Window treatment calculators

Four calculators: the size to order for a blind or shade, the rod width, panels and length for curtains, the fabric to buy for making them, and the weight a rod and its brackets have to hold. Each shows its rule and where the rule comes from.

Lengths are typed as you read them off the tape: 34 3/8, 34.375 or, in centimeters, 87.5. Results are shown to the nearest 1/8 inch or 0.5 cm. Every box starts with a fictional example; replace the figures with yours. Write what you get on the window survey or the orders sheet.

1. Blind and shade size

For a blind, shade or shutter. Inside mount sits within the frame; outside mount covers the opening from the wall or trim. The method is in Measuring for blinds and shades.

Assumptions

• Inside mount: the width is the narrowest of the three and the height the longest, as Bali's wood blind measuring guide and Levolor's 2022 cellular shade guide ask; one shutter seller asks for the shortest height instead, so the box lets you choose. Your maker's guide wins.
• No deductions are taken: the factory makes its own (Levolor gives 3/8 in for an inside-mount cellular shade; The Shade Store about 1/8 to 1/2 in).
• A spread of more than 1/4 in between the widths is flagged as an observation. That threshold is ours, not a maker's.
• Depth: the minimum differs by product and operation, from about 1/2 in to over 2 in for motorized shades in Levolor's 2022 guide, so the box asks for your maker's figure rather than assuming one. A lock or crank standing proud of the frame is taken off the depth.
• Outside mount: 1 1/2 in of overlap each side is Bali's and Levolor's figure (The Shade Store gives 1 to 1 1/2 in; Bali's vertical cellular 3 in). The 2 in added above is ours, from the 2 in of flat surface Bali and Hunter Douglas ask for outside-mount brackets; change it to suit your trim.

2. Curtain rod, panels and length

For curtains on a rod or track. The rod runs past the opening so the open curtains stack on the wall; the stack depends on the fabric and the fullness. The method is in Measuring for curtains.

Assumptions

• The one-third rule: a 1977 Michigan State extension bulletin, How to Make Curtains and Draperies, says the stack takes about a third of "the fabric width", more for heavy fabric and less for light. We read that as a third of the finished width, about the rod width, in all: that reading gives 16 in a side on a 96 in rod at double fullness, close to Kirsch's 14 in, while the flat-fabric reading would give 32 in a side. With the stack equal to the extension, the rod comes out at 1 1/2 times the opening and each extension at a quarter of it on a two-way draw. This is our reading of the bulletin's wording, checked against one maker's chart.
• Kirsch's chart: the pleated stack guide in the Kirsch Wrought Iron 2024 Price Guide, for 2, 2 1/2 and 3 times fullness on tracks of 18 to 240 in, read between its rows. Kirsch says the stack size is estimated and the fabric can change it. The box solves for the track width at which the chart's stack equals the extension.
• Returns of 4 in are The Shade Store's standard; a 5 in center overlap comes from Kirsch's Superfine installation sheet. Fullness is applied to the rod width plus returns and overlap.
• Panel count is rounded up, and up again to an even number for a pair. A ready-made panel's stated width is assumed to be its flat width.
• Length: rod height, less any ring or clip drop, less 1/2 in of clearance for curtains opened often (The Shade Store), or plus a 1/2 in break or a puddle (makers give 1/4 to 3/4 in for a break and 1 to 4 in or more for a puddle). The 2 in default puddle is ours.

3. Fabric for making curtains

For curtains you sew. How the allowances are used is in the making section of Buying, ordering or making.

Assumptions

• Widths: rod width times fullness, divided by the fabric width, rounded up, with 2 1/4 times as the default fullness, from Rowley's grommet tape and pinch pleat tape instruction sheets.
• Allowances: 17 in for a doubled 4 in header and a doubled 4 in hem with 1 in for shrinkage (Rowley's grommet sheet; the 1971 Illinois extension circular gives the same 17 in for unlined panels); 9 1/2 in for a pleat tape heading (Rowley's pinch pleat sheet).
• Pattern: the cut length is rounded up to whole repeats so every width starts at the same point in the pattern. Rowley adds one repeat to each cut instead; the box offers one extra repeat for placing the pattern.
• Lining: same widths and allowance, no repeat. Many workroom methods cut lining shorter than the face; follow yours.
• An odd number of widths is split with a half width at each outer edge, as the Illinois circular places them.

4. Curtain weight and hardware

For any curtain on a rod or track. It estimates the load and sets it beside the figures the rod and anchor makers publish. What to do with the answer is in Hardware and installing.

Assumptions

• Weight formula and fabric weights from the Kirsch Wrought Iron 2024 Price Guide: square yards are yards times 36 times the fabric width in inches, divided by 1,296; pounds are square yards times ounces per square yard, divided by 16. The fabric choices use the top of each of Kirsch's ranges (for example linen 6 to 10 oz is taken as 10), which is our choice; a figure from your fabric's maker is better.
• Brackets: one at each end and supports no more than 48 in apart, from Kirsch's spacing guidance; Kirsch puts the next bracket within 24 in of a stacking end and asks for closer spacing for heavy drapery. Load per bracket assumes the brackets share the load evenly, which is ours.
• Anchors: TOGGLER publishes ultimate loads and says safety standards recommend a quarter of that as a working load; another maker publishes lab averages and says to apply a safety factor without naming one. The box divides by four only when you say the figure is ultimate.
• No anchor maker we found publishes values for plaster. The comparison here is two numbers side by side; it does not say a wall will hold.