Measuring windows for blinds is where most DIY blind projects go wrong — and where most homeowners end up with coverings that don't fit, don't look right, or can't be returned. This guide walks you through exactly how to measure for both inside and outside mount installations, what deductions mean, and the common mistakes that cost Vancouver homeowners money every year.

Inside Mount vs. Outside Mount: The First Decision

Before you measure anything, you need to decide how your blind will be mounted. This affects everything — what you measure, how you measure it, and what the finished result looks like.

Inside mount means the blind sits inside the window frame, in the recess of the window. This gives a clean, built-in appearance and is the preferred choice in most modern Vancouver condos and renovated homes with deep window frames.

Outside mount means the blind bracket is attached to the wall or window trim outside the frame, and the blind hangs in front of the window. This is the right choice when your window frame is too shallow for an inside mount (less than 2 inches of depth), when you want to make a window appear larger, when you need to block light that bleeds around the edges of an inside-mounted blind, or when the window has obstructions inside the frame like handles or cranks.

How to Measure for an Inside Mount

You will need a steel measuring tape — not a fabric tape measure, which can flex and give inaccurate readings. Measure in inches to the nearest 1/8 inch.

Width: Measure the inside width of the window frame at three points — top, middle, and bottom. Use the narrowest of the three measurements. This accounts for window frames that aren't perfectly square, which is extremely common in Vancouver's older housing stock. The blind manufacturer will apply a deduction (typically 3/8 to 1/2 inch) to ensure the blind fits inside the frame without binding.

Height: Measure the inside height of the window frame at three points — left side, centre, and right side. Use the longest of the three measurements. This ensures the blind covers the full opening even if the sill isn't level.

Minimum depth: Check that your window frame has at least 2 inches of depth (front to back) for a standard inside mount. Some blinds require 3 inches or more. Motorized systems typically need more depth than manual ones. If your frame is shallower, switch to an outside mount.

How to Measure for an Outside Mount

Width: Decide how far beyond the window frame you want the blind to extend on each side — typically 1.5 to 3 inches per side to minimize light gaps and create the illusion of a wider window. Add twice that number to the actual frame width. For example: frame width 36 inches + 3 inches left + 3 inches right = order a 42-inch blind.

Height: Measure from where you want the top of the bracket (typically 2–4 inches above the frame or at the ceiling) to where you want the bottom of the blind (typically the window sill or 1/2 inch below it).

The Deduction — What It Means and Why It Matters

Blind manufacturers apply a deduction to inside mount orders — they make the blind slightly narrower than the measurement you provide so it fits inside the frame without rubbing. The deduction is typically 3/8 to 1/2 inch total and is applied by the manufacturer automatically.

This means: do not apply your own deduction when ordering. Give the manufacturer your actual measured width. If you deduct yourself and the manufacturer also deducts, your blind will be too narrow and light will bleed around both sides. For outside mounts, there is no deduction — you order the exact size you want.

Common Measuring Mistakes in Vancouver Homes

• Measuring only once. Window frames in Metro Vancouver — particularly in homes built before 1980 — are almost never perfectly square. Always measure at three points.
• Using the wrong tape. A flexible tape measure can sag mid-span on wide windows. Use a rigid steel tape for widths over 36 inches.
• Forgetting window obstructions. Crank handles, tilt latches, and window locks inside the frame affect minimum depth requirements for inside mounts.
• Not accounting for stacking. When a roller or zebra blind is rolled up, the fabric creates a roll at the top. On inside mounts, the stack reduces the height of the opening that gets covered. If you need coverage all the way to the top of the frame, discuss this with your supplier.

When to Call a Professional

For standard rectangular windows, measuring is straightforward if you follow the steps above. Call a professional for: windows that are not square (bay windows, angled ceilings, dormers), arched or curved windows, motorized systems requiring precise clearances, or any window over 96 inches wide where installation requires two people and proper equipment.

Swati Contracting offers free in-home measuring across Greater Vancouver. We measure to 1/16 inch precision and handle the full order and installation. Call 604-712-9252.