How to Measure for Blackout Blinds: A Step-by-Step Guide

Custom blackout blinds are only as good as the measurements behind them. A blind can use the highest quality fabric available, but if the width or height is off by even a centimetre, you will end up with the exact same light gaps as a standard, off-the-shelf option.

The good news is that measuring for custom blackout blinds is not complicated. It just needs to be done carefully, with the right tools, and with a clear understanding of what actually gets measured. This guide walks through the process step by step.

Why Accurate Measuring Matters More Than Fabric Quality

A blackout blind blocks light in two ways: the fabric itself, and the seal around the edges. Fabric quality is largely fixed once you choose a product, but the seal depends entirely on how closely the blind matches your window's actual dimensions.

Even a small measuring error, a few millimetres on one side, can leave a visible gap once the blind is installed. This is the same issue that causes many standard "blackout" blinds to underperform: not a fabric failure, but a fit failure that starts at the measuring stage.

What You Will Need Before You Start

Measuring for a custom blind takes about ten minutes per window and requires only a few basic tools:

• A steel tape measure, rather than a fabric or retractable cloth one, for accuracy
• A pencil and paper, or a notes app, to record each measurement immediately
• A step stool if your window is tall enough that you cannot comfortably reach the top
• Good lighting, so you can clearly see the frame edges and any unevenness

It helps to measure alone rather than relying on memory, since even a short delay between measuring and recording can lead to mixed-up numbers, especially if you are measuring multiple windows in one session.

Step 1: Decide Between an Inside Mount and an Outside Mount

Before measuring, you need to know whether your blind will sit inside the window recess or mount outside it, over the frame. This decision affects every measurement that follows.

Inside Mount

An inside mount sits within the recess itself, giving a cleaner, flush appearance. This works well if your recess has enough depth to hold the blind and its mounting hardware without interference from handles or window cranks.

Outside Mount

An outside mount is fixed to the wall or frame surrounding the window, extending beyond the recess on all sides. This is often the better choice for shallow recesses, uneven frames, or windows where maximum light blockage is the priority, since the blind can extend further than the opening itself.

If you are unsure which mount type suits your window, it is worth reviewing how installation works before measuring, since the mounting style changes where you take each measurement from.

Step 2: Measure the Width

Width is typically the measurement most affected by uneven frames, so this step needs extra attention.

• For an inside mount, measure the width at the top, middle, and bottom of the recess
• For an outside mount, measure from where you want the blind to start on one side to where it ends on the other, allowing extra coverage beyond the frame edges
• Record all three measurements separately if they differ, and use the smallest one for an inside mount to avoid ordering a blind that does not fit

Older homes especially tend to have recesses that are not perfectly rectangular, sometimes by several millimetres from top to bottom. Catching this at the measuring stage prevents a gap from appearing later.

Step 3: Measure the Height

Height follows a similar process to width, with measurements taken from multiple points rather than a single reading.

• For an inside mount, measure the height at the left, center, and right of the recess
• For an outside mount, measure from where the blind will start above the frame to where it will end below the sill
• As with width, use the smallest measurement for an inside mount, and the largest for an outside mount, to ensure full coverage

If your window has an angled or uneven sill, note this separately, since it can affect how flush the bottom edge of the blind sits once installed.

Step 4: Measure the Depth (Inside Mount Only)

If you are planning an inside mount, you also need to measure the depth of the recess, meaning how far back the window sits from the front of the wall.

This measurement determines whether there is enough clearance for the blind's mounting hardware, and whether it will clear any handles, locks, or cranks on the window itself. A recess that is too shallow may not be suitable for an inside mount at all, in which case an outside mount is usually the better option.

Step 5: Note Any Obstructions

Before finalizing your measurements, check for anything that could interfere with the blind once it is installed:

• Window handles, locks, or cranks that protrude into the recess
• Blinds, curtains, or hardware from a previous installation that has not been removed
• Trim, molding, or decorative elements that reduce usable space for an outside mount
• Nearby furniture or fixtures that could limit how far the blind opens

Noting these details alongside your measurements helps avoid a mismatch between what fits on paper and what actually works once the blind arrives.

Common Measuring Mistakes to Avoid

A few mistakes come up repeatedly, even among people measuring carefully:

• Measuring only once. Taking a single measurement at one point on the window, rather than at multiple points, is the most common cause of a poor fit on uneven frames.
• Rounding numbers. Rounding to the nearest half centimetre or inch might seem harmless, but on a custom product, that rounding is exactly the kind of small gap that leads to a visible light leak.
• Confusing recess measurements with frame measurements. Measuring the outer frame instead of the inner recess, or vice versa, produces a blind that does not match the intended mount type.
• Forgetting to account for obstructions. A measurement that is technically accurate but ignores a window crank or handle can still result in installation problems later.

What Happens After You Submit Your Measurements

Once your measurements are recorded, they are used to manufacture your custom-fit blackout blinds to those exact dimensions, rather than adjusting a standard size to approximate your window. This is the step that directly addresses the edge-gap problem common with off-the-shelf blackout products.

If your window has an unusual shape, a shallow recess, or any feature that makes measuring feel uncertain, it is reasonable to double-check before submitting. A short consultation can confirm whether your measurements and mount choice are a good match for your specific window.

When to Get Help Instead of Measuring Alone

Most windows are straightforward to measure independently, but a few situations are worth a second opinion before ordering:

• Bay windows, angled windows, or windows with irregular shapes
• Very old homes where frames may be significantly out of square
• Windows with unusual hardware, such as built-in blinds between glass panes
• Uncertainty about whether an inside or outside mount is the better choice

In these cases, a free 15-minute call with a specialist can help confirm your approach before you commit to an order, which is often faster than measuring, ordering, and discovering an issue after the blind arrives.

FAQ: How to Measure for Blackout Blinds

How do I measure my window for custom blackout blinds?

Measure the width and height at multiple points, using the smallest measurement for an inside mount or the largest for an outside mount. This accounts for the fact that many window recesses are not perfectly square, and using a single measurement point risks ordering a blind that does not seal properly along one or more edges.

Should I measure the window frame or the recess?

This depends on your mount type: measure inside the recess for an inside mount, or the surrounding wall area for an outside mount. Mixing up the two is one of the most common measuring errors, since it produces a blind sized for a different installation method than the one you actually plan to use.

What tools do I need to measure for blackout blinds?

A steel tape measure is recommended over a fabric or cloth tape for accuracy, along with a way to record each measurement immediately. A step stool may also be necessary for taller windows, and good lighting helps you clearly see the edges of the frame or recess while measuring.

What if my window frame is not perfectly square?

This is common, especially in older homes, and is exactly why measurements should be taken at multiple points rather than just once. Recording the width and height at the top, middle, and bottom (or left, center, and right) allows you to identify unevenness and choose the correct measurement for a proper fit.

Can I measure for an inside mount if my recess is shallow?

It depends on the depth available, since inside mounts need enough clearance for the blind and its hardware to sit within the recess without interference. If your recess is too shallow, an outside mount is usually the more reliable option, and measuring the depth in Step 4 will tell you which approach makes sense for your window.