How this calculation works
Find enough full fabric widths for each equal panel after join and outside hem consumption. Add top and bottom allowances to the finished drop, round each cut drop to a straight pattern repeat, then multiply by all drops and add one initial alignment allowance.
Usable panel width = full widths × roll width − 2 × join allowance × joins − 2 × side hem; cut drop rounds upward to repeatA worked example
A 200 cm span at 2× fullness with two panels needs 200 cm flat width per panel. Two 140 cm fabric widths leave 268 cm after the entered joins and hems. Four drops at a repeat-rounded 270 cm require 10.8 m before any further allowance.
Good to know
Equal panels and full-width drops. Straight vertical pattern match only; no half-drop motif, railroaded fabric, lining, shrinkage, offcut optimization or supplier-specific pleat header design.
Reference: Wayfair: curtain fullness and pattern allowance (opens in a new tab) ↗
Begin with the track span and flat fullness
The span is the pole or track width the curtains must cover. Multiplying by the chosen fullness gives the target flat fabric width before pleating. Divide that target between the equal panels to find the minimum width each panel needs.
Returns, overlaps and extra side coverage belong in the measured span or a separate design decision. The calculator does not infer a header type or select how much fabric a supplier’s specific pleat system consumes.
Account for both sides of every internal join
Joining two fabric widths consumes seam allowance on both adjoining edges. The outside side hems are charged only at the outside edges of each completed panel. Confusing a panel edge with every fabric-width edge can overstate the required material.
The tool adds whole full-width drops until the usable panel width meets the target. Any extra usable width remains visible as achieved fullness. It does not claim to optimize narrow offcuts or trim every panel to exactly the target width.
Round the cut length for a straight repeat
Finished drop plus top and bottom allowances gives the raw cut length. A straight vertical repeat rounds that length upward to a complete repeat so successive drops can start at the same motif position.
An additional initial alignment allowance is entered once at the start of the roll. Half-drop patterns, offset motifs, fabric defects, shrinkage and directional restrictions need a supplier-specific cutting plan. Matching a repeat in arithmetic alone does not verify those constraints.
Turn a theoretical length into a purchase
The cut list distinguishes drop count, cut length and the supplier’s purchase increment. Rounding the final order to a larger increment does not change the cut geometry; it creates spare material that should remain accounted for.
The wallpaper full-drop roll calculator has a related vertical-cut idea but a different installation task. Curtains add fullness, sewn joins and outside hems. Check all measurements and pattern instructions before cutting material that cannot be returned.
A few questions, answered
Do I enter a doubled side-hem allowance?
Enter the amount consumed at one outside edge; the tool accounts for the two edges per panel.
Does it support half-drop patterns?
No. The repeat calculation assumes a straight vertical match.
Why is achieved fullness above my target?
The model purchases and joins complete fabric widths instead of optimizing narrow offcuts.