Measuring Bay, Arched, and Tilt-Turn Windows

The standard measuring method — three widths, three heights, narrowest and longest — assumes a single flat, rectangular opening. Bay windows, arched windows, and tilt-turn casements all break that assumption in different ways, and each needs a small adaptation before you run the numbers through our Blind & Shade Size Calculator. Here’s how to handle each one, with worked examples.

Bay windows: treat every panel as its own window

A bay window is really two or three separate window openings joined at an angle, not one wide window that happens to bend. The single biggest mistake is assuming the panels are identical and measuring only one of them — in practice, a classic three-panel bay very often has a wider centre panel flanked by two narrower, angled side panels, and even a bay built with nominally matching panels can differ by half an inch or more between them once the house has settled. Measure and calculate each panel completely independently, as if it were an unrelated window elsewhere in the house.

Take a typical three-panel bay: two side panels each with a narrowest recess width of 24 in, and a wider centre panel at 30 in, all sharing a 54 in drop. Run each through the calculator for an inside mount separately, and the two side panels each return an order width of 23.75 in, while the wider centre panel returns 29.75 in — three different order sizes for what looks, from across the room, like one continuous window. Ordering all three panels at the centre panel’s width, on the assumption that “close enough” would do, would leave both side blinds roughly six inches too wide to fit their actual recesses.

Depth is worth a second look on a bay too: the angled side panels sometimes sit in a shallower reveal than the flat centre panel, especially in older houses where the bay was added or altered after the original build, so check each panel’s depth individually against your chosen headrail’s minimum-depth spec rather than assuming the centre panel’s depth applies to the sides.

Bow windows: the same rule, with more panels and shallower angles

A bow window is a close cousin of the bay — typically four or five panels curving out from the wall in a gentler arc, rather than the bay’s usual three panels at sharper angles. The measuring principle is identical: every panel gets its own set of three width readings and three height readings, run through the calculator as its own independent window. Where a bow window adds a wrinkle is that its shallower angles between panels sometimes make the width differences between panels smaller and easier to miss visually, which paradoxically makes it more important to actually measure each one rather than eyeball them as matching — a half-inch difference is much harder to spot by eye across a gentle five-panel curve than across a sharply angled three-panel bay, even though the consequence of missing it is exactly the same ill-fitting blind.

Arched windows: measure the rectangle you can actually cover

Most blinds, whatever the style, are built as a straight-edged rectangle, which means a true arched or Palladian window — one with a curved top — can’t be fully covered edge-to-edge by a standard product without custom fabrication. The common, practical approach is to treat the window as two zones: the rectangular section below the springline (the point where the curve begins), which takes a standard blind sized to that lower rectangle, and the arch itself above it, which is either left as a fixed, uncovered feature, fitted with a custom arch-shaped treatment ordered separately from a specialist, or covered by an outside mount that extends further up the wall to visually absorb part of the curve into its coverage.

Take an arched window whose lower rectangular section measures 36 in by 48 in up to the springline. Mounted inside that lower rectangle, the calculator returns an order size of 35.75 in by 48 in, covering the straight-edged portion cleanly while leaving the arch above it as an uncovered architectural feature — a common and often deliberate look, since many homeowners like the arch left visible. If instead you want the blind to visually reach further up toward the arch’s springline, an outside mount with a larger-than-default top overlap does that: entering a 6 in top overlap instead of the standard 2 in default, with the usual 1.5 in side overlap, on that same 36 in by 48 in base rectangle returns an order size of 39 in by 54 in — six inches taller than the plain inside-mount version, giving the visual impression of reaching higher into the arch without attempting to actually follow its curve.

Tilt-turn windows: measure around the handle, not through it

Tilt-turn windows open two ways from the same handle — tilted inward from the top for ventilation, or swung fully open on hinges from the side — and that handle, along with the hardware it operates, often protrudes into the reveal in a way a standard sash window’s hardware doesn’t. Measuring the raw opening width without accounting for the handle can produce a blind that technically fits the recess but fouls the handle every time someone tries to operate the window, or one that can’t tilt or turn at all once the blind is fitted.

Take a tilt-turn window with a narrowest recess width of 32 in and a 60 in drop, where the handle protrudes roughly a quarter-inch into the reveal when the window is closed. Measured and calculated as a standard inside mount, the calculator returns an order width of 31.75 in — a figure that accounts for bracket clearance but not for the handle specifically, since the handle isn’t a figure the size calculator takes as an input. If the handle’s protrusion would foul that inside-mounted headrail or the tilted blind itself, the practical fix is usually an outside mount instead, which clears the recess entirely: on the same 32 in by 60 in opening, the calculator’s outside-mount default returns 35 in by 62 in, mounted on the wall or frame beyond the recess where the handle has no way to interfere with it.

If you want to keep the cleaner look of an inside mount on a tilt-turn window, measure the specific clearance the handle needs when the window is both closed and mid-operation, and check that figure against your chosen headrail’s own projection into the room — a slim, low-profile headrail can sometimes clear a handle that a bulkier one wouldn’t, even at the same calculated order width.

Labelling multi-panel windows so nothing gets mixed up

Once you’re measuring three, four, or five panels for a single bay or bow window, the biggest practical risk shifts from mismeasuring to mislabelling — recording six or more correct numbers and then losing track of which order width belongs to which panel by the time you actually place the order. Number or letter each panel physically (a small piece of masking tape in the corner of each pane works well) and use the same labels consistently on paper, in the calculator, and on the final order form. This matters even more on a bow window, where four or five very similar-looking panels make it easy to accidentally swap two panels’ figures without noticing until the blinds arrive and don’t quite fit.

What all three shapes have in common

Bay, arched, and tilt-turn windows each break a different assumption behind the standard measuring method — one shared opening, one continuous curved edge, one unobstructed reveal — but the fix in every case is the same underlying habit: identify exactly which assumption doesn’t hold for your specific window, adapt the measurement to work around it, and then let the Blind & Shade Size Calculator handle the arithmetic on whatever adapted figures you feed it, whether that’s three separate panel widths, a springline-limited base rectangle, or a handle-clearing outside-mount overlap. Our broader guide to common blind measuring mistakes covers the errors that trip up even a standard rectangular window, and every one of those still applies on top of the shape-specific adjustments here — measure each panel or section three times, use one unit throughout, and never guess where a precise figure is available instead.

When it’s worth calling in a professional measure

Complex shapes are exactly where a professional in-home measuring visit earns its cost most clearly, particularly for a bay window with more than three panels, a genuinely irregular arch, or a tilt-turn window where the handle clearance is tight enough that a small measuring error would mean an unusable blind rather than just a slightly visible gap. Our guide to costing a whole-house project includes a measuring fee as one of its standard line items precisely because shapes like these are common enough that most retailers price a professional measure as a routine option, not an unusual upsell. Paying for that one professional visit on a genuinely awkward window, and doing the straightforward rectangular windows elsewhere in the house yourself, is often the most sensible split of effort and cost for a mixed-window home.