Curtain Stack-Back Calculator
Enter the window width, how far the rod runs past it, the curtain fabric, and the heading to estimate how wide the open curtains stack, how much glass they cover, and how far the rod should extend to keep it clear.
Curtain Stack-Back Calculator calculates in your browser.
Open curtains still take up room — the rod has to make space for them
Pushed fully open, curtains don’t disappear: their fabric folds into a stack at the ends of the rod. If the rod stops at the window frame, that stack sits over the glass and blocks light and view. Rods are extended past the window to give the stack somewhere else to go.
This calculator estimates the stack from the fabric and a heading-style factor, shows how much glass it covers with your rod, and works out the extension that clears it. It uses the same rod, fullness, and fabric arithmetic as the Curtain Rod Length, Curtain Width, and Curtain Panel Count calculators, so the four agree.
How to use the Curtain Stack-Back Calculator
- Choose inches or centimeters and enter the window’s outside width.
- Enter how far the rod runs past each side of the window (enter 0 for a rod as wide as the window).
- Choose a pair that opens from the middle, or one side only.
- Give the fabric as a fullness, or as the number and width of the panels you have.
- Pick the heading style, or choose your own estimate and enter the maker’s figure.
How the calculation works
Rod = window + 2 × extension; fabric = rod × fullness + returns + overlap (or panels × panel width)
Stack = (fabric − returns) × stack factor; half each side for a pair
Glass covered on a side = stack on that side − extension, if more
Extension to clear (pair, by fullness) = k × (fullness × window + overlap) ÷ (2 × (1 − k × fullness))
Because a longer rod needs more fabric for the same fullness, the stack grows as the rod does. The extension is solved for directly: 20% of 2× fullness on a 48 in window gives 16 in each side, and on that 80 in rod the 160 in of fabric stacks exactly 16 in each side.
| Window and rod | Fabric and heading | Stack / glass clear | Extension to clear the glass |
|---|---|---|---|
| 48 in window, rod 8 in past each side, pair | 2× fullness, pinch pleat | 12.8 in / 38.4 in of 48 in (80%) | 16 in |
| 72 in window, rod 10 in past each side, pair | 4 panels of 54 in, grommet | 16.2 in / 59.6 in of 72 in (82.8%) | 16.2 in |
| 120 cm window, rod 15 cm past each side, one way | 2.5× fullness, wave | 56.3 cm / 78.8 cm of 120 cm (65.6%) | 81 cm |
Calculated on your device
- The estimate is worked out in your browser as you type; nothing is sent to a server.
- Your measurements and results are not sent to Looty Tools or stored.
- The calculator area, including the drawing, is masked from session-recording analytics.
Limitations
- Stack-back is an estimate. Fabric thickness, lining, heading, fullness, rings or carriers, and how the folds are trained all change it; use the maker’s figure when you have one.
- Returns are treated as wrapping to the wall and not stacking across the rod.
- It covers rods and tracks that draw sideways. Swags, tie-backs, Roman shades, and blinds aren’t calculated.
- Fullness 1–4×, stack 5–40% of the flat fabric, extension up to 48 in (121.9 cm) each side.
Related calculators
- Curtain Width CalculatorEstimate how wide curtains should be for a window.
- Curtain Panel Count CalculatorEstimate how many curtain panels a window needs.
- Curtain Rod Length CalculatorEstimate how long a curtain rod should be.
Guides
- Curtain Rod ExtensionWhy curtain rods extend past the window, common extensions from a few inches to a foot, how stack-back sets the minimum, and what to do when space is tight.
- Stack-Back by HeadingHow much pinch pleat, rod pocket, tab top, grommet, and wave curtains stack when open, with worked widths and what else makes a stack wider.
- Calculating Stack-BackWork out curtain stack-back step by step and solve for the rod extension that clears the glass, allowing for the extra fabric a longer rod needs.
- Open Curtains Blocking GlassWhy open curtains still cover the window, and practical ways to fix it: a longer rod, a tighter-stacking heading, less fullness, or a one-way draw.
Curtain Stack-Back Calculator FAQ
- What is curtain stack-back?
- The width curtains take up when they are fully open, gathered at the ends of the rod. A pair of pinch-pleat curtains at 2× fullness on a 64 in rod is 128 in of fabric; at about 20%, it stacks about 12.8 in each side.
- How far past the window should the rod go so curtains clear the glass?
- At least as far as the stack on each side. With a set fullness a longer rod needs more fabric, which stacks wider, so the calculator solves for the extension that just clears: a 48 in window at 2× in pinch pleats needs about 16 in each side — an 80 in rod.
- How much glass do open curtains cover?
- Whatever the stack is wider than the extension. On that 48 in window with the rod 8 in past each side, 12.8 in of stack each side covers 4.8 in of glass on each side, so 38.4 in (80%) stays clear.
- Do grommet curtains stack tighter than pinch pleats?
- Usually a little. About 15% of the flat fabric width is a common estimate for grommet and wave headings, and about 20% for pinch pleats, rod pockets, and tab tops. Lined or heavy fabric stacks wider.
- Is the stack-back exact?
- No — it is an estimate. Fabric weight, lining, the heading, rings or carriers, and how the folds are dressed all change it. If the maker gives a stack figure, choose “My own estimate” and enter it.
- Are the measurements I enter sent anywhere?
- No. The check runs in your browser as you type. Your measurements and results are not sent to Looty Tools or stored, and the calculator area is masked from session-recording analytics. Refreshing the page clears it.
