For inside mount: measure width at three points and use the smallest, measure height at three points and use the largest, and never make your own deductions. For outside mount: use the largest measurements and add at least 4 inches to both width and height for proper coverage and light blockage.
Ordering custom blinds is one of the best upgrades you can make for your home. But there's one step that trips up almost every first-time buyer: taking accurate measurements.
Get it right, and your blinds fit perfectly — no gaps, no guesswork, no returns. Get it wrong, and you're looking at a frustrating reorder process.
This guide walks you through exactly how to measure your windows for custom blinds, whether you're going with an inside mount or outside mount installation. We'll cover everything from the tools you need to the common mistakes that cause sizing errors. If you're specifically ordering cellular honeycomb shades, we also have a dedicated professional measuring guide for cellular shades with additional depth-check requirements and a built-in reference calculator.
Before You Start: Inside Mount vs. Outside Mount
Inside mount gives a clean, built-in look that showcases window trim — requires at least 1.5–2 inches of frame depth and precise measurements. Outside mount covers the full frame, is more forgiving to measure, and provides better light blockage by eliminating side gaps.
Inside Mount
An inside mount means your blinds sit inside the window recess, flush with the frame. The result is a clean, built-in look that showcases your window trim and gives the room a more polished appearance.
Inside mount works best when your window frame has enough depth to accommodate the headrail — typically at least 1.5 to 2 inches. It also requires more precise measurements, since the blinds need to fit within the frame without rubbing the sides.
Outside Mount
An outside mount means your blinds are installed on the wall or trim above the window, covering the entire frame. This is a great option when your window frame is too shallow for an inside mount, when you want to make a window appear larger, or when you want maximum light blockage by eliminating side gaps.
Outside mount is generally more forgiving for measurements, since you're adding coverage rather than fitting inside a specific space.
Not sure which to choose? A good rule of thumb: if you want a sleek, minimalist look, go inside mount. If you want maximum privacy and light control, go outside mount.
What You'll Need
Use a rigid steel tape measure — not a fabric tape, which can stretch and give inaccurate readings. Also have a pencil and paper to record measurements, and a level for installation.
A steel tape measure is essential — not a fabric measuring tape, which can stretch and give inaccurate readings. You'll also want a pencil and paper to record your measurements, and ideally a level to confirm your brackets are even during installation.
How to Measure for Inside Mount Blinds
Measure width at three points (top, middle, bottom) and use the smallest. Measure height at three points (left, center, right) and use the largest. Round to the nearest 1/8". Do not make your own deductions — UAM Blinds handles all clearance calculations from your exact measurements.
Measuring the Width
Measure the width of your window opening at three points: near the top, in the middle, and near the bottom. Windows aren't always perfectly square, so these three measurements may differ slightly.
Use the smallest of the three width measurements as your order width. This ensures your blinds will fit inside the frame without binding on the sides.
If your smallest measurement includes a fraction of 1/16" or 1/32", round down to the nearest 1/8". For example, if your smallest width measurement is 24 9/16", round down and order 24 1/2".
Measuring the Height
Measure the height of your window opening at three points: from the top left corner down to the sill, from the top center down to the sill, and from the top right corner down to the sill.
Use the largest of the three height measurements as your order height. This ensures full coverage without leaving any gap at the bottom.
For height fractions: if your largest measurement includes a 1/16" or 1/32" increment, round up to the nearest 1/8". For example, if the largest height is 36 9/16", round up and order 36 5/8".
Important: Do Not Make Your Own Deductions
You do not need to subtract anything from your measurements to account for clearance. When you place your order with UAM Blinds, we handle all necessary deductions based on the exact measurements you provide. If you subtract from your measurements yourself — even by a small amount — your blinds may end up too narrow or too short.
How to Measure for Outside Mount Blinds
Use the largest of three width measurements and add at least 4 inches total (2 inches per side). Use the largest of three height measurements and add at least 4 inches total (2–3 inches above the frame, 1–2 inches below the sill).
