Inside vs Outside Mount: Which Is Right for Your Window?

Before you measure a single window, you have to answer one question: will the blind sit inside the recess or outside it, on the wall or frame? It is the decision that shapes everything else — how you measure, how much light escapes, how the finished window looks, and even how the blind sounds when it swings against a draught. Neither option is universally better; each suits particular windows and particular goals, and the right answer changes from room to room in the same house.

What each mount actually means

An inside mount fixes the blind within the window recess, so the headrail, slats, or fabric sit flush inside the opening, level with the wall around it. An outside mount attaches above the opening, on the wall or the window frame itself, with the blind overlapping the recess on every side rather than sitting inside it. The same physical blind can often be ordered either way — what changes between the two isn’t the product, but the bracket placement and, critically, the measurements you supply. Order the wrong figures for the mount you actually intend to fit, and even a well-made blind will be the wrong size for the job.

The case for an inside mount

Inside mounts give the cleanest, most built-in look of the two. Because the blind sits within the frame rather than in front of it, it shows off any decorative moulding or architrave around the window, leaves the sill completely clear for ornaments, plants, or a window seat, and layers neatly under a pair of curtains if you decide to add them later without either treatment fighting for the same wall space. Designers reach for it by default for exactly this tailored, architectural appearance — it reads as part of the window rather than something hung in front of it.

The trade-off is twofold. First, an inside mount needs a recess deep enough to take the headrail without the blind protruding past the trim; a shallow recess simply rules it out regardless of how much you like the look. Second, it inevitably leaves small light gaps down each side and sometimes along the top, wherever the blind has to clear the reveal to tilt, roll, or lift. For most living spaces that’s a non-issue — a sliver of light at the edge of a kitchen or living-room window rarely matters — but in a bedroom where you want genuine darkness, it’s a real limitation, not a cosmetic quibble.

The case for an outside mount

An outside mount is the more forgiving and more light-blocking of the two, and it earns that reputation directly from the arithmetic. Take a 36 in by 48 in opening: mounted inside, the calculator’s standard clearance brings the order size down to 35.75 in by 48 in — a blind slightly smaller than the recess, by design. Mounted outside on the same opening, the default overlap pushes the order size up to 39 in by 50 in — a blind more than three inches wider and two inches taller than the recess itself. That extra 3.25 in of width swing between the two mounts, on an identical window, is what closes the light gaps an inside mount leaves and is the whole reason outside mounts are the default recommendation for bedrooms and nurseries.

Beyond darkness, an outside mount is also the more forgiving choice for a difficult window: it can make a small window look larger and taller since the blind extends past the actual glass, it hides an out-of-square, chipped, or unattractive recess behind a clean overlap, and it’s the only realistic option when the recess is too shallow to take an inside-mounted headrail at all. The downsides are mostly aesthetic and practical rather than functional: the blind projects further into the room, it covers some of the surrounding moulding rather than showing it off, and you need a flat, solid section of wall or frame above the opening strong enough to anchor the brackets.

How to choose: work through it in order

Rather than guessing, run each window through the same short sequence of questions:

• Is the recess deep enough? Measure the depth and compare it against the blind’s minimum-depth specification. If the recess falls short, an outside mount has already been decided for you — there’s no workaround for insufficient depth.
• How dark does the room actually need to be? For a nursery, bedroom, or media room, an outside mount paired with a blackout or room-darkening fabric blocks meaningfully more light than the same fabric mounted inside. Run the room and your priority through our Light-Blocking Opacity Selector to see the band it recommends before you commit to a mount.
• Is the window itself square and attractive? A neat, deep, well-proportioned recess flatters an inside mount and shows it off; a shallow, uneven, or plain one is usually better hidden behind the cleaner line of an outside mount.
• What sits on the sill or frame? Handles, cranks, deep tile reveals, and a sill you want to keep permanently clear for ornaments or a coffee cup all push the decision toward one option or the other before light and looks even enter the picture.

You don’t have to pick one mount for the whole house

It’s worth saying plainly: this decision is made window by window, not once for the entire home. A deep, square living-room recess with moulding worth showing off is a natural inside mount, while the bedroom next door with a shallower recess and a strong need for darkness is a natural outside mount — and there’s nothing inconsistent about running both in the same house, or even the same room, if one window has a radiator under the sill and another doesn’t. Treat each opening on its own merits rather than forcing a single rule across a house full of different windows.

Measuring follows the mount, not the other way round

Once you’ve chosen, the measuring method is fixed and the two are not interchangeable. For an inside mount, measure the exact opening — width in three places, taking the narrowest — and let the standard clearance be deducted from that raw figure. For an outside mount, measure the same opening and then add your chosen overlap on top of it; you are never deducting for an outside mount, only adding. Our companion guide on how to measure windows for blinds walks through the full technique step by step once you’ve made this call. The Blind & Shade Size Calculator handles both directions itself: pick the mount type, enter the raw opening, and it applies the correct deduction or overlap automatically, so the order size you get out the other end always matches the window you actually have.

Special cases: no recess, a very shallow one, or a very deep one

Not every window even offers a real choice. Some modern builds have windows set almost flush with the interior wall, with no meaningful recess at all — here an outside mount isn’t just preferred, it’s the only option, since there’s nowhere for an inside-mounted headrail to sit. At the other extreme, a very deep reveal — common in older stone or thick-walled brick construction — can comfortably take an inside mount and even leave room for a second, outside-mounted treatment layered in front of it on the same wall, which is how some period homes end up with both a fitted blind and full curtains on the same window without either one crowding the other. And a recess that’s deep but narrow can sometimes fit an inside mount for a slim roller while still ruling one out for a bulkier venetian headrail, so always check the specific product’s minimum-depth figure rather than assuming all inside-mount blinds need the same clearance.

Cost and installation differences between the two mounts

The blind itself typically costs the same whether it’s ordered for an inside or an outside mount — you’re paying for the same fabric or slats either way. What differs is the size you end up ordering and, occasionally, the bracket hardware. Because an outside mount always adds coverage rather than deducting a small clearance, it tends to land you a size or two larger than the equivalent inside mount on the same opening, which can nudge the price up slightly on a per-square-foot pricing model. Installation is broadly similar in effort for both, though an outside mount needs a suitably solid, flat section of wall or frame above the opening to anchor into, which isn’t always a given on a stud wall without adding a mounting board first.

Checking your own choice against a range of window sizes

If you’re weighing this decision across several windows at once — a whole bedroom wing, say, where some recesses are deep and some are shallow — it helps to see how the inside- and outside-mount order sizes actually diverge across a range of common openings rather than reasoning about just one window in isolation. Our Standard Window Size Reference runs both mount types across nine common sizes side by side, which makes it easy to spot that the inside-to-outside size gap grows with the window rather than staying fixed, purely because the outside-mount overlap is added per side and compounds with width.

A quick rule of thumb

If you want a crisp, architectural, tailored look and the recess is deep and square, mount inside. If darkness, forgiving coverage, or a difficult or shallow window is your priority, mount outside. When you genuinely can’t decide either way, default to an outside mount — it forgives more measuring imprecision, blocks more light, and covers more cosmetic sins than an inside mount ever will, which is exactly why it’s the standard safe choice for bedrooms.