How to Measure for Venetian Blinds: Step-by-Step UK Guide

Venetian blinds are unforgiving of measurement errors. A blind that is 5 mm too wide will catch on the recess and fail to tilt properly. A blind that is 5 mm too narrow will allow a visible stripe of light down each side that defeats the purpose of fitting a blind at all. Unlike curtains, where a few centimetres either way rarely matters, venetian blinds are precision products — and the measurement process needs to reflect that. This step-by-step guide walks you through measuring a venetian blind correctly for both recess and face fit installations.

Before You Start: Tools You Need

A steel tape measure is essential — do not use a fabric dressmaking tape or a retractable plastic rule, both of which can flex and give inaccurate readings. A pencil and notepad to record measurements as you take them. A spirit level is useful to check whether the top of the recess is truly horizontal before you take your drop measurement.

Step 1: Decide on Recess Fit or Face Fit

The first decision is whether you want to fit the venetian blind inside the window recess (recess fit) or in front of it, on the wall or window frame (face fit). The two methods produce different measurements and different visual results.

A recess fit gives a neat, integrated appearance where the blind sits flush within the window opening. The blind is hidden within the recess when raised and completely contained when lowered. This requires a recess depth of at least 55–65 mm (to accommodate the headrail, the slat stack when raised, and sufficient clearance for the slats to tilt without touching the glass).

A face fit positions the blind on the wall or frame above and around the window opening. It requires less recess depth (none at all if fitted to the wall), allows the blind to overlap the window surround for better light blocking, and is the appropriate choice where the recess is too shallow for a recess fit.

Step 2: Measure the Width

For a Recess Fit

Measure the internal width of the recess at three points: near the top, in the middle, and near the bottom. Record all three measurements. Use the smallest figure — this is your ordering width. Most venetian blind manufacturers automatically deduct a small amount (typically 10–12 mm in total) from your stated width to ensure the finished blind fits without the slats catching on the recess sides. Always check whether the supplier you are ordering from applies this deduction, or whether you need to make it yourself before ordering.

For a Face Fit

Measure the width of the window opening (the glass, not the frame), then add at least 50 mm to each side — giving a total addition of 100 mm. This overlap prevents light from entering around the sides of the blind. For a venetian blind where light blocking is particularly important, consider adding 75 mm each side rather than 50 mm.

💡 Always measure at three points and use the smallest figure. Window recesses in older UK homes are rarely perfectly square — top-to-bottom variation of 5–10 mm is common, particularly in period properties.

Step 3: Measure the Drop

For a Recess Fit

Measure the internal drop of the recess at three points — left side, centre, and right side — from the top of the recess to the windowsill. Use the smallest figure. The finished blind should reach to within 10–15 mm of the sill to allow the slats to tilt freely at the bottom. If your measurements show significant variation (more than 10 mm) between left and right, the sill or top of the recess may not be level — check with a spirit level and plan accordingly.

For a Face Fit

Decide how far above the window opening you want to position the top bracket, and measure from that point to the sill or to the floor if you want a floor-length drop. Most face-fixed venetian blinds above a window are positioned 50–75 mm above the opening to leave room for the bracket and allow the headrail to clear the window frame.

Step 4: Check the Recess Depth

If you are planning a recess fit, measure the depth of the recess — the distance from the front of the window frame to the glass surface. This measurement is critical for venetian blinds because the slats need room to tilt without touching the glass, and the headrail needs space to sit without projecting beyond the front of the frame.

As a minimum, you need 55 mm of recess depth for a 25 mm slat venetian blind and around 75 mm for a 50 mm slat blind. If your recess depth is less than this, a face fit is the right approach. If the depth is borderline, check the specific minimum depth stated by your blind supplier, as this varies by product.

Step 5: Record and Double-Check

Write down: the ordered width (after deductions if applicable), the ordered drop, and the fitting type (recess or face). Before submitting your order, measure everything a second time independently — do not simply re-read your original notes, as a misread figure can introduce the same error twice. Double-check that the measurements are in millimetres and that you are ordering the correct fitting type for your window.

Common Measuring Mistakes

Measuring the Frame Rather Than the Recess

For a recess fit, always measure the internal recess width and height — not the outer frame dimensions. The outer frame is wider and taller than the recess opening, and ordering to those dimensions will give you a blind that is too large to fit inside.

Forgetting the Manufacturer's Deduction

If the supplier states that they will deduct 10 mm from the supplied width (to allow for clearance), and you already deduct 10 mm from your measurement before ordering, the finished blind will be 10 mm narrower than you intend. Read the manufacturer's instructions carefully and only deduct what they ask you not to deduct.

Measuring Across the Glass Rather Than the Recess

The glass pane is always narrower than the recess opening. If you measure the glass width rather than the recess internal width, you will order a blind that is too narrow. Always measure from recess side to recess side, not from glass edge to glass edge.

Once your measurements are confirmed, browse our full range of venetian blinds in wood, faux wood, and aluminium — all available in a wide range of sizes.

Measuring for a Bay Window

For a bay window, each pane must be measured independently. Do not assume that the three panels of a bay are all the same size — they almost never are. Measure each pane separately for width and drop, record each set of dimensions clearly (left panel, centre panel, right panel), and order each blind individually to the correct dimensions.

Summary Checklist

Before placing your order: decide on recess or face fit, measure width at three heights and use the smallest figure, measure drop at three positions and use the smallest figure, check recess depth against the manufacturer's minimum, confirm whether the supplier applies a deduction, and double-check all figures by measuring again. Following these steps will give you a venetian blind that fits correctly first time.