Roman Blind Fabric Formula: Complete Calculation Guide

The Roman Blind Formula

A roman blind calculation has two independent dimensions: cut width and cut length.

Cut Width

Finished width   = window width ± mount adjustment
Cut width        = finished width + (side hem × 2)

Mount adjustments:

• Inside recess: finished width = window width − 2 cm (−0.8”) — clearance prevents binding
• Outside mount: finished width = window width + 10 cm (+4”) — 5 cm overlap each side

Side hem: Standard 2.5 cm (1”) each side, folded double = 1.25 cm (0.5”) visible.

Cut Length

The cut length formula has three components added together:

Cut length = finished drop
           + fold allowance
           + top allowance
           + bottom hem

Each component explained:

Fold Allowance Formula

The fold allowance is the most important (and most misunderstood) part of the calculation.

Step 1 — Calculate Number of Sections

Sections = round(finished drop ÷ fold spacing)

Round to the nearest whole number. Never leave a fraction — each section must be equal.

Step 2 — Calculate Number of Folds

Folds = sections − 1
Rods  = folds     (one rod at each fold line)

The top section of the blind has no rod above it (it goes up to the batten). The bottom section has no rod below it (it forms the hem). All intermediate sections need one rod at their lower boundary.

Step 3 — Calculate Fold Allowance

Flat roman blind:

Extra per fold = fold depth × 2
Total extra    = folds × fold depth × 2

Cascade / hobbled roman blind:

Extra per fold = fold depth × 3
Total extra    = folds × fold depth × 3

Why the difference?

In a flat roman, each fold stacks up cleanly behind the face when raised. The fold fabric goes back one fold depth and returns — consuming 2× fold depth in cut length.

In a cascade blind, the folds hang permanently as visible front loops. Each loop protrudes forward, hangs down, and returns — consuming approximately 3× fold depth.

Complete Worked Example

Flat blind, inside recess:

• Window width: 90 cm
• Drop: 120 cm
• Fold spacing: 25 cm
• Fold depth: 12 cm
• Top allowance: 5 cm
• Bottom hem: 10 cm
• Side hem: 2.5 cm each

Width calculation:

Finished width = 90 − 2 = 88 cm
Cut width      = 88 + 2.5 + 2.5 = 93 cm

Length calculation:

Sections        = round(120 ÷ 25) = round(4.8) = 5
Folds           = 5 − 1 = 4
Actual spacing  = 120 ÷ 5 = 24 cm (not 25 — rounding adjusted it)
Fold allowance  = 4 × 12 × 2 = 96 cm
Cut length      = 120 + 96 + 5 + 10 = 231 cm = 2.31 m

Total fabric: 2.31 metres (2.53 yards)

The Same Blind in Cascade Style

Same measurements, cascade instead of flat:

Fold allowance (cascade) = 4 × 12 × 3 = 144 cm
Cut length               = 120 + 144 + 5 + 10 = 279 cm = 2.79 m

Total fabric: 2.79 metres (3.05 yards) — 48 cm / 0.48 m more than the flat version for the same window. For the full cascade fold formula and loop depth guide, see How to Calculate Cascade Roman Blind Fabric.

Lining Cut Dimensions

Lining is cut smaller than face fabric on both dimensions. For lining types and attachment methods, see the Roman Blind Lining Guide:

For the 90 cm × 120 cm flat example:

Lining width  = 88 − 2 = 86 cm
Lining length = 120 + 96 + 2 = 218 cm = 2.18 m

Fabric Yardage Summary Table

54” (137 cm) fabric, 12 cm fold depth, standard allowances:

Bold = most common bedroom / sitting room blind size.

Use the Roman Blind Calculator to calculate your exact fabric requirement.

See also: Roman Blind Types Guide and How to Calculate Roman Blind Folds.