How to Measure a Window for Blackout Shades

TL;DR: Learning how to measure windows for curtains and blackout shades takes about five minutes per window. Measure width and height at three points each, use the smallest width and tallest height, then choose inside mount or outside mount based on your window depth. Most measurement mistakes come from rounding up instead of down, skipping the three-point check or confusing mount types. This guide covers both methods plus the extra steps required for sealed-track blackout shades.

You have been staring at your tape measure for ten minutes. The window looks simple enough, but the numbers need to be exact because custom-made shades cannot be returned if the measurements are wrong.

That anxiety is real. "Ordering was a little anxiety inducing" is one of the most common phrases UBlockout customers use to describe the measuring process. And according to This Old House, "Way too many window-replacement projects come to grief because the opening wasn't measured properly before the order was placed."

The good news: measuring a window is not complicated. It is just precise. This guide walks through how to measure windows for curtains, drapes, roller shades and sealed-track blackout shades with step-by-step instructions for both inside and outside mount.

What Tools Do You Need to Measure a Window?

You do not need specialty equipment. Four items handle every window in your house.

• Steel tape measure (25 ft). A flexible fabric tape stretches and bends. Steel stays rigid across the full width of the opening. This matters when you are measuring solo.
• Pencil and notepad. Write down every measurement the moment you take it. Label each window by room and position (e.g., "Primary bedroom, left of bed"). Do not trust your memory.
• Step stool. Measuring the top of a tall window from an angle introduces errors. Get your eyes level with the point you are measuring.
• Level (optional but helpful). A 2-foot level tells you instantly whether your window frame is square. If the bubble drifts, your window is out of plumb and the three-point measuring method becomes critical.

Skip the laser measurer for windows. Laser tools are accurate to 1/16 inch on long distances but awkward to position inside a window frame where you need edge-to-edge readings.

How Do You Measure a Window for Inside Mount?

Inside mount means the shade or curtain rod sits inside the window frame. It looks clean and shows off decorative trim. This is the preferred mount for most blackout shade installations because the shade sits closer to the glass.

Here is the step-by-step process:

Step 1: Measure width at three points. Place your tape measure at the top of the window opening, jamb to jamb. Record that number. Then measure at the middle and bottom. Windows are rarely perfectly square, so these three numbers will often differ by 1/8 to 1/4 inch.

Step 2: Use the narrowest width. Record the smallest of the three width measurements. A shade built to the widest measurement will not fit into the narrower section of the frame. As Bob Vila notes, "If you find that the measured widths are not uniform, keep the narrowest measurement to ensure that once installed, the blinds are able to operate freely, without obstruction."

Step 3: Measure height at three points. Measure from the top of the opening to the sill on the left side, center and right side. Again, these numbers will often vary.

Step 4: Use the tallest height. Record the longest of your three height measurements. A shade that is slightly too tall can be adjusted. A shade that is too short leaves a gap at the bottom that leaks light.

Step 5: Check the frame depth. This step is critical and most curtain guides skip it. Measure the depth of your window frame from the front edge to the glass. Standard roller shades need at least 1.5 inches. Sealed-track blackout shades like UBlockout need at least 2.75 inches for a flush inside mount. If your frame is shallower than required, outside mount is the better option.

Step 6: Record measurements to the nearest 1/8 inch. Never round up. Always round down to the nearest eighth. A shade that is 1/8 inch too narrow is invisible. A shade that is 1/8 inch too wide will not fit.

How Do You Measure for Outside Mount?

Outside mount places the shade or curtain rod on the wall above and around the window frame. This is the go-to option when frame depth is too shallow for inside mount or when you want maximum light blocking and maximum coverage to make your room pitch black.

Step 1: Mark where the brackets will go. For curtains, the standard recommendation is to position the rod 4-6 inches above the window frame. For roller shades or blackout shades, 2-3 inches above is typical. The higher you mount, the taller the window appears.

Step 2: Measure width from bracket to bracket. Add 2-3 inches on each side of the window frame for light overlap. If you are measuring for curtains, this overlap prevents light gaps at the edges. For sealed-track blackout shades, the aluminum channels mount directly against the wall so you measure the full width you want to cover.

Step 3: Measure height from bracket position to the sill. For curtain length, you have three standard options: sill length (ending at the windowsill), below-sill length (4 inches past the sill) or floor length (1/2 inch above the floor). For blackout shades, measure to 2 inches below the sill to eliminate the bottom light gap.

Step 4: Check for obstructions. Look for anything within the mounting zone: electrical outlets, light switches, vent covers, adjacent window trim or door frames. Mark these on your notepad so you can adjust bracket placement before ordering.

What Are the 5 Most Common Measuring Mistakes?

After 30,000+ installations, these are the errors that cause the most problems:

Mistake 1: Measuring only once. Every guide says measure three times. Most people measure once at the center and call it done. Windows in homes built before 1990 are frequently out of square by 1/4 inch or more. That 1/4 inch is the difference between a shade that fits and one that jams or leaves a gap.

Mistake 2: Rounding up. Your instinct says "better too big than too small." For inside-mount shades, this is backwards. A shade that is too wide will not slide into the frame. Round down to the nearest 1/8 inch.

Mistake 3: Confusing inside and outside mount dimensions. Inside mount measures the opening. Outside mount measures the area you want to cover. These are different numbers. Mixing them up is the single fastest way to get a shade that does not fit.

