How to Measure for Cellular Shades: A Step-by-Step Checklist

Cellular shades — the honeycomb pleats that trap air to insulate — are one of the more forgiving products to measure, because they need very little frame depth and their soft edges hide minor imperfections. But forgiving is not foolproof, and a cellular shade is still custom-made to your numbers and generally non-returnable. This checklist walks the exact sequence, with the cellular-specific details — depth, headrail size, top-down/bottom-up — that generic guides skip.

What you need

• A steel tape measure — never cloth, which stretches and sags.
• A pencil and paper, or the printable checklist from our Measure & Mount Fit Checker.
• A step stool for tall windows.
• Your seller's measuring guide open on your phone.

Write every measurement as width × height (W × H), in that order, to the nearest 1/8 inch. Reversing W and H is a classic remake, and it costs nothing to label each number as you take it.

Step 1: Decide inside or outside mount

Cellular shades take either mount, and the choice changes every number that follows. An inside mount gives the clean, built-in look and shows your trim; an outside mount covers the opening for better darkness and suits shallow or out-of-square frames. Decide first, because you measure the two completely differently. If you are unsure, work through inside vs outside mount before you pick up the tape.

Step 2: Check the depth (cellular's easy win)

Cellular shades have one of the shallowest depth requirements of any product, which is why they fit windows other blinds cannot. For a flush inside mount — the headrail sitting even with the wall — you generally need only about 3/4 inch of flat depth. For the headrail to sit fully recessed inside the frame, you will want more, often around 1 to 2 inches depending on the headrail size and whether you add top-down/bottom-up hardware.

Measure depth from the glass (or the frontmost obstruction — a crank, handle, or lock) to the front edge of the opening. If you clear roughly 3/4 inch, a flush inside mount is on the table; if you do not, plan an outside mount. Depth minimums for every product are collected in minimum mount depth by product, and your seller's page lists the exact figure for the headrail you choose.

Step 3: Measure the width at three points

Window openings are rarely perfectly square. Measure the width in three places — across the top, the middle, and the bottom of the opening — and record all three.

• For an inside mount, use the narrowest of the three. Example: 34 1/8, 34, and 34 1/16 → your width figure is 34 inches.
• For an outside mount, use the widest span you want to cover and add overlap (commonly about 1.5 to 2 inches per side) so light does not sneak around the edges.

Read to the nearest 1/8 inch. Do not round to the nearest inch — that lost eighth is exactly the tolerance the fit depends on.

Step 4: Measure the height at three points

Measure the height at the left, center, and right of the opening.

• For an inside mount, most sellers want the longest of the three so the shade reaches the sill, but some specify otherwise — check the guide.
• For an outside mount, measure from where the top bracket will sit (typically 2 to 3 inches above the opening) down to where you want the shade to end, and include that top allowance.

Cellular shades stack into a small bundle when raised; if a fully-open view matters, note the seller's published stack height for your chosen cell size, since a taller shade stacks taller.

Step 5: Enter the opening — let the seller take the deduction

For an inside mount, the factory shaves a small amount off the width — typically 1/4 to 1/2 inch — so the shade clears the frame. Almost always, you enter your exact opening and the seller applies this deduction automatically. Do not subtract it yourself unless the seller's guide explicitly tells you to, or you will double-deduct and receive a shade that is too narrow. The deep dive is in deduction rules explained; the short rule is that the seller's own guide decides, and it wins over any generic figure.

Cellular-specific options that affect measuring

• Single-cell vs double-cell. A double-cell shade is a bit deeper and stacks slightly taller; confirm your depth and note the difference if headroom is tight. Which to buy is covered in single-cell vs double-cell shades.
• Top-down/bottom-up (TDBU). This hardware lets the shade lower from the top or raise from the bottom, and it usually adds to the headrail depth and the stack. If you want TDBU, check the deeper depth requirement before committing to an inside mount.
• Cordless vs continuous-loop. Cordless is the safer default around young children, consistent with CPSC guidance and the industry cord-safety standard, and it does not change your measurements — but a continuous-loop or motorized option might add a little side clearance, so confirm on the product page.
• Headrail size. Larger shades and blackout cellulars sometimes use a bigger headrail that needs more depth to sit flush. The seller lists headrail dimensions; match them to your measured depth.

Worked example, start to finish

A bedroom window, inside mount, cellular blackout shade. You measure width at 35 15/16, 35 7/8, and 36 (top, middle, bottom) and take the narrowest: 35 7/8. You measure height at 47 3/4, 47 7/8, and 48 and take the longest for an inside mount: 48. Depth is 1 1/4 inches — comfortably past the 3/4-inch flush minimum. Your seller's guide says to enter the exact opening and they deduct, so you order 35 7/8 × 48 and leave the deduction to them. No arithmetic on the width, no double-deduction.

Before you click order

Cellular shades are forgiving, but the order is still custom and generally not returnable — confirm the seller's remake policy, and know that coverage for a measuring mistake varies from full remakes to paid add-ons to nothing at all. Order fabric swatches to settle opacity and color against your actual glass, run your numbers through the Measure & Mount Fit Checker, and if anything about the depth or deduction is ambiguous, ask the seller before you commit. Measure twice, order once.