How to Measure Your Windows for Blinds

Accurate measurements are the foundation of well-fitting blinds. Whether you're ordering custom blinds online or purchasing from a local supplier, getting your measurements right the first time saves money, prevents frustration, and ensures your window coverings look and function as intended. This guide will walk you through the measurement process for all common scenarios.

Before you begin, gather your tools: a steel tape measure (cloth measures stretch and are less accurate), a pencil, paper to record your measurements, and a step ladder if you have high windows. Allow yourself plenty of time—rushing measurements leads to errors.

Understanding Mounting Options

Before measuring, you need to decide how you want to mount your blinds. This decision affects both how you measure and the final appearance of your window treatment.

Inside Mount (Recess Mount)

Inside mounting places the blind within the window recess, creating a clean, built-in appearance. This option works best when you have an attractive window frame you want to display, sufficient depth within the recess (typically 50mm or more, depending on the blind type), and relatively square window openings.

The key advantage of inside mounting is the streamlined look it creates. The blind sits flush within the window opening, leaving the surrounding wall and frame visible. However, it requires precise measurements since there's minimal margin for error, and some light will typically leak around the edges.

Outside Mount (Face Mount)

Outside mounting attaches the blind to the wall or window frame face, extending beyond the window opening. Choose this option when your window recess is too shallow for inside mounting, you want maximum light blocking (blinds can extend past the window edges), the window opening isn't square, or you want to make windows appear larger.

Outside mounting is more forgiving with measurements and offers better light control, but the blind hardware will be visible against the wall.

Measuring for Inside Mount

Inside mount measurements require precision because the blind must fit within the window recess without rubbing or leaving gaps.

Step 1: Measure the Width

Measure the width of the window opening at three points: the top, the middle, and the bottom. Window recesses are often not perfectly square, so measuring at multiple points is essential. Record the smallest of the three measurements—this ensures your blind will fit without jamming.

Measure from the inside edge of the window frame on one side to the inside edge on the other side. Don't include any protruding handles, locks, or trim in your measurement.

Step 2: Measure the Height

Measure the height of the window opening at three points: the left side, the centre, and the right side. For height, record the largest measurement to ensure your blind covers the entire opening.

Measure from the inside top of the window frame to the sill or to where you want the blind to end if there's no sill.

Step 3: Check the Depth

Measure the depth of the window recess from the glass to the front edge of the frame. Different blind types require different minimum depths:

• Roller blinds: 50-70mm minimum depth
• Venetian blinds: 40-75mm depending on slat size
• Vertical blinds: 75-90mm minimum depth
• Cellular blinds: 40-50mm minimum depth

If your recess doesn't have sufficient depth, consider outside mounting instead.

Measuring for Outside Mount

Outside mount measurements are more flexible because you're determining how much of the wall the blind will cover, not fitting into a specific opening.

Step 1: Measure the Width

Decide how far you want the blind to extend beyond the window on each side. A common approach is to add 50-100mm on each side of the window opening. This provides better light blocking and creates the illusion of a larger window.

Measure the width you want the blind to cover, from the left edge to the right edge of your desired coverage area. For example, if your window opening is 1000mm wide and you want 75mm overlap on each side, your blind width would be 1150mm.

Step 2: Measure the Height

Determine where you want the blind to start (your mounting point) and where you want it to end. For maximum light blocking, mount the blind at least 100mm above the window opening and extend it 50mm below the sill or window edge.

Measure from your intended mounting point to your intended endpoint. This is the height you'll order.

Step 3: Check for Obstructions

Before finalising outside mount measurements, check for any obstructions that might interfere with the blind or its mounting:

• Air conditioning units
• Window winders or handles
• Light switches or power points
• Curtain rods or tracks
• Picture rails or cornices

Special Window Situations

Bay Windows

Bay windows require individual measurements for each section. Measure each panel separately as if it were an independent window. Pay special attention to the angles where panels meet—you may need special brackets or consideration for blind overlap.

Sliding Doors

For sliding doors, measure the entire door opening if you want one blind to cover everything (common with vertical blinds). Alternatively, measure each door panel separately for individual roller blinds. Consider which direction the door slides and ensure your chosen blind style won't obstruct door operation.

French Doors

French doors typically work best with individual blinds mounted on each door. Measure each door glass panel, and consider how the blind will move with the door. Magnetic or tension-mounted blinds designed for doors prevent the blind from swinging when the door opens.

Skylights

Skylight blinds require precise measurements due to their angled installation. Measure the glass area width and height, and note the angle of the skylight. Most skylight blinds require custom ordering and professional installation.

Recording Your Measurements

Organised record-keeping prevents confusion, especially when measuring multiple windows:

• Label each window clearly (e.g., "Master Bedroom - Left Window")
• Record width first, then height (W x H is the standard convention)
• Note whether measurements are for inside or outside mount
• Include any obstructions or special considerations
• Take photos of each window for reference

When to Call a Professional

While measuring for blinds is a straightforward DIY task for most windows, some situations benefit from professional measurement:

• Complex window shapes (arches, circles, angled tops)
• Very large windows or sliding doors
• Historical homes with non-standard window constructions
• When ordering expensive blinds where mistakes are costly
• Multiple windows throughout a home (bulk orders)

Many blind suppliers offer free measure-and-quote services. The measurer's expertise can be valuable, especially for complex installations, and the supplier takes responsibility for measurement accuracy.

Common Mistakes to Avoid

Learning from others' errors can save you time and money:

• Measuring only once: Always measure at least twice, and check at multiple points
• Using a cloth tape measure: Cloth stretches; use a steel tape for accuracy
• Rounding measurements: Record exact measurements to the millimetre
• Forgetting to check depth: Insufficient recess depth is a common oversight
• Not accounting for obstructions: Window handles, locks, and other hardware can interfere with blind operation
• Mixing up width and height: Clearly label each measurement as you record it

Taking your time with measurements pays dividends when your perfectly-fitted blinds arrive and install smoothly. With careful attention to detail, you'll achieve professional-looking results that enhance your home for years to come.