The most frequent error people make when ordering blinds is the confusion between width and height. This is not a close call, just a huge misunderstanding. A customer may write 84 x 72 instead of the correct 72 x 84 dimensions, causing problems with their window treatment.

When it comes to how to measure for vertical blinds, the process becomes a lot easier once people establish which mounting style they want to use. Inside mount and outside mount require different approaches, with the differences resulting from the need for manufacturer's deductions while measuring for the former.

Decide the Mount Before You Write Down a Single Number

Inside mount refers to the situation when blinds are mounted inside the window frame. On the other hand, outside mount is used to refer to the cases when the headrail is affixed to the wall or trim above the window.

The rule that trips up most buyers is the deduction. On an inside mount, the factory subtracts the clearance the blind needs to operate inside the frame. You submit the raw opening size. On an outside mount, no deductions are taken at all, so the number you order is the finished size of the blind.

That's why the same window gives you two different orders. Inside mount: a 48-inch opening is ordered as 48 inches. Outside mount: the same window is ordered at 54 inches, because you're adding overlap yourself and need about 3 inches extra each side

Pick inside mount when the recess is deep enough and the frame is square, since it looks cleaner and leaves the trim visible. Pick outside mount vertical blinds when the recess is shallow, when the frame has gone out of square, or when you want to cover extra glass for light control. On sliding doors, outside mount wins almost every time.

What to Get Right Before You Start

Sort these four things before the first window opening measurement, because each one causes returns:

• Utilize a metal tape measure. Fabric and vinyl tapes stretch and therefore using a stretched tape on an 84-inch drop is a huge mistake.

• Make measurements to the nearest 1/8 of an inch rather than rounding the measurement to the nearest inch.

• Measure every window. Framing drifts so do not rely on sight when windows “look the same size”.

• Write all orders as width first and height second.

Write the numbers on paper at the window. Reconstructing vertical blind dimensions from memory at the checkout screen is how width and height get swapped. Anyone who has had to measure vertical blinds twice for the same room learned this the expensive way.

Minimum Mounting Depth for Inside Mount Vertical Blinds Is 1 3/8 Inches

A 3.5-inch vertical blind with a valance generally needs at least 1 3/8 inches of usable depth inside the frame to mount. That is the number to check first, and it answers how deep does an inside mount vertical blind need to be without any guesswork.

Measure the depth from the face of the glass to the front edge of the frame, at the top where the headrail will sit. Watch for anything that eats into that space: a crank handle, a security sensor, a deep glazing bead, or a window that opens inward. Any of those reduces your usable vertical blind mounting depth even when the frame looks deep enough.

If you come up short, do not force it. Go outside mount. A headrail crammed into a shallow recess either fouls the window operation or leaves the vanes rubbing the glass every time they rotate.

How to Measure for Inside Mount Vertical Blinds

Take six measurements per window, then submit two. Here is the sequence for inside mount vertical blinds:

1. Measure the width across the top of the opening, then the middle, then the bottom. This is your vertical blind width measurement.

2. Use the narrowest of those three. The blind has to clear the tightest point or it will bind.

3. Measure the height on the left side, the center, and the right side, from the top of the opening to the sill.

4. Use the shortest of those three for your vertical blind height measurement.

5. Round both to the nearest 1/8 inch.

6. Submit those exact numbers. Do not deduct anything.

That last step is the one people override. The factory already subtracts the operating clearance for you, so a buyer who helpfully takes off half an inch first gets a blind that is half an inch too narrow, with a light gap down one side. How to measure for vertical blinds inside mount comes down to reporting the opening honestly and letting the factory do the math.

How to Measure for Outside Mount Vertical Blinds

Add at least 3 inches of overlap on each side of the opening, and mount at least 3 inches above it. That answers how much overlap for outside mount vertical blinds, and it is the spec that keeps light from leaking around the edges. Vertical blind overlap is the whole difference between how to measure for vertical blinds outside mount and the inside mount method above.

The arithmetic runs like this:

• Width: opening width plus 3 inches on the left plus 3 inches on the right, so 6 inches total added. On a 48-inch opening, order 54 inches.