Mistake 4: Ignoring frame depth. You measured width and height perfectly but forgot to check whether your window frame is deep enough for an inside mount. Now your shade sticks out past the trim. Measure depth before you order.

Mistake 5: Using a cloth tape measure. Fabric tape measures stretch over time. A 36-inch cloth tape can read 36 inches when the actual distance is 35 7/8 inches. Always use a steel tape measure for window measurements.

How Do You Measure for Sealed-Track Blackout Shades?

Sealed-track shades like UBlockout add two considerations that standard curtains and roller shades do not require.

Side channel clearance. UBlockout's patented sealed track technology uses aluminum side channels that snap against the wall or window frame to seal every edge. For inside mount, your frame depth must accommodate the shade cassette plus the side channels. For outside mount, the channels mount directly to the wall surface so depth is not a factor.

Deduction table. UBlockout provides specific deductions based on your mount type. For inside mount, UBlockout deducts the necessary clearance from your raw measurements so the shade fits perfectly within the frame. You submit your raw measurements and UBlockout handles the math. This is different from standard blinds where you need to apply your own deductions.

The easiest way to confirm your measurements is through UBlockout's Fit Guarantee. For $40 per shade, a measurement expert reviews your numbers on a video call that takes about 10-15 minutes. They confirm accuracy, answer questions and cover any issues if the measurements are off.

With 30,000+ installs and a 4.9/5 rating from 839+ reviews, most UBlockout customers report that the shade arrives fitting "perfectly." One reviewer put it simply: "The marketing is 100% accurate."

How High Should You Hang Curtains and Blackout Shades?

Where you position the top of your window treatment changes both the look of the room and how much light gets blocked.

For aesthetics: Mount curtain rods 4-6 inches above the window frame. This makes the window appear taller and the room feel larger. The wider you extend past the frame edges, the wider the window appears when the curtains are open.

For light blocking: Mount as close to the ceiling as possible. Every inch of gap between the top of the shade and the ceiling is an inch where light can enter. For blackout curtains, this top gap is the hardest leak to eliminate. For sealed-track shades, the cassette housing at the top seals this gap regardless of mounting height, but closer to the ceiling still looks cleaner.

For function: If you have crown molding, mount just below it. If the window is close to the ceiling (less than 4 inches of wall space above the frame), inside mount is often the only practical option.

The key rule: there is no single correct height. The right position depends on your ceiling height, window size and how much darkness you need. For sleep-focused rooms, prioritize light blocking over aesthetics.

Key Takeaways

• Measure at three points. Width at top, middle and bottom. Height at left, center and right. Use the narrowest width and tallest height.
• Round down, never up. For inside mount, a shade that is 1/8 inch too narrow fits. A shade that is 1/8 inch too wide does not.
• Check frame depth before choosing mount type. Inside mount needs sufficient depth for the shade hardware. Most standard windows work fine. Shallow frames require outside mount.
• Use a steel tape measure. Cloth tapes stretch. Laser measurers are overkill for window-sized openings.
• Standard curtain sizes do not apply to custom-made blackout shades. Custom shades are built to your exact measurements in 1/8-inch increments.
• For sealed-track shades, submit raw measurements. UBlockout handles the deductions. The $40 Fit Guarantee includes a video call to confirm accuracy before production begins.

Ready to measure with confidence? Start with UBlockout's measuring guide and Fit Guarantee, or book a free consultation to get expert help before you order. Every shade comes with a 30-Night Free Trial, so you can test total darkness risk-free.

Frequently Asked Questions

How accurate do my measurements need to be?

Within 1/8 inch is the goal. Most windows have slight variations across the frame, which is why you measure at three points and use the most conservative number. For context, 1/8 inch is roughly the thickness of two stacked credit cards.

Can I measure my windows myself or do I need a professional?

You can absolutely measure yourself. The three-point method described in this guide is the same technique professionals use. If you want extra confidence, UBlockout's Fit Guarantee ($40 per shade) includes a video call where a measurement expert walks through your numbers before production starts.

What if my window is not square?

Most windows are not perfectly square, especially in older homes. The three-point measuring method accounts for this. Use the narrowest width so the shade fits through the tightest point and the tallest height so the shade covers the full opening. If your measurements vary by more than 1/4 inch across the three points, consider outside mount for the most forgiving fit.

Do I measure differently for blackout curtains vs blackout shades?

Yes. Blackout curtains on a rod need width measurements multiplied by 1.5-2x for fabric fullness. Blackout shades are built to the exact window dimensions with no multiplier. The measuring process for shades is actually simpler because you record the raw opening size and the manufacturer handles the rest.

What is the difference between inside mount and outside mount?

Inside mount sits within the window frame. It looks sleek and shows off decorative trim but requires sufficient frame depth. Outside mount attaches to the wall above and around the frame. It covers more area, blocks more light and works on any window regardless of frame depth. For complete blackout in a bedroom, outside mount with sealed side channels provides the most complete light seal.

Does UBlockout deduct for inside mount or do I need to calculate that myself?

UBlockout handles the deductions. Submit your raw inside-mount measurements (narrowest width, tallest height) and UBlockout applies the necessary clearance during production. You do not need a deduction table or any math beyond the three-point measuring method. The Fit Guarantee video call confirms everything before your shade is custom-made.