Measuring for blinds looks simple until you notice that your window frame is a quarter inch wider at the top than at the bottom. Custom blinds and shades are cut to the measurements you give, so a small error can mean a blind that will not fit inside the frame, or one that leaves a bright line of light down each side.
This guide explains how to measure the way professionals do, the difference between inside and outside mounting, and the details that most often go wrong. We take these measurements ourselves during in-home consultations, but knowing the process helps you plan, compare options, and spot problems before anything is ordered.
What to know first
- Use a steel tape measure and record every number to the nearest eighth of an inch.
- For inside mounts, give the exact frame measurements and do not make your own deductions.
- For outside mounts, add about 2 to 3 inches on each side and above the frame for better coverage.
- Check the depth of the frame before choosing an inside mount, especially for wood blinds and cassette shades.
| Question | Inside mount | Outside mount |
|---|---|---|
| Look | Clean and built-in, shows the trim | Covers the trim and makes the window look larger |
| Light gaps | Small gaps at the sides | Overlap on the wall reduces gaps |
| Depth needed | Enough frame depth for the headrail | None inside the frame |
| Best for | Deep frames, attractive trim, layering under curtains | Shallow frames, bedrooms, out-of-square windows |
| What you measure | The opening itself | The area you want to cover |
Tools and preparation
Use a steel tape measure. Cloth tapes stretch and give inconsistent readings. Measure to the nearest eighth of an inch, and write the width first and the height second, every time. Mixing up the order is one of the most common reasons a custom blind arrives the wrong size.
Measure each window separately, even if they look identical. Windows in the same room are often slightly different, especially in older homes or where trim was added later. Label every measurement by room and position, for example “primary bedroom, left window,” so nothing gets mixed up when you order.
- A steel tape measure
- A pencil and a notepad, or a note on your phone
- A step stool for tall windows
- A small level if you plan an outside mount
Inside mount or outside mount: which should you choose?
An inside mount sits within the window frame. It looks clean and built-in, shows off good trim, and leaves room for curtains in front. The trade-off is a small gap of light on each side, since the blind has to be slightly narrower than the opening to move freely.
An outside mount attaches to the wall or the trim above the window and covers the whole opening plus some of the wall around it. It blocks more light, hides frames that are not square, and can make a small window look larger. It is often the better choice in bedrooms and on shallow frames.
When an inside mount works best
Choose an inside mount when most of these are true:
- The frame is deep enough for the headrail.
- The trim is attractive and you want to see it.
- Curtains or drapery will hang in front.
- Full darkness is not required.
When an outside mount works best
Choose an outside mount when one or more of these apply:
- The frame is shallow, or a crank handle is in the way.
- You want a darker room with fewer light gaps.
- The window is noticeably out of square.
- You want the window to look taller or wider.
How to measure for an inside mount
For an inside mount, you measure the opening itself, from one side of the frame to the other and from the top of the opening down to the sill.
Step 1: Measure the width in three places
Measure the inside width at the top, middle, and bottom of the opening. Record the narrowest of the three. Window frames are rarely perfectly square, and the blind has to fit through the narrowest point without rubbing.
Step 2: Measure the height in three places
Measure the inside height at the left, center, and right, from the top of the opening to the sill. Record the longest of the three so the blind reaches the sill along its whole width.
Step 3: Measure the depth
Measure from the glass, or from anything that sticks out such as a crank handle, lock, or alarm sensor, to the front edge of the frame. This tells you whether the headrail will fit inside. Different products need different depths, and a fully recessed, flush mount needs more than a partial one.
Step 4: Do not deduct
Give the exact narrowest width. The manufacturer makes a small deduction so the blind clears the frame. If you subtract your own allowance as well, the blind ends up too narrow, with larger gaps at the sides.
How to measure for an outside mount
For an outside mount, you measure the area you want the blind to cover rather than the window itself.
Step 1: Decide how much to cover
Most outside mounts extend about 2 to 3 inches past the frame on each side, which reduces light gaps and the view in from an angle. In a bedroom where darkness matters, a little more overlap helps.
Step 2: Measure the width
Measure the width of the frame including the trim, then add the overlap for both sides. Check that nothing on the wall, such as a light switch, a corner, or a neighboring window, gets in the way.