Measuring the Width
Measure the width of the area you want covered at three locations: left, center, and right. Use the largest of the three width measurements, then add a minimum of 4 inches (2 inches on each side). For bedrooms where maximum light blockage is the priority, you can add up to 3 to 4 inches on each side for a total addition of 6 to 8 inches.
Measuring the Height
Measure the height of the area you want covered at three locations: left, center, and right. Use the largest of the three height measurements, then add a minimum of 4 inches — typically 2 to 3 inches above the window frame and 1 to 2 inches below the sill.
Room-by-Room Measurement Tips
Bedrooms: outside mount for maximum blackout. Living rooms: inside mount for clean aesthetics. Bathrooms and kitchens: inside mount with moisture-resistant materials. Nurseries: inside mount with cordless blackout cellular shades.
Bedroom
Bedrooms benefit from maximum light blockage, which makes outside mount the better choice in most cases. If you're ordering blackout cellular shades or blackout roller shades for a bedroom, always consider outside mount for the best results. Not sure whether blackout or light filtering is right for each room? See our Blackout vs Light Filtering Blinds guide.
Living Room
Inside mount tends to look cleaner and more intentional in living rooms, especially if you have decorative window trim you want to showcase. For living rooms where you want both natural light and privacy simultaneously, a Top-Down Bottom-Up (TDBU) cellular shade gives you independent control from both directions.
Bathroom and Kitchen
Moisture-prone rooms call for waterproof or moisture-resistant materials. Faux wood blinds are a popular choice here. Consider inside mount if your window frame allows it — the recessed look works especially well in smaller spaces.
Nursery and Children's Rooms
For children's rooms, cordless designs are non-negotiable for safety. Consider blackout cellular shades to support better sleep schedules. See our complete guide on safe cordless blinds for nurseries.
The 5 Most Common Measuring Mistakes
Using a fabric tape measure. Measuring only once. Making your own deductions for inside mount. Forgetting to add overlap for outside mount. Measuring old blinds instead of the window opening directly.
Using a fabric tape measure. Fabric stretches. Always use a rigid steel tape measure.
Measuring only once. Always measure at three points and use the appropriate value.
Making your own deductions for inside mount. Don't subtract anything. Submit exact measurements.
Forgetting to add overlap for outside mount. Always add at least 4 inches total to both width and height.
Measuring the old blinds instead of the window. Measure the window opening directly — the old blinds may have been the wrong size.
Need Help?
Have questions about measuring or installation? Contact us at support@uamblinds.com — we're happy to help you get the perfect fit before you order.
Frequently Asked Questions
Should I measure inside mount or outside mount?
Choose inside mount if your window frame has at least 1.5–2 inches of depth and you want a clean, built-in look. Choose outside mount if your frame is too shallow, if you want to make the window appear larger, or if maximum light blockage is the priority — outside mount eliminates the side gaps that inside mount leaves.
Do I need to subtract anything from my inside mount measurements?
No. Do not make any deductions. UAM Blinds handles all necessary clearance calculations based on the exact measurements you submit. If you subtract from your measurements yourself, your blinds may end up too narrow or too short.
What if my three width or height measurements are different?
This is normal — most windows are not perfectly square. For inside mount width, use the smallest of your three measurements. For inside mount height and all outside mount dimensions, use the largest measurement.
How much should I add for outside mount blinds?
Add a minimum of 4 inches total to the width (2 inches on each side) and a minimum of 4 inches total to the height (2–3 inches above the frame, 1–2 inches below the sill). For bedrooms, adding 6–8 inches total to the width provides even better light blockage.
Further Reading
- Professional Measuring Guide for Cellular Shades — deeper guide with frame depth requirements and reference calculator
- Honeycomb Cellular Shades: What to Know Before You Buy — operating systems, materials, and energy efficiency
- Blackout vs Light Filtering Blinds — choose the right fabric for each room before you order
- TDBU vs. Standard Cellular Shades — understand operating systems before measuring
- Safe Cordless Blinds for Nurseries — child safety guide