Inside vs Outside Mount: Which to Choose and Why It Changes Your Measurements

Every custom-blind order form starts with one question that quietly decides everything after it: inside mount or outside mount? Pick inside and the blind sits recessed in the window frame, the factory shaves a deduction off your width, and the depth of your frame suddenly matters. Pick outside and the blind mounts on the wall or trim above the opening, you add overlap instead of subtracting a deduction, and depth stops mattering almost entirely. The two mounts are measured differently, priced similarly, and fail differently — so the choice is not cosmetic. Here is how to make it, and exactly how each path changes the numbers you write down.

The 30-second version

Inside mount: recessed, clean, and depth-dependent

An inside mount drops the blind into the window box so the trim frames it. It is the tidier look and the usual first choice — but it comes with two requirements.

First, depth. The frame has to be deep enough for the product's headrail to sit inside, either flush with the wall or recessed a little. A cellular shade needs only about 3/4 inch of flat depth for a flush mount; a roller needs roughly 2 inches so the fabric roll clears; wood and faux-wood want about 2 inches or more; a roman about 1.5 inches. Measure the depth from the glass to the front edge of the opening, and note any handles, cranks, or locks that eat into it. The full reference is in minimum mount depth by product; when in doubt, defer to the depth chart on your seller's product page.

Second, the deduction. Because the blind has to fit inside the opening with clearance for its brackets and to travel without scraping, it is manufactured slightly narrower than the opening — typically 1/4 to 1/2 inch off the width, though the exact figure is set by each seller. Here is the part people get wrong: most sellers ask for your exact opening measurement and apply that deduction themselves at the factory. Do not subtract it yourself unless the seller's guide explicitly tells you to, or you will double-deduct and end up with a blind a half-inch too narrow, leaking light on both sides. Read why inside-mount blinds are cut narrower, then follow the seller's own measuring guide on whether to enter the opening or the deducted size.

Outside mount: coverage, darkness, and forgiveness

An outside mount fastens above the opening — to the wall, the trim, or a mounting board — and hangs down over it. It is the answer when an inside mount will not work or will not perform:

• Shallow frames that fail the depth test.
• Out-of-square openings, where an inside blind would show a wedge of daylight; an outside mount hides the irregular edges.
• Maximum darkness, because covering the opening plus overlap closes most of the side light gaps an inside mount leaves.
• Small windows you want to look larger, since a wider, taller outside mount makes the window read bigger.

Outside mounts are also more forgiving of a measuring error — there is no tight opening to fit, so being a fraction off is invisible. The trade is the look (you cover the trim) and the need to plan overlap and clearance around handles, cranks, and adjacent walls.

How the choice changes your measurements

This is where the two paths diverge on the order form. Same window, two very different sets of numbers.

Inside mount, worked example. Measure the width at the top, middle, and bottom of the opening and use the narrowest. Say you get 36 1/8, 36, and 36 1/16 inches — you write down 36 inches as your opening width. Measure the height at the left, center, and right and use the longest for most inside products (defer to your seller). Then you enter 36 as the exact opening and let the factory take its deduction. If — and only if — your seller instructs you to deduct yourself at, say, 1/2 inch, you would order 35 1/2 inches. You never do both.

Outside mount, worked example. Start from the opening you want to cover — say a 36-inch-wide opening — and add overlap so light does not sneak around the sides. A common allowance is about 1.5 to 2 inches per side, so you might order a 39 to 40 inch wide blind. For height, plan the brackets 2 to 3 inches above the opening and measure down to where you want the blind to end (sill, apron, or below), folding that top allowance into your ordered height. Nothing is deducted; you are building in coverage, not subtracting clearance.

Notice the reversal: inside mount is exact-opening-minus-a-deduction; outside mount is opening-plus-overlap. Mixing the two logics is a leading cause of remakes.

The decision checklist

Run these in order:

1. Measure the depth. Does the frame meet the product's minimum for a flush inside mount? If not, go outside mount (or a low-profile headrail if the seller offers one).
2. Check square. Is the opening within about 1/4 inch top-to-bottom and side-to-side? Badly out-of-square favors an outside mount.
3. Rank your priority. Cleanest built-in look points inside; maximum darkness or a bigger-looking window points outside.
4. Look for obstructions. Cranks, handles, alarm sensors, or a tight sill can force an outside mount or a specific projection bracket.
5. Confirm the seller's rule. Whether to enter the exact opening or a deducted size — and the overlap they recommend for outside mounts — is on their measuring guide. It wins over any generic advice, including this page.

Our Measure & Mount Fit Checker walks the same logic interactively: enter the width, height, mount, and product type and it shows the deduction range and the minimum depth for that combination, then prints a checklist you can carry to the window.

One caveat that applies to both

Whichever mount you choose, a custom blind is made to your size and is generally not returnable — confirm the seller's remake policy before you order, because that policy is your only real safety net if a number is wrong. Fit coverage varies widely: some sellers cover a small measuring mistake, some sell that coverage as a paid add-on, and some publish none at all. Order fabric swatches, measure twice, and let the seller's guide settle the deduction. For the errors that most often trigger a remake, see the most common measuring mistakes. Measure twice, order once.