Step 3: Measure the height
Measure from where the headrail will mount, usually 2 to 3 inches above the frame or at the top of the trim, down to where you want the blind to stop. That might be the sill, just below the apron, or a set distance below the bottom of the trim.
Step 4: Check the mounting surface
Make sure there is a flat surface for the brackets above the frame. Crown molding, a soffit, or an existing curtain rod can change where the brackets go and how far the blind sits from the wall.
Product-specific details to check
Each type of treatment has its own needs, so confirm these details for the product you choose.
Cellular and honeycomb shades
They have compact headrails and fit many frames. For a top-down, bottom-up shade, the fit at the top of the frame matters, because the shade moves at both ends.
Zebra blinds and roller shades
The fabric roll and any cassette take up more depth than a cellular headrail. Check the depth carefully for an inside mount, and keep in mind that the fabric is narrower than the brackets, so the side gaps are slightly larger.
Wood and faux wood blinds
These have deeper headrails and slats that need room to tilt. Make sure handles and locks do not block the slats. Faux wood blinds are heavier, which can limit how wide a single blind can be.
Curtains and drapery
Drapery is measured from the rod or track down to the floor, not from the window frame. Our curtain length guide explains how to measure the finished drop.
Read the curtain length guideExplore Honeycomb ShadesExplore Wood Blinds
Common measuring mistakes
Most of these are small slips, but on a custom product a small slip can mean a remake.
- Switching width and height. Always record the width first.
- Rounding to the nearest inch instead of the nearest eighth.
- Measuring one window and assuming the others match.
- Making your own deductions on an inside mount.
- Forgetting to check depth for handles, cranks, and sensors.
- Measuring the glass instead of the frame opening.
- Ignoring the stack, the height the raised blind takes up at the top, which can cover part of the glass on wood blinds.
Bay, corner, arched, and extra-wide windows
Bay and corner windows need each section measured separately, and the blinds have to be planned so they do not collide where they meet. Arched windows can take a specialty shape, or a standard blind below the arch with the arch left open. Wide windows and patio doors may need two or more shades side by side, especially with heavier products.
These are the situations where a professional measurement saves the most trouble. The angle of a bay, the clearance at a corner, and the weight of a large shade all affect what will work.
Choose the control side and lift type
Measurements are only half of an order. You also need to decide how each blind will be operated and from which side. Look at where furniture sits, which side of the window you approach from, and whether a lamp, a bed, or a sofa blocks one edge.
Cordless lifts keep the window clean and are the safer choice in homes with children and pets. Wand controls suit tall windows where reaching the bottom rail is awkward. Motorized lifts make the most sense for high, wide, or hard-to-reach windows, and for rooms where you want several shades to move together.
On wide windows split into two or more blinds, decide whether the controls should sit on the outer edges, the center, or the same side on every blind. It is a small detail that makes the window easier to use every day.
Record your measurements clearly
A clear record prevents most ordering errors. For each window, write down the room and position, the mount type, the width, the height, the frame depth, and anything that sits in the way, such as a handle, a sensor, or a vent below the sill.
Note the control side and lift type on the same line, and add a quick phone photo of each window with the tape measure visible. If someone else places the order, they can check your numbers against the photo instead of guessing.
When to have a professional measure
You can measure a standard window yourself with care. For a whole house, tall windows, bays, motorized shades, or any room where light gaps matter, a professional measurement is worth it. It also means the person ordering the product has seen the window, the trim, and the obstructions in person.
During our in-home consultations we measure each opening, check depth and obstructions, and help you choose between an inside and an outside mount with samples in hand.
We can measure, sample, and style the room with you.
Our in-home consultations across Texas help you decide fabric, lining, hardware, and final drop with the room in front of us.
Frequently asked questions
Do you measure the glass or the frame for blinds?
Measure the frame opening, not the glass. For an inside mount, measure inside the frame from side to side and from the top to the sill. For an outside mount, measure the area you want to cover around the frame.
How much should an outside-mount blind overlap the window?
About 2 to 3 inches on each side is a common starting point. Add a little more in bedrooms if you want fewer light gaps.
Should I deduct from my measurements for an inside mount?
No. Give the exact narrowest width and the longest height. The manufacturer applies the small deduction the blind needs to fit and operate.
How deep does a window frame need to be for an inside mount?
It depends on the product. Cellular shades usually need the least depth, while wood blinds and cassette roller or zebra shades need more. Check the specification for your exact product before ordering.
