Getting the wrong measurements for blinds or curtains is one of the most frustrating DIY mistakes you can make. I have been there, measuring a window quickly and ordering custom blinds, only to find they do not fit. The blinds end up too tight, too loose, or letting light peek through the edges. Learning how to measure windows for blinds and curtains the right way saves you money, time, and a whole lot of frustration.
The good news is that measuring is not complicated once you know the process. Whether you are installing vertical blinds, roller shades, or drapes, the same basic principles apply. You need a steel tape measure, three reference points for width and height, and an understanding of your mount type.
In this guide, I will walk you through every step from choosing your mount style to recording your final numbers. I will also cover curtain-specific measuring that most guides skip entirely. By the end, you will have the confidence to measure any window in your home accurately.
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The single most important tool for measuring windows is a steel measuring tape. Cloth or fabric measuring tapes stretch and bend, which leads to inaccurate readings. I learned this the hard way when a cloth tape gave me a measurement that was off by a quarter inch, and my blinds would not fit into the frame.
Here is what you should gather before you start measuring:
Steel measuring tape, at least 25 feet long
Pencil and a notepad or printable measurement worksheet
Step stool or ladder for reaching high windows
Someone to help hold the tape for wide or tall windows
A level if you want to check whether your window frame is square
A printable worksheet is extremely helpful. You can sketch each window, label the measurements, and keep everything organized. Many homeowners I have talked to on DIY forums swear by writing down every single number twice, once at the window and once on their order form.
If you do not have a steel tape measure handy, some smartphone apps can give you rough measurements using augmented reality. These are fine for getting a ballpark figure, but you should always confirm with a physical tape before ordering custom window treatments.
Before you measure anything, you need to decide on your mount type. This decision changes everything about how you measure, so it must come first. The two options are inside mount, where blinds fit within the window frame, and outside mount, where blinds attach to the wall or ceiling above the frame.
Inside mount blinds sit inside the window casing, giving a clean, built-in look. This is the most popular choice for homeowners because it shows off the window frame and looks tailored. The blinds sit flush with or slightly recessed into the frame, creating a streamlined appearance.
For inside mounts to work, your window frame needs sufficient depth. Most blinds require at least 1 to 3 inches of depth depending on the product type. Faux wood blinds typically need about 2 inches of depth, while blackout roller shades can sometimes fit in shallower frames.
The main advantage of inside mounting is appearance. The downside is that even small measurement errors are noticeable because the fit is precise.
Outside mount blinds attach to the wall above the window or to the ceiling. They cover the entire window opening and overlap onto the surrounding wall. This option is ideal when your window frame is too shallow for an inside mount or when you want to make a small window look larger.
Outside mounts are more forgiving with measurements because they overlap the opening. They also do a better job of blocking light since there are no gaps between the blind and the frame. Many homeowners choose outside mounts for smart blinds and shades because the headrail has more room for motors and wiring.
Inside mount gives you a polished, custom look but requires precise measuring and adequate frame depth. Outside mount is more forgiving and better for light blocking, but it covers your window trim and extends beyond the frame. When in doubt, check your frame depth first, as that will often make the decision for you.
This is where most people make mistakes, so I will break down every single step. The key principle is to measure in three places for both width and height. Windows are rarely perfectly square, especially in older homes, so taking measurements at multiple points catches these variations.
Before anything else, measure the depth of your window frame from the front edge to the glass. This tells you whether an inside mount is even possible. Use your steel tape and measure from the inner edge of the frame straight back to the window pane.
Different blind types have different depth requirements. Roller shades may need only 1.5 inches, while wood blinds might need 2.5 inches or more for a flush fit. Write this number down because you will need it when ordering.
Open your tape measure inside the window frame and measure the width at the top, near the headrail. Keep the tape taut and level. Record this number to the nearest eighth of an inch.
Now move to the middle of the window and measure across again. Then measure at the bottom, close to the sill. You now have three width measurements.
For inside mount blinds, always use the narrowest of these three measurements. This ensures the blinds will fit in the tightest part of the frame without scraping or sticking. If your top measures 35 inches, the middle is 34.75, and the bottom is 35.125, your width is 34.75 inches.
