Calculate the correct blind or shade size for your windows. Compare inside vs outside mount dimensions, costs by blind type, and depth requirements.
Ordering custom blinds or shades with the wrong measurements is an expensive mistake - most custom window treatments aren't returnable. The difference between inside and outside mount sizing, combined with the precise deductions and additions required, trips up even experienced homeowners. A small measurement error can also change how well the blind blocks light or clears the frame. That is why it helps to convert measurements into order dimensions before you buy.
This blind size calculator converts your raw window measurements into exact ordering dimensions for both inside and outside mounts. It handles the standard deductions (1/4" for inside mounts) and additions (3" per side for outside mounts) automatically, checks whether your window frame has enough depth for your chosen blind type, and estimates costs across six popular blind styles.
Whether you're outfitting a single window or an entire house, the calculator helps you determine the right type, confirm proper measurements, and budget accurately before placing your order.
Use this calculator to turn window measurements into order dimensions that actually fit the frame and the mounting style you want. It is useful when you need to compare inside and outside mounts, check depth requirements, and estimate costs before ordering custom blinds or shades. That makes it easier to avoid a wrong-size order before you spend money on custom treatments.
Inside mount: order_width = window_width − 0.25", order_height = window_height − 0.25". Outside mount: order_width = window_width + 6", order_height = window_height + 6". Cost = (order_width × order_height / 144) × price_per_sq_ft.
Result: Order: 35.75" × 47.75", $59.97 per blind
A 36" × 48" window with inside mount requires blinds ordered at 35.75" × 47.75" (1/4" deduction). At $5/sq ft for cellular shades, each blind costs about $59.97.
Inside mount provides a clean, built-in look that works beautifully with decorative window trim. The blind fits flush within the frame, leaving the trim visible. However, it requires minimum frame depth (varies by blind type) and provides less complete light blocking since small gaps exist at the edges. Outside mount is the better choice for shallow frames, maximum light blocking, and making windows appear larger — the blind covers the entire frame plus wall overlap.
Cellular shades offer the best energy efficiency with their honeycomb air pockets. Venetian blinds provide the most adjustable light control with tiltable slats. Roller shades are the most affordable and streamlined. Wood and faux wood blinds add warmth and texture. Roman shades offer a soft, fabric look ideal for formal spaces. Vertical blinds are the standard for sliding doors and very wide windows.
The number one mistake is measuring once and assuming all windows are identical. Windows in the same room can vary by 1/2 inch or more due to settling, construction tolerances, and humidity changes. Other common errors include measuring at only one point (instead of three), not checking frame depth before ordering inside mount blinds, and forgetting to account for handles, cranks, or sensors that might interfere with blind operation.
Inside mount fits within the window frame for a clean, built-in look. Outside mount attaches to the wall above the frame, covering more area and blocking more light. Inside mount requires sufficient frame depth (usually 2-3").
Measure width at top, middle, and bottom of the window opening. Use the narrowest measurement. Measure height on left, center, and right. Use the longest measurement. Most manufacturers want exact measurements — they'll apply deductions.
It varies by blind type: roller shades need 1.5", cellular shades 2", venetian blinds 2.5", and wood blinds 2.75". If your frame is too shallow, use outside mount instead.
For inside mount, measure precisely to the nearest 1/8 inch — don't round up or the blinds won't fit. For outside mount, it's safer to add a bit extra since the blind sits on the wall outside the frame.
Cellular (honeycomb) shades are the most energy efficient, with their air-trapping honeycomb structure providing R-values of 2-5. They can reduce heat loss through windows by 40-50% compared to unshaded windows.
Yes, most blinds come with installation hardware and instructions. Inside mount blinds are generally easier — just screw brackets into the top of the window frame. Outside mount requires drilling into the wall and may need anchors.