Deduction Rules, Explained: Why Your Inside-Mount Blind Is Cut Narrower

The deduction is the most misunderstood number on a blinds order, and the most expensive to get wrong. When you order an inside-mount blind, the finished product comes back narrower than the opening you measured — on purpose. That gap is the manufacturer's deduction, and understanding who applies it, how big it is, and why it exists is the difference between a blind that glides in cleanly and one that either jams or leaks a half-inch of daylight down each side. This guide explains the rule in plain terms, then shows you the one mistake that turns it into a remake.

Why an inside-mount blind is cut narrower

A blind is not just fabric; it is a headrail with brackets on each end, a bottom rail, and moving parts that have to travel up and down without scraping the frame. If the factory built the blind to the exact width of the opening, there would be no room for the mounting brackets, no clearance for the headrail to seat, and no tolerance for a frame that is a hair out of square. So the manufacturer subtracts a small, deliberate amount from your measured width. The result slides in, the brackets have somewhere to live, and the shade lifts and lowers without rubbing.

That subtraction is the deduction (some sellers call it a factory deduction or a manufacturing take-up). It applies to width. Height is usually left at or near your measured figure, because a blind hanging in an opening does not need side clearance top-to-bottom — though a few products trim the height slightly or ask you to account for a sill, so this is product-specific.

How big is the deduction?

Across the industry, inside-mount width deductions typically run 1/4 to 1/2 inch. That is a range, not a rule, because every seller sets its own figure based on its hardware and tolerances. One seller might take 3/8 inch on a cellular shade; another might take 1/2 inch on a faux-wood blind because the tilt mechanism needs more room. The exact number lives in the seller's own measuring guide, and it is the only number that matters for your order.

This is why our standing advice is: the seller's deduction wins. Generic charts — including this one — tell you what to expect; the seller's guide tells you what is true for the blind you are actually buying. If the two disagree, follow the seller.

The one question that prevents a remake: who subtracts?

Here is the trap that causes more inside-mount remakes than any other. There are two conventions, and you must know which one your seller uses:

1. Enter the exact opening — we deduct. Most major sellers work this way. You measure the opening, type in that number, and the factory quietly subtracts its deduction. You do nothing to the number.
2. Enter your finished blind size — you deduct. A minority ask you to do the subtraction and enter the already-reduced figure.

If you assume the second convention when your seller uses the first, you double-deduct: you subtract, then the factory subtracts again, and your blind arrives roughly a half-inch too narrow — inside a non-returnable order. This is the single most common self-inflicted measuring error, and it is entirely avoidable by reading one line on the seller's measuring page before you type anything.

Worked examples

Assume a clean 36-inch opening (you measured top, middle, and bottom and took the narrowest).

• Seller deducts (the common case). You enter 36. The factory takes, say, 3/8 inch, and your blind is built to 35 5/8. It slides in with clearance. You never type the deduction.
• You deduct (the minority case). The seller's guide says to subtract 1/2 inch yourself. You enter 35 1/2. The factory builds exactly 35 1/2. Correct.
• The double-deduction error. The seller deducts, but you also subtract 1/2 inch and enter 35 1/2. The factory takes another 3/8, building 35 1/8 — nearly 7/8 inch under your opening. Too narrow, light leaks, remake.

A second size for practice: a 42 1/4-inch opening where the seller tells you to deduct 1/2 inch yourself becomes an order of 41 3/4 inches. If instead the seller deducts for you, you enter 42 1/4 and leave it alone.

What about height, depth, and square?

Height. Most inside-mount products use your measured height as-is or trim a token amount; some ask which measurement to favor. Measure at the left, center, and right, and follow the seller on whether to use the longest, the shortest, or a specific point.

Depth. The deduction assumes the blind can seat inside the frame, which only works if the frame is deep enough. Confirm your opening meets the product's minimum — see minimum mount depth by product. If it does not, you are looking at an outside mount, where width deductions do not apply; see inside vs outside mount.

Square. The deduction also buys tolerance for an out-of-square opening. Measure the width at three heights; if the spread is more than about 1/4 inch, the opening is noticeably out of square and an outside mount may look and seal better.

Factory tolerance: blinds are built to a window, not a micron

Custom blinds are manufactured to a tolerance — commonly around 1/8 inch — so a finished blind can legitimately arrive an eighth off its target size and still be in spec. That tolerance is exactly why the deduction exists and why you should never measure to a rougher precision than the eighth. Use a steel tape, read to the nearest 1/8 inch, and do not round to the nearest inch; rounding is itself a common error covered in the most common measuring mistakes.

Find your seller's rule before you order

Every seller publishes a measuring guide, and every one states its deduction convention and figure. Before you commit a single window:

• Open the seller's measuring guide and find the inside-mount deduction and whether they apply it.
• Cross-check with our Measure & Mount Fit Checker, which shows the standard 1/4-to-1/2-inch range and the minimum depth for your product so you know what normal looks like.
• Remember the safety net: a custom blind is made-to-size and generally not returnable, so confirm the remake policy. Coverage for measuring mistakes varies — some sellers offer it, some charge for it as a paid add-on, and some publish none — which makes getting the deduction right the first time the cheapest path by far.

Get the deduction convention right, measure to the eighth, and defer to the seller's guide, and the narrower-than-the-opening blind will do exactly what it is designed to do: slide in clean and cover the glass. Measure twice, order once.