Width Deduction

The width deduction is the amount a manufacturer subtracts from your stated inside-mount width so the finished blind clears the frame and operates freely — commonly 1/4" to 1/2", but it varies by seller and product. The critical detail for buyers: most sellers make the deduction for you (you give the exact opening size and they subtract), while a few expect you to deduct yourself — getting this backwards double-deducts and leaves a gappy blind. This is the single most consequential number in a custom order, and it is the seller’s to define. Read their measuring guide, confirm who deducts, and remember custom sizes are generally not returnable for a mismeasure.