Now measure the height from the inside top of the frame down to the sill. Do this on the left side, in the center, and on the right side. Record each measurement to the nearest eighth of an inch.
For inside mount, always use the tallest of these three measurements. Using the tallest height ensures full coverage and prevents light gaps at the bottom. If left is 58 inches, center is 58.25, and right is 57.875, your height is 58.25 inches.
Measure diagonally from corner to corner. If the two diagonal measurements differ by more than a quarter inch, your window is significantly out of square. This does not mean you cannot install blinds, but it does mean you should be extra careful with your measurements.
Look for anything that might interfere with the blinds. Check for window cranks, door handles, moulding, or security sensors. These obstructions might require spacer blocks or projection brackets. Note them on your worksheet so you can discuss them when ordering.
Write down your final width as the narrowest measurement and your final height as the tallest measurement. Do not make any deductions yourself. The manufacturer will take the necessary deductions to ensure proper clearance inside the frame. This is a critical point that trips up many DIYers who try to subtract an eighth inch themselves.
Outside mount measuring is simpler and more forgiving, but it still requires care. Since the blinds overlap the window opening, you have more flexibility in sizing.
Decide how far beyond the window frame you want the blinds to extend. A good rule of thumb is to add at least 1.5 to 3 inches on each side for light control and privacy. More overlap means less light leakage around the edges.
Measure the full width of the area you want the blinds to cover, including the overlap on both sides. If your window opening is 36 inches and you want 2 inches of overlap on each side, your width measurement is 40 inches. Record this to the nearest eighth inch.
For outside mount, measure from where the headrail will sit down to where you want the bottom of the blinds to end. The headrail typically mounts 2 to 3 inches above the window frame. The bottom can end at the sill, extend below it, or go all the way to the floor.
Write down the total height from the mounting point to the desired bottom position. For example, if you mount 3 inches above a 60-inch window and want the blinds to end at the sill, your height is 63 inches.
Make sure the surface where you plan to mount the brackets is flat and wide enough. Outside mount brackets need a solid surface to attach to. If you have decorative moulding or trim around the window, you may need spacer blocks to create a flat mounting area.
Curtain measuring is different from blind measuring, and most guides skip it entirely. Since curtains hang from a rod rather than fitting inside a frame, the measurements focus on rod placement and fabric coverage rather than frame dimensions.
Curtain rods typically mount 4 to 6 inches above the top of the window frame. Mounting higher creates the illusion of taller windows and adds visual height to the room. Measure up from the top of the window frame to your desired rod height and mark it on both sides.
For ceiling-length curtains, the rod should sit just below the ceiling or crown moulding. For sill-length curtains, the rod still goes above the frame but the fabric ends at the windowsill.
Measure the width of your window and add 8 to 12 inches on each side for the rod extension. This extra width allows curtains to stack back fully off the glass when open, letting maximum light in. A 36-inch window with 10 inches on each side needs a 56-inch rod.
The curtain rod should always be wider than the window. This is one of the most common mistakes I see, where people buy a rod exactly the window width and the curtains never fully clear the glass.
For floor-length curtains, measure from the top of the rod down to the floor. Subtract 1 inch if you want the curtains to hover just above the floor, which prevents wear and makes vacuuming easier. This is called a floating hem and it looks intentional and clean.
For sill-length curtains, measure from the top of the rod to the windowsill. Add or subtract based on where you want the hem to land. Insulated blackout curtains for drafty rooms should always go floor-length to block cold air effectively.
Curtain panels should be 1.5 to 2 times the width of the rod for proper fullness. Flat panels look cheap and do not fold nicely. If your rod is 56 inches, you need combined panel width of 84 to 112 inches. Two 50-inch panels give you 100 inches, which works well for that rod size.
This is the area that causes the most anxiety among homeowners I have talked to on forums. The rounding rules are actually straightforward once you understand the logic behind them.
For inside mount width, always round down to the nearest eighth inch. If your measurement is 35 and 3/16 inches, round down to 35 and 1/8 inches. The reasoning is simple: you can always add a little clearance, but you cannot make a blind wider if it is too tight for the frame.
