Blind Size Calculator

Understanding Window Blinds

Window blinds are window coverings with adjustable slats made from wood, plastic, metal, or composite materials. A control mechanism at the side lets you tilt or raise the slats to regulate light and privacy. Unlike curtains or shades, blinds offer precise light control through their angled design.

Two primary mounting options exist: inside mounts position blinds within the window frame for a clean, recessed look, while outside mounts cover the frame entirely and can make windows appear larger. Your choice depends on frame condition, window shape, and aesthetic preference.

Measuring Your Window Correctly

Accurate measurement is essential before ordering blinds. Take three width readings—at the top, middle, and bottom of the window opening—using a steel measuring tape. Similarly, measure height at three points: left edge, center, and right edge. Record all six measurements to the nearest eighth of an inch.

For inside mounts, measure from the inner edges of the window frame. For outside mounts, measure from the outer edges of the trim or from the points where you plan to mount the brackets. Measuring in multiple locations accounts for windows that may be slightly out of square, which is common in older homes.

• Use a steel tape, not a cloth one, for accuracy
• Measure during daylight to see clearly into corners
• Write down each measurement immediately to avoid confusion

Calculating Blind Dimensions

Your raw window measurements must be converted into blind specifications based on mounting style. The calculator applies these adjustments automatically:

• widthTop, widthMiddle, widthBottom — The three horizontal measurements across your window opening
• heightLeft, heightCenter, heightRight — The three vertical measurements along your window opening
• 0.25 inches — Standard clearance from the window sill for inside mounts
• 3 inches — Overlap allowance per side for outside mounts
• 1.5 inches — Overlap allowance above the window for outside mounts

Inside vs. Outside Mount Specifications

Inside Mount Blinds fit snugly within the window frame. Use the smallest width measurement to ensure the blind fits without binding. Use the largest height measurement to guarantee full coverage. If you want a small gap above the sill, subtract 0.25 inches from the height. Inside mounts work best for square, plumb windows in good condition.

Outside Mount Blinds overlay the window frame and surrounding wall or trim. Adding 3 inches to the width (1.5 inches overlap per side) ensures the blind completely covers the window and adjacent frame. Adding 1.5 inches to the height covers above the window opening. Outside mounts are ideal for oddly-shaped windows, uneven frames, or when you want to make windows appear larger.

To determine which suits your window, measure the diagonals from corner to corner. If they differ by more than 1 inch, an outside mount will better disguise the irregular shape.

Common Measurement and Ordering Pitfalls

Avoid these frequent mistakes when determining your blind size:

1. Overlooking out-of-square windows — Many windows are slightly twisted or leaning. By measuring in three locations for width and height, you catch these discrepancies. Ignoring them leads to blinds that bind or leave gaps. Always use the most conservative measurement (smallest for width, largest for height) on inside mounts.
2. Forgetting brackets in the equation — Outside-mount brackets add depth. Ensure your blind headrail clears any obstacles above the window, such as ceiling trim or recessed lighting. Ask the manufacturer for exact bracket dimensions before ordering if space is tight.
3. Confusing inside and outside mount requirements — Inside mounts measure from inside the frame; outside mounts measure from the visible frame edges. Using inside measurements for an outside mount, or vice versa, results in incorrect sizes. Double-check your mounting style before entering measurements into the calculator.
4. Neglecting future sill contact — If you think you might want blinds to touch the sill later but don't now, order for a non-sill configuration. Shortening blinds is often impossible, whereas adjusting a non-sill installation is simple.