Blinds in Windows: Measuring, Mounting, and Light Gaps

Getting blinds in windows right comes down to three things. Measure to the narrowest point of the opening. Choose inside or outside mount based on your window’s actual depth. Then expect a small light gap as part of normal, correct operation.

What Do Measuring, Mounting, and Light Gaps Actually Mean for Blinds in Windows?

These three factors decide how well any blind performs, regardless of fabric quality. Measuring determines whether the blind fits the opening precisely. Mounting determines where the hardware sits relative to the window frame. Light gaps are the small, expected spaces where daylight can still enter, even on a correctly fitted blind.

Homeowners often assume a light gap means something went wrong. Usually, it didn’t. Blinds need a small clearance to operate smoothly without rubbing against the frame. Consequently, understanding this from the outset prevents disappointment once the blind is installed and daylight is visible along the edges.

How Do You Measure Blinds in Windows for an Accurate Fit?

Accurate measuring starts with three measurements, not one. Measure the width at the top, middle, and bottom of the opening, then use the narrowest figure. Measure the height at the left, centre, and right, then use the shortest figure.

This matters because very few window openings are perfectly square, especially in older homes. Additionally, depth matters just as much as width and height. The depth measurement is taken from the glass to the front of the architrave. It determines whether an inside mount is even possible for that particular window.

Because Australian window frames vary in size and reveal depth under the AS 2047 building standard, no two homes measure quite the same. A professional measure catches variations that a quick DIY check often misses. This is especially true where a window has been replaced or repaired at some point, since the replacement frame may not match the original reveal exactly.

Inside Mount or Outside Mount: Which Suits Your Windows?

Inside mount fits blinds within the window recess, giving a clean, built-in look. This works well when the recess has enough depth to clear obstructions like handles or locks.

Outside mount fixes the blind to the wall or architrave above and around the window instead. This suits shallow recesses, windows with obstructions, or rooms where extra light blockage matters, such as bedrooms and media rooms. Outside mount also makes a window appear larger, since the blind extends beyond the frame on all sides. For most homes, the choice depends on recess depth first, and personal preference second. A window with only 40mm of usable depth, for example, may simply not suit certain inside-mount hardware, regardless of how the room is styled.

Why Do Blinds in Windows Always Have Some Light Gap?

Every inside-mounted blind needs a small gap between the fabric and the window frame. This gap lets the blind operate without rubbing or jamming against the recess. Without it, the mechanism would bind, and the fabric would wear out faster than expected.

For most living areas, this gap is barely noticeable. In bedrooms and media rooms, however, even a small gap can be enough to disturb sleep. Homeowners who need maximum darkness should consider an outside mount with extra overlap. Alternatively, side channels can block light along the edges without changing the fabric itself. Some fabrics also come with a fully enclosed cassette, which encloses the roll at the top and reduces light spill from above. Our roller blinds range can be fitted either way, depending on how much light control your room actually needs.

How Do Older Brisbane Homes Affect Measuring and Mounting?

Older Queenslander and character homes often have less consistent window openings than modern builds. Timber frames shift slightly over decades, and reveal depths can vary from window to window in the same house.

This is a detail most measuring guides skip entirely, since they’re written for standard, modern frame sizes. In practice, a Queenslander’s front bedroom window might need an inside mount. The window right beside it, meanwhile, might need an outside mount instead. On our own jobs across Brisbane, this variation is common enough that we treat it as the rule rather than the exception. We measure every window individually, rather than assuming one mounting method suits the whole house. Our guide to inside-mount shutters explains recess depth requirements in more detail, and the same principles apply directly to blinds.

Common Mistakes When Measuring and Mounting Blinds in Windows

The most common mistake is measuring only once, at a single point on the window. This misses the narrowest or shortest dimension. As a result, the blind can end up slightly too wide or too short once it’s made.

Another frequent issue is choosing inside mount without checking for obstructions first. A window handle or security stay can prevent the blind from closing fully, even when the recess depth looks adequate on paper. Homeowners also sometimes expect zero light gap from an inside-mounted blind, which isn’t realistic given how the mechanism actually works.

Getting the Mounting Method Wrong

A third mistake is assuming every window in the house needs the same treatment. Older homes especially often need a mix of inside and outside mounts across different rooms, not one blanket approach.

Our Roman blinds guide covers measuring principles that apply across most fabric-based window coverings. Meanwhile, our room-by-room blinds guide explains which fit and fabric combination suits each space in the home.

 

FREQUENTLY ASKED QUESTIONS

How do you measure blinds in windows for an accurate fit?
Measure width and height at three points each, using the narrowest and shortest figures. Depth also matters, since it determines whether inside mount is even possible.

Should I choose inside mount or outside mount?
Inside mount suits windows with enough recess depth for a clean, built-in look. Outside mount suits shallow frames or rooms needing extra light blockage instead.

Why do blinds in windows always have a small light gap?
Inside-mounted blinds need a small clearance so the mechanism can operate without rubbing. For maximum darkness, an outside mount with extra overlap works better.

Do older Brisbane homes need different measuring approaches?
Often, yes, since Queenslander and character homes can have inconsistent reveal depths window to window. Measuring each one individually avoids costly mounting mismatches.

What’s the most common mistake when measuring blinds?
Measuring only once, at a single point on the window. Since few openings are perfectly square, this misses the narrowest or shortest dimension.

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