For inside mount height, always round up to the nearest eighth inch. If your measurement is 58 and 1/16 inches, round up to 58 and 1/8 inches. This ensures the bottom of the blind reaches the sill and does not leave a visible gap.
Most retailers and manufacturers work in eighth-inch increments. Getting a measurement like 35 and 3/32 inches is too precise for their system. Round to the nearest eighth using the rules above, and the manufacturer will handle the rest.
Reddit users who shared their experiences confirmed that being off by an eighth inch rarely causes problems. One user measured at 25 and 5/8 inches and found that a quarter inch of total clearance worked fine. The key is consistency and following the rounding rules.
I have scoured DIY forums and home improvement communities to find the most common measuring mistakes. Here are the ones that come up repeatedly.
Even windows that look identical can vary by a quarter inch or more. This is especially true in older homes where settling has occurred. Always measure every window individually, even if they appear to be the same size.
Single measurements are risky because a slight angle in your tape or a small shift in your hand can throw off the reading. Measure twice for every dimension and compare the numbers. If they do not match, measure a third time.
Cloth tapes stretch and sag, leading to measurements that are off by an eighth inch or more. Always use a steel tape measure. This is mentioned by every professional and manufacturer guide, and it is the single most common source of measuring errors.
Many people subtract an eighth inch from their width to account for clearance. Do not do this. The manufacturer already makes deductions based on the product specifications. Provide your exact measurements and let them handle the math.
Nothing is worse than ordering inside mount blinds only to find your frame is too shallow for them. Check depth first, before you even measure width and height. If your depth is insufficient, you need to switch to an outside mount or choose a different product.
Bay windows require measuring each individual section separately. A bay window with three panels needs three separate sets of measurements. Measure each section as its own window, following the inside or outside mount steps above. The angles where sections meet mean each opening will have different dimensions.
Homes built decades ago often have windows that have shifted out of square. The three-point measurement system handles this naturally because you use the narrowest width and tallest height. If your window is significantly out of square, consider outside mounting which is more forgiving of irregular shapes.
Arched, circular, or triangular windows need special measuring techniques and often custom-ordered treatments. For these, I recommend consulting with the manufacturer directly or hiring a professional measurer. Some companies offer templates you can trace and send in for custom shapes.
If you feel uncertain about your measurements, many retailers offer professional measuring services. The cost is typically modest and often credited toward your purchase. For expensive custom treatments, professional measuring provides peace of mind and a guarantee against measuring errors.
For a 35-inch inside mount window, order blinds at 35 inches. The manufacturer makes the necessary deductions to ensure proper fit inside the frame. For outside mount, add 3 to 6 inches total for overlap, so order blinds 38 to 41 inches wide.
Yes, several smartphone apps use augmented reality to estimate window measurements. Apps like Measure on iOS and AR Ruler on Android can give rough figures. However, always verify with a steel tape measure before ordering custom blinds, as app accuracy varies.
The correct method is to measure width at three points (top, middle, bottom) and height at three points (left, center, right) using a steel tape measure. For inside mount, use the narrowest width and tallest height. Round width down and height up to the nearest eighth inch.
No, blinds should not be the exact same size as the raw window opening. For inside mount, the manufacturer deducts slightly from your measurements so the blinds fit without scraping. For outside mount, blinds should be wider than the window to provide overlap and light control.
Measurements should be precise to the nearest eighth of an inch. This level of accuracy ensures a proper fit without being so precise that minor variations cause problems. Manufacturers work in eighth-inch increments, so rounding to this level matches their production standards.
Yes, Home Depot offers professional in-home measuring services for blinds and window treatments. The service is often free when you purchase blinds through them. This is a good option if you are unsure about DIY measuring or have unusually shaped windows.
Knowing how to measure windows for blinds and curtains comes down to a few core principles. Always use a steel tape measure. Take three width measurements and three height measurements. Use the narrowest width and tallest height for inside mounts. Follow the rounding rules of down for width and up for height.
Double-check every number before you place your order. Measure each window separately even if they look identical. And remember that the manufacturer handles deductions, so give them your exact measurements.
Once you have your numbers recorded and verified, you are ready to shop for blinds, shades, or curtains with confidence. Accurate measuring is the foundation of a great-looking window treatment installation, and now you have the process down.