How to Calculate Drapery Stack Back: Rod Extension Guide

What Is Stack Back?

Stack back is the horizontal depth each drapery panel occupies when fully open. When a drapery panel is drawn open, all the fabric compresses into a stack on one side. That stack has a fixed depth determined by:

• How many fabric widths are in the panel
• The fabric bolt width
• The pleat style (determines how tightly the fabric compresses)

Your rod or track must extend at least as far as the stack depth past the window edge on each side — otherwise open panels block part of the glass.

Stack Back Formula

Stack per side = widths per panel × fabric bolt width × stack factor

Stack factors by pleat style:

Step-by-Step Example

Given: 120” track, 2 panels, pinch pleat (2.5×), 54” fabric, 3” side returns each.

Step 1 — Find widths per panel:

Finished panel width = 120 ÷ 2 = 60"
Cut panel width      = 60 × 2.5 + 3 + 3 = 156"
Widths per panel     = ⌈156 ÷ 54⌉ = 3 widths

Step 2 — Calculate stack per side:

Stack = 3 × 54 × 0.33 = 53.5"

Each open panel stacks to approximately 54” deep. Your rod must extend 54” past the window frame on each side.

Step 3 — Calculate minimum rod width:

Minimum rod width = window frame width + (stack per side × 2)

For a 72” window frame:

Minimum rod width = 72 + 54 + 54 = 180"

This is why the track width is NOT the window width — it must include room for both stacked panels.

Ripplefold Stack Back

Ripplefold is the most compact pleat style. The S-wave carriers compress tightly when opened.

Example: 84” window, 2 panels, ripplefold (1.8×), 54” fabric, no returns.

Finished panel width = 84 ÷ 2 = 42"
Cut panel width      = 42 × 1.8 = 75.6" → round up for widths
Widths per panel     = ⌈75.6 ÷ 54⌉ = 2 widths
Stack per side       = 2 × 54 × 0.25 = 27"

Ripplefold needs only 27” of extension vs 54” for pinch pleat on comparable windows. This makes ripplefold ideal for narrow rooms where rod extension is limited. See Pinch Pleat vs Ripplefold Drapery for a complete hardware and cost comparison. For ripplefold-specific yardage calculation, see How to Calculate Ripplefold Drapery Yardage.

Stationary Panels: Stack Back Is Decorative

Stationary (non-drawing) panels — goblet pleat, box pleat, and some formal styles — don’t need full stack clearance. The rod only needs 6–8” past the frame per side for the return to wall.

Stationary panels are hung and never drawn:

• Goblet: cylindrical heads crush if traversed
• Box pleat: very stiff heading, not designed to traverse
• Formal valances: by definition not drawn

For stationary panels, ignore the stack calculation. Only traversing panels need stack clearance.

Common Stack Back Mistakes

Mistake 1: Calculating stack from finished width (not cut width)

Stack forms from the cut fabric, not the finished width. Always use widths × bolt width × factor.

Mistake 2: Not allowing for fabric weight

Heavy fabric (upholstery-weight linen, velvet) stacks looser than lightweight silk or voile. Add 10–15% to the stack factor for very heavy fabrics.

Mistake 3: Ordering the rod after measuring the window only

Always calculate stack before selecting hardware. A rod ordered to window width + 6” each side will leave panels blocking 40–50% of the glass when open.

Use the Drapery Yardage Calculator — it calculates stack back automatically for every pleat style.

See also: Drapery Yardage Formula and Drapery Pleat Types Guide.