Custom blinds are made to your numbers and cannot be returned. That is the whole reason learning how to measure for blinds matters so much more than picking the colour. Get the fabric wrong and you live with it. Get the width wrong by half an inch and you own an expensive object that does not fit any window in your house.
The measuring itself takes about four minutes per window. Almost every failure comes from one of three things: measuring in a single spot, deducting for the hardware when the factory already does it, or choosing the mount type after taking the measurements instead of before.
Work through it in this order and the window you measure is the window the blind fits.
Decide inside or outside mount first
This is step zero, and skipping it is why people end up measuring twice. Inside and outside mount use completely different numbers, so the decision has to come first.
An inside mount sits within the window opening, inside the frame. An outside mount sits on the wall or on the face of the trim, covering the opening and some of the surrounding wall.
When inside mount is the better choice
Pick inside mount when the window has an attractive frame you want visible, when the opening is deep enough for the hardware, and when the window is reasonably square. It gives a clean, built in look and leaves the trim on show.
The tradeoff is light. An inside mount leaves a small gap on each side where light leaks in, usually a quarter inch or so per side. In a living room that is invisible. In a bedroom where someone sleeps past sunrise, those two thin strips of light become a nightly irritation.
When outside mount is the better choice
Pick outside mount when the opening is too shallow for the hardware, when the window is noticeably out of square, when there are obstructions inside the frame, or when you need real darkness.
It also has a useful side effect. Mounting high and wide makes a window look larger than it is, which is the same trick covered in our guide to how high to hang curtains. In a small bedroom that illusion is worth more than the neat built in look, and it pairs well with the other ideas in our guide to making a tiny bedroom feel bigger.
Check the depth before you commit to inside mount
Measure from the face of the glass to the front edge of the frame. That is your available depth, and it decides whether an inside mount is even possible.
Every product lists a minimum depth for a flush inside mount and a smaller minimum for a partial one, where the headrail projects slightly. Those figures vary a lot between a slim roller shade and a chunky wood blind or a cellular shade with a deep headrail, so read the specification for the exact product rather than relying on a general rule.
If you are short on depth, you have three ways out: choose a shallower product, accept a partial inside mount where the hardware sticks out a little, or switch to outside mount.
Inside mount measurements
Use a steel tape, never a cloth one. Cloth tapes stretch, and an eighth of an inch matters here.
Width
Measure the inside of the opening in three places: at the top, at the middle and at the bottom. Write all three down.
Use the narrowest of the three, and round down to the nearest eighth of an inch. A blind sized to the widest point will not go in at all. A blind sized to the narrowest point fits everywhere along the opening.
Height
Measure from the top of the opening to the sill in three places: left, centre and right. Use the longest of the three, and round up to the nearest eighth.
Height works the opposite way to width because the consequence is reversed. A blind an eighth too long just rests on the sill. A blind an eighth too short leaves a visible gap of daylight at the bottom of the window.
Do not subtract anything
This is the single most expensive mistake in the whole process. Most custom blind retailers apply the fitting deduction at the factory, usually a quarter to half an inch off the width you submit, so the blind has room to operate. If you subtract that yourself before ordering, the deduction happens twice and you receive a blind that is visibly narrow with a light gap down each side.
Submit the exact opening measurement and let the manufacturer handle it. If you are buying from a retailer you have not used before, confirm on their measuring page that they deduct, because a small number of suppliers expect you to do it. Reading one line on their site is cheaper than reordering.
Outside mount measurements
Outside mount is more forgiving because you are choosing the size rather than matching an opening. The numbers below are overlap targets, not fixed rules.
- Width: measure the opening and add at least 2.5 inches of overlap on each side, so 5 inches of extra width in total.
- Height above: mount at least 3 inches above the top of the opening. This hides the headrail and gives the window more apparent height.
- Height below: either stop at the sill, or run at least 2 inches below it if you want maximum light blocking.
Before you settle on those figures, check what is actually on the wall. You need solid mounting surface at the chosen height, and you need the blind to clear any door or window that swings. A blind mounted 3 inches high is useless if the crown moulding starts at 2 inches.
For a bedroom where you want real darkness, go generous on all four sides. Light finds every gap, and the extra inch costs nothing at the ordering stage.
The obstructions that ruin an inside mount
Depth is not the only thing inside the frame. Before committing, look for:
- Crank handles on casement windows, which stand well proud of the frame
- Window locks and tilt latches on double hung sashes
- Alarm sensors and contacts in the top corners
- Trim or stops that reduce the usable opening partway down
- Handles on a sliding door within the track opening
A folding crank handle solves the casement problem for a few pounds. For everything else, an outside mount is usually simpler than fighting the obstruction.
Windows that are not square
Older houses settle, and almost no window is perfectly rectangular. Measuring in three places tells you how bad it is.
If the difference between your largest and smallest width reading is under a quarter inch, carry on with an inside mount. The factory deduction absorbs it.
If the difference is half an inch or more, switch to outside mount. An inside mount in a crooked opening leaves a wedge shaped light gap that gets wider down the window, and no amount of adjustment hides it. Sloped ceilings and awkward openings in converted spaces have the same problem, which is why our guide to attic bedroom ideas leans on wall mounted treatments rather than fitted ones.
Measure every window separately
Two windows built on the same day by the same crew to the same drawing will still differ by an eighth or a quarter of an inch. Walls move, frames shim differently, paint builds up unevenly.
Measure each one. Label each set of numbers with the room and the position, something like “back bedroom, left of the bed”, and tape the label to the window itself. When four identical looking boxes arrive, you will be glad you did.
A short checklist before you order
- Mount type chosen and written down
- Depth checked against the product specification, for inside mount
- Three width readings taken, narrowest used, rounded down
- Three height readings taken, longest used, rounded up
- No deductions applied by you
- Obstructions checked inside the frame
- Each window measured and labelled individually
- Measurements entered as width first, then height
That last point sounds trivial and it is not. Width before height is the universal convention in the window covering trade. Enter them the other way round on a tall narrow window and you will receive something unusable.
Frequently Asked Questions
Should I subtract anything from my inside mount measurement?
Usually no. Most custom retailers deduct a quarter to half an inch at the factory so the blind can operate inside the opening. Subtracting it yourself as well produces a blind that is clearly too narrow. Check the retailer’s measuring page to confirm their policy before you order.
Why do I round width down but height up?
Because the two errors have different consequences. A blind slightly too wide will not fit into the opening at all. A blind slightly too long simply rests on the sill, while one slightly too short leaves a permanent strip of daylight at the bottom.
How much overlap does an outside mount need?
At least 2.5 inches past the opening on each side, at least 3 inches above the top, and either level with the sill or 2 inches below it. Go further on all sides in a bedroom where you want genuine darkness.
Can I use an inside mount on a window that is out of square?
If the variation between your widest and narrowest reading is under a quarter inch, yes. At half an inch or more, switch to outside mount, because an inside mount will show a wedge of light that widens down the window.
Do I need to measure windows that look identical?
Yes. Windows installed at the same time routinely differ by an eighth or a quarter of an inch once the house has settled and the paint has built up. Measure and label each one separately.
Four minutes a window
Nothing in this process is difficult. It is just unforgiving, because the order is final and the tolerance is an eighth of an inch.
Take a steel tape, decide the mount type before you measure anything, take three readings in each direction, and resist the urge to be clever about deductions. Do that and the blinds arrive, go up in ten minutes, and you never think about them again.