• Height: measure from 3 inches above the top of the opening down to wherever the blind should end. That top 3 inches gives the mounting brackets somewhere to land.

Where the blind ends depends on the sill. If the sill protrudes, run the measurement down to the top of the sill. If it does not protrude, carry it about 2 inches below the opening so the vanes cover the gap.

No deductions apply here. What you order is the finished size, which means an error in your blind mounting measurements shows up at full scale. Measure the wall space twice, and check that your overlap does not collide with a light switch, a door casing, or an adjacent window.

Go generous on this one. Wider than 3 inches per side costs little and improves both privacy and the finished look, so how far vertical blinds should extend past window openings is worth answering with more rather than less.

How to Measure Vertical Blinds for a Sliding Glass Door or Patio Door

When measuring for a sliding glass door, measure the height from the floor as opposed to measuring from the sill and allow the blind to hang half an inch above. This is the only rule that allows you to distinguish between measuring for sliding glass door vertical blinds and measuring for regular blinds. A blind that reaches the floor drags, bends at the bottom, and collects dirt all the time, thus making the half-inch vertical blind clearance matter, since it determines whether a blind will last or wear out during the course of two years.

Width is measured according to the outside mount rule, allowing for 3 inches of overlap on each side. Go larger in windows than in sliding doors. Vertical blind stack width increases with the increase of number of vanes, thus allowing for enough space for parking in case of the wide opening.

Stack direction is the decision people forget until the blind arrives. You choose left stack, right stack, center stack, or split stack, and the right answer is whichever keeps the stack off the operating panel. Note which panel of the slider moves before you order, since a stack parked in front of it makes the door annoying every single day. For a full run of patio glass, the sliding glass door collection is filterable by width.

Size Limits and Standard Vertical Blind Sizes

Factory Direct Blinds manufactures vertical blinds within these dimensions, and the dimensions should be confirmed prior to liking a design.

• Minimum width: 5 inches

• Minimum height: 8.5 inches

• Maximum width: 192 inches

• Maximum height: 144 inches

• Slat width: 3.5 inches

FactoryDirectBlinds.com has a 192-inch maximum width is 16 feet on one vertical blind headrail, which covers almost any residential slider or picture window without splitting the order. That single spec is the reason verticals get specified on wide openings where horizontal blinds have to be broken into multiple units. The 3.5-inch vertical blind slat width is standard across PVC, faux wood, and fabric vanes, so switching material later does not change your carrier spacing.

There is no such thing as a standard vertical blind size in a custom order, which is the point. What size vertical blinds do I need has one honest answer: whatever your opening measures. Every blind is cut to the numbers you submit, so a nonstandard opening is fine as long as it falls inside those limits, and vertical blind sizing stops being a guessing game.

Measuring an Existing Vertical Blind for Replacement Vanes

To get started with the measuring process, take the dimensions of only one existing vane, excluding the window itself, and then place an order for the new ones indicating that you need "VANES ONLY." Take one vane off the carrier clip, place it on a flat surface, and measure its total length and width. You will now find out how to carry out the measuring process for your vertical blind if it is the vanes that have gone bad, but the headrail system is in perfect condition.

Match the width exactly. Carriers are spaced to the vane width, so a 3-inch vane on a headrail built for 3.5-inch vanes leaves gaps between slats even when the length is right. Check the top of the vane too, since the hole or slot has to match your clip type. When the headrail itself is worn, the vertical blinds installation guide covers replacing the whole assembly instead.

Where Measuring Goes Wrong

Two errors account for most bad orders, and neither one is a fraction of an inch. The first is swapping width and height. The second is taking your own deduction on an inside mount when the factory was going to take one anyway. Fix those two habits and your measurements will be fine even if your tape work is only average.

Check your numbers against the official vertical blind measuring instructions before ordering, then browse the vertical blinds range and place the order.

On a large or awkward opening, the MeasureSafe guarantee covers a remake on a mismeasure, which is worth adding to any window you only get one shot at.

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