A custom shutter is cut to the numbers you send. If those numbers describe the wide part of an opening and the opening is narrower somewhere else, the panel comes back too big to install. If you subtract an allowance the factory also subtracts, the panel comes back too small. The steps below are the ones a major retailer publishes for interior shutters, plus the reason each step exists. They are not a universal building-code tolerance, and they are not a substitute for the worksheet that ships with a particular product.
Tools and habits
Blinds.com’s interior-shutter guide says to use a steel tape, to measure to the nearest 1/8 inch, and, if the mark falls on a 1/16 inch, to round down to the next 1/8 inch. It says to write width first and height second, and to measure every window instead of assuming two openings match because they look the same. A cloth tape stretches. A single measurement at eye level misses a bow in the jamb. Those are the practical reasons for a steel tape and for three readings, not a preference for one brand of tape.
Inside mount
An inside-mount shutter sits inside the window frame or casing. The panel has to pass the narrowest width and the shortest height of that hole. The retail steps are:
- Check squareness by measuring the two diagonals. If the opening is out of square by more than 1/4 to 1/2 inch, that guide recommends an outside mount instead. The range is theirs. A different maker may accept a different amount. The point to keep is that a twisted opening is a poor hole for a rigid panel that was built square.
- Measure the depth of the frame. An inside mount needs enough recess for the shutter’s frame and the tilt of the louvers. The guide says to compare that depth with the mounting requirement for the product you chose. It does not publish one depth for every louver size on the page that was read, so this site does not invent a minimum depth.
- Measure width at the top, the middle, and the bottom. Submit the narrowest of the three.
- Measure height at the left, the center, and the right. Submit the shortest of the three.
- Order the exact opening size, with no deduction of your own. The factory takes the deductions in production.
Why the narrowest and shortest: the panel is a rigid rectangle. It has to fit the smallest cross-section of the opening, not the average. A width taken only at the middle of a bowed jamb can be wider than the top. The shutter then stops on the tight spot, or it forces the frame. Why you do not deduct: if the factory already plans to make the panel a fraction smaller than the number you send, and you have already sent a smaller number, the gap around the panel is twice the allowance. Light leaks and a shutter that looks undersized are the usual result. The size of the factory deduction was not printed as a single figure on the page that was read, so it is not printed here. Use the maker’s sheet.
If the opening is 60 inches or taller, that same guide requires a divider rail and asks you to measure from the bottom of the window up to where the center of the rail should fall. The rail, it says, will be positioned within 1.5 inches of that mark, and on some of their shutters it is centered automatically. Again, that is one factory’s rule. See interior styles for what a divider rail does.
Outside mount
An outside-mount shutter is fastened to the trim or to the wall around the opening, so the frame can be larger than the hole. The retail steps flip the “which of the three readings” rule:
- Measure width at the top, middle, and bottom, and submit the widest.
- Measure height at the left, center, and right, and submit the longest.
- If there is trim, measure the flat face of that trim. The guide says most shutter frames need at least 1 1/2 inches of flat surface to mount properly. That is “most” frames in their line, not a code minimum for every hinge on the market.
- If there is no trim, the guide says not to add extra inches to the opening size yourself. The product is made with the frame’s own added width and height for the frame style you picked.
Why the widest and longest: the outside frame has to cover the opening even where the opening is largest. A short height taken at one side of a sloping sill leaves a gap at the tall side. Adding your own overlap, on top of the overlap the factory builds into that frame style, can make the shutter collide with casing, a handle, or the next window.
Why a small mistake is not small
Fabric drapes can be hemmed. A shutter frame, louver bank, and hinge layout are cut before they reach the house. The costly part of a wrong order is not the tape. It is a panel that will not go into the opening, or a panel that rattles because it was ordered an inch shy. Three widths and three heights take a few minutes. Ordering from one casual reading is how custom work misses.
Stock exterior pairs, including the Amazon listings in the side column, are a different problem. You choose a published width and height. Several of those listings also say the piece you receive can be off the named size by a fraction of an inch: Ekena says widths and lengths are nominal plus or minus 1/2 inch; Ortus and Pontiac Trail each say about plus or minus 3/4 inch. That is a manufacturing tolerance on a stock panel, not the three-point method above. A stock panel that is a half inch off will still not become a custom fit. If the window needs a size that is not in the list, the stock pair is the wrong product.
Two of those listings also put “Inside Mount” in a technical field while describing exterior window shutters. That conflict is noted under the products. Do not treat the field as measuring instructions.
Storm panels are measured differently again. A storm panel has to cover the opening and land its fasteners in framing, which is a layout of holes and anchors, not only a width and a height. FEMA says to follow the manufacturer’s instructions and not to fasten storm shutters to the window frame or to brick veneer. Bring those instructions to the wall before you drill. This measuring page does not redraw fastener schedules.
What was left out on purpose
No clearance such as “always deduct 1/4 inch” is stated as a universal rule, because the retailer above says the opposite for inside mounts: send the opening, and let the factory deduct. No depth minimum is stated for every louver size. No claim is made that two windows in one room match. If a worksheet conflicts with the steps here, the worksheet for that product wins.
Sources
- Blinds.com, How to Measure for Interior Shutters — steel tape, 1/8 inch, three-point width and height, inside versus outside mount, squareness range, 1 1/2 inch flat surface, 60-inch divider rail, factory deductions
- FEMA 499, Technical Fact Sheet No. 26, Shutter Alternatives (August 2005) — attach storm shutters to framing, not to the window frame or brick veneer
- Ekena Millwork listing, Ortus Collective listing, and Pontiac Trail listing — the size-tolerance lines quoted above