Blind Size Calculator - Inside/Outside Mount Dimensions

A blind size calculator is a measurement tool that turns the width and height of a window opening into the exact blind dimensions you should order or cut. It accepts three width readings (top, middle, bottom) and three height readings (left, center, right), then applies inside or outside mount rules to return a recommended blind width, length, and matching centimeter length.

Common uses include replacing old blinds on an out-of-square window, sizing inside mount installations that sit flush inside the frame, sizing outside mount installations that need 3 in of width overlap and 1.5 in of top overlap to hide the trim, and ordering a single blind or multi-panel configuration that matches the calculated width down to the nearest 1/8 in.

Three readings per axis catch a frame that is not perfectly square: the smallest width and largest height drive an inside mount, while the largest readings feed an outside mount with overlap built in.

If you are dressing the same window with curtains as well as blinds, the Curtain Size Calculator returns the curtain width, length, and fabric yardage from the same three-point width and height readings.

The calculator applies two different formulas depending on whether the blind sits inside the frame or overlaps the trim, and converts the length to centimeters for international product listings.

• Wt, Wm, Wb: Window width at top, middle, and bottom (inches).
• Hl, Hc, Hr: Window height at left, center, and right (inches).
• sillClearance: Optional gap between the bottom of the blind and the sill for inside mount, in inches.

Both width and length round to the nearest 1/8 inch because most retailers cut blinds in eighth-inch increments. Rounding before converting to centimeters keeps the metric value consistent with what the supplier will ship.

According to Omni Calculator, an inside mount blind uses the smallest of three width measurements and the largest of three height measurements, while an outside mount blind uses the largest width plus 3 inches of overlap and the largest height plus 1.5 inches of top overlap.

According to NIST, the international inch is defined as exactly 2.54 centimeters, which is the conversion factor the calculator uses for the centimeter length output.

If you are rough-framing a new opening and need to size a replacement unit before the trim goes on, the Window Door Calculator applies the same three-point width and height workflow to a window or door unit, including rough opening, frame size, and shim allowance.

Four ideas come up again and again when you measure for blinds, and each one changes the number you should order.

Knowing which concept applies to your window matters more than the numbers: a deep inside mount frame rewards precision, while a shallow frame usually forces a switch to outside mount even when the room would look better with the trim exposed.

If the same wall is also getting a wallpaper refresh, the Wallpaper Calculator turns the wall widths and ceiling height into a rolls estimate with pattern repeat and waste built in, which makes it easy to subtract each window opening from the total before ordering.

Run through these steps in order and the calculator returns a width, length, and centimeter value you can copy straight into an order form.

1. 1 Pick the mount type: Choose inside mount if the frame hides the headrail, otherwise pick outside mount.
2. 2 Measure width three times: Measure the inside edge of the frame at the top, middle, and bottom to the nearest 1/8 inch.
3. 3 Measure height three times: Measure the inside edge of the frame at the left, center, and right to the nearest 1/8 inch.
4. 4 Set the sill clearance: Use 0 if the blind should kiss the sill, or 1/4 in for the most common gap on inside mount.
5. 5 Read the result panel: Note the recommended blind width, length, centimeter value, and which reading (min, max, left, center, right) was used.

For a 36 x 60 in double-hung window with readings 36/35.875/35.75 in width and 60/60.25/60 in height, an inside mount produces a 35-3/4 in x 60 in blind (152.4 cm). Switch to outside mount and the recommendation becomes a 38-7/8 in x 61-5/8 in blind so the frame is fully covered.

If you are also sewing a fabric valence, cornice, or matching curtain to hide the headrail, the Fabric Calculator takes the cornice length, width, and pattern repeat to return the yardage you need to buy in one step, so the window treatment stays in proportion to the new blind.

The calculator removes the two failures behind most blind returns: a blind that is too wide to fit, and a blind too narrow to block light.

• • Avoids misorders: The min-for-inside and max-for-outside rules come from established retailer guidance, so the size you order matches the size you install.
• • Saves a second trip to the store: A clear centimeter result lets you cross-check international listings without converting inches by hand.
• • Catches out-of-square windows: The width-source and height-source rows show exactly which reading was used, so a wide spread between top and bottom widths becomes obvious before ordering.
• • Works for inside and outside mounts: Toggle the mount type and the same measurements recompute with the right overlap and clearance rules applied.
• • Supports cut-to-size and custom orders: Outputs round to the nearest 1/8 inch, the standard cutting increment at Home Depot, Lowe's, IKEA, and most online blind vendors.

Most blind sizing mistakes are quiet: a 1/4 in misread on a 36 in window becomes a blind that scrapes the frame. The calculator makes those silent mistakes visible by showing the underlying min or max reading next to the rounded result.

The same room with a confirmed blind size also feeds into cooling load math: the AC Tonnage Calculator uses window count, sun exposure, and shading to set the right cooling capacity, and a well-fitted outside mount that blocks solar gain can lower the recommended tonnage.

Window type, depth, and material change both the size and the mount style you should pick, so use these factors to sanity-check the calculator's output before you order.

• • Standard blind sizes are not universal: vendor cut increments range from 1/8 in to 1/4 in, and the calculator rounds to 1/8 in. Always confirm the supplier's minimum and maximum cut size before ordering.
• • The sill clearance field is intended for inside mount and is ignored when you select outside mount, where the 1.5 in top overlap is fixed by the standard practice.

Tape measures are accurate to 1/16 in at best; if your readings jump between 1/8 in marks, re-measure with a partner and a rigid tape to avoid pulling sideways across the frame.

According to The Home Depot, customers should record width at top, middle, and bottom and height at left, center, and right, then choose inside mount dimensions based on the smallest width and largest height.

Window orientation and shading also change the lighting bill. Once the blinds are ordered, the LED Savings Calculator takes the bulb count, daily run hours, and electricity rate to estimate savings from leaning on daylight control with the new blinds instead of leaving the lights on.