To measure for blinds, first decide on an inside or outside mount. For an inside mount, measure the width inside the frame at the top, middle and bottom and use the narrowest, measure the height at the left, centre and right and use the longest, and check the frame depth. For an outside mount, measure the area you want to cover, including the overlap past the frame on every side. Use a steel tape, measure to the nearest 1/8 inch (or millimetre) and always write width before height.
Quick Answer: How to Measure for Blinds
- Choose the mount (inside or outside) before you measure anything.
- Inside mount width: measure at the top, middle and bottom of the opening; record the narrowest.
- Inside mount height: measure at the left, centre and right; record the longest.
- Depth: measure from the glass or window frame to the front edge of the opening, and note any cranks or handles.
- Outside mount: measure the opening, then add the overlap you want on each side and above.
- Record width x height, label each window and don’t make your own deductions unless the order form asks for them.
What You Need
- A steel tape measure (cloth tapes stretch and give poor readings)
- A pencil and a sheet to note each window by room and position
- A step ladder
- A level, if you plan to install the blinds yourself
Measure every window separately, even ones that look identical. Frames in the same room are often slightly different sizes, especially in older homes.
Step 1: Decide Inside or Outside Mount
An inside mount sits within the window opening. It looks clean and built-in and shows off the trim, but it needs enough flat depth at the top of the frame for the headrail and it always leaves a thin gap at each side.
An outside mount fixes to the wall or trim above the window and overlaps the opening. It hides shallow or out-of-square frames, cuts light around the edges and can be mounted higher to make the window look taller. For a bedroom that needs darkness, or a sunny room where edge glare bothers you (see why light filtering blinds still let in glare), it’s often the better choice.
Obstacles can decide for you: crank handles, locks, alarm sensors and a shallow frame all push toward an outside mount.
Step 2: How to Measure for Inside Mount Blinds
Width
Measure the inside width of the opening from one side of the frame to the other at the top, the middle and the bottom. Record the narrowest of the three. If you use the widest, the blind will bind on the narrow spot.
Height
Measure from the underside of the top of the frame to the sill at the left, centre and right. Record the longest, so the fabric reaches the sill everywhere. A roller shade can simply stay slightly lowered where the opening is shorter.
Depth
Measure the flat surface at the top of the frame, from the window (or anything protruding, like a crank) to the front edge. Every product has a minimum depth for mounting, and a larger depth still for the headrail to sit fully recessed. Check the depth before choosing the product, not after; it’s the most common cause of install-day surprises.
Check for square
Measure both diagonals, corner to corner. If they differ noticeably, the frame is out of square, and an inside mount will show uneven gaps. An outside mount hides that.
Don’t deduct it yourself
For custom-made blinds, you normally give the exact opening size and the factory takes off the small clearance the blind needs to operate. If you subtract that clearance yourself as well, the blind ends up too narrow and leaks light at both sides. Read the order form: if it asks for the exact opening, give exactly that.
Step 3: How to Measure for Outside Mount Blinds
Measure the width of the opening (or the outside edges of the trim, if you want to cover it) and add overlap on each side. Around 1 1/2 to 3 inches per side is common; more overlap means less light at the edges, which matters in bedrooms.
For height, decide where the headrail will go. Mount it a few inches above the opening so the brackets have a solid surface and the rolled-up blind sits above the glass instead of blocking the top of your view. Measure from that line down to where you want the blind to end: the sill, or a little below it if there’s no sill in the way.
Check that the wall above the window is flat and clear of mouldings or vents where the brackets will go, and that the blind won’t hit a nearby wall, cabinet or ceiling.
Common Measuring Mistakes
- Measuring the glass instead of the opening or the area to cover.
- Rounding to the nearest half or whole inch. Stick to 1/8 inch or millimetres.
- Swapping width and height on the order. Always write width first.
- Measuring one window and copying it to the rest.
- Double deductions on inside mounts, as described above.
- Forgetting obstacles like cranks, handles and tile edges.
Bay, arched and angled windows follow their own rules; see our guide to custom blinds for arches, bays and odd-shaped windows before measuring those.
Installing Light Filtering Shades Yourself
Once the sizes are right, mounting a light filtering roller shade on a standard window is a realistic DIY job:
- Mark the bracket positions near each end of the headrail, with any middle brackets spaced evenly.
- Check level across the full width with an actual level, not by eye. Window frames are often slightly out of level; if so, split the difference.
- Pre-drill and fasten into the frame, a stud, the header or proper wall anchors.
- Hang the shade with the brackets snug but not fully tightened, run it up and down a few times, then tighten.
- Install the cord or chain safety device supplied with the shade, well out of reach of children.
Level matters more with light filtering fabric than with blackout. Because light comes through the fabric, a shade that hangs slightly crooked shows an uneven bright line at the bottom and uneven gaps at the sides. For more detail on brackets, anchors and when a DIY install stops being simple, read our step-by-step DIY blind installation guide.
The Alternative: Free In-Home Measurement
If you’d rather not risk a custom blind arriving the wrong size, we’ll measure for you. Our free in-home measurement covers every window, including the tricky ones, and our certified technicians handle the installation, typically within 3 to 4 days. Our checklist on getting your home ready for installation day helps the visit go smoothly.
Frequently Asked Questions
Do you measure the window or the frame for blinds?
Neither the glass nor the outside of the window unit. For an inside mount, measure the inside of the opening, frame to frame. For an outside mount, measure the area you want the blind to cover, including overlap.
Should I deduct from my inside mount measurements?
Usually not for custom blinds. Give the exact opening and let the manufacturer take the operating clearance. Only deduct if the order form specifically tells you to.
How much wider should an outside mount blind be than the window?
A common rule is around 1 1/2 to 3 inches past the opening on each side. Go wider in bedrooms or wherever light around the edges bothers you.
Is inside or outside mount better for light filtering shades?
Inside mount looks cleaner and suits most living areas. Choose outside mount if the frame is shallow or out of square, or if light at the edges is a problem.
Get It Measured Right the First Time
Browse our custom roller blinds in light filtering and blackout fabrics, then decide whether to measure yourself or have us do it. Either way, we’re happy to help across Mississauga, Toronto and the GTA. Request your free quote and book your free in-home measurement.