a general industry rule, the rough opening is about a half inch larger than the unit in both width and height, leaving roughly a quarter-inch shim gap on each side. Manufacturers like Andersen and Pella publish exact rough-opening dimensions for each product, and those published numbers always take priority over any estimate.
Measure the existing opening width in three places — top, middle, and bottom — and use the smallest number. Then measure the height in three places — left, center, and right — and again use the smallest. Older Florida homes settle, so openings are rarely perfectly square, and the smallest dimension is what the new unit has to clear. For a true retrofit replacement, the new unit is ordered slightly smaller than the measured opening (commonly about a half to three-quarter inch less in width and height) so it slips in and can be sealed. A free professional measure confirms these numbers before anything is ordered.
It depends on whether you are replacing an existing window or building a new opening. For a replacement, you start from the measured opening and order the unit slightly smaller so it fits the existing frame. For new construction, you choose a nominal unit size and frame the rough opening about a half inch larger. Impact (hurricane-rated) units are sized the same way as standard units — the difference is in the laminated glass and the anchoring, not the nominal dimensions. Our calculator gives you a working estimate, but the exact size for an impact unit should always be confirmed against the manufacturer's spec sheet and a professional in-home measure.
Not necessarily a bigger one, but the rough opening does have to be correct and structurally sound. Impact and hurricane-rated windows rely on a precise, code-compliant rough opening because the frame has to be anchored to the structure to resist wind load and pressure. The shim gap stays in the normal range (about a quarter inch per side), but the surrounding framing, the fasteners, and the buck (in masonry) must meet Florida Building Code so the unit performs in a storm. That is why exact sizing and proper anchoring matter more on impact units than on ordinary replacements.
No. It is an estimate based on standard industry clearances, and it is meant to help you understand sizing — not to place an order. Real rough-opening and masonry-opening dimensions vary by manufacturer, product line, frame type, and how your specific wall is built. Always confirm the final numbers with the manufacturer's published specifications and a free in-home professional measure. Final sizing is the installer's responsibility. Call American Impact Windows and Doors Orlando at (689) 219-5507 to schedule a free, exact measure.
An estimate is a great start — an exact measure is what gets your impact windows and doors ordered right the first time. Schedule a free, no-obligation in-home measure with a licensed local installer. We confirm every opening, match the manufacturer's spec, and handle the permitted, code-compliant install. Call (689) 219-5507 today.
