Fabric Formulas: Yardage, Area & Weight Math

Fabric Formulas Used Across Fabric Formula

This page collects the core equations behind fabric yardage, width, weight, quilting and curtain calculations. Use it to check the math manually or understand what an interactive calculator is doing.

Real sewing layouts can require whole pieces, seams, pattern matching and purchase rounding, so a simple area formula is often only the first step.

Basic Fabric Length and Area

Rectangular areaArea = width × length

Use matching units before multiplying.

Inches to yardsYards = inches ÷ 36

Useful for converting total cut length into purchase yardage.

Running-yard areayd² per running yd = fabric width (in) ÷ 36

A 54-inch running yard contains 1.5 square yards.

Usable widthUsable width = nominal width − excluded edges

Exclude selvage or any edge you will not cut.

Fabric Weight Formulas

GSM to oz/yd²oz/yd² = GSM ÷ 33.9057

Converts areal fabric mass from metric to U.S. units.

oz/yd² to GSMGSM = oz/yd² × 33.9057

Inverse areal-weight conversion.

Total massgrams = width (m) × length (m) × GSM

Calculates net fabric mass from area.

Roll length from weightlength (m) = net grams ÷ [GSM × width (m)]

Reverse the total-mass equation.

Quilting Formulas

Quilt backing sizeBacking dimension = quilt dimension + 2 × side overage

Apply independently to width and length.

Binding perimeterPerimeter = 2 × (quilt width + quilt length)

Add joining and finishing extra before cutting strips.

Unfinished block sizeUnfinished = finished block + 2 × seam allowance

For a 1/4-inch seam on both sides, add 1/2 inch total.

Square countTotal squares = rows × columns

Add planned extras separately.

Curtain and Drapery Formulas

Finished coverageCoverage = rod face + returns + overlap

Use the finished-width extras required by the treatment.

Flat curtain widthFlat width = finished coverage × fullness

Fullness determines how much fabric width is gathered or pleated into the finished treatment.

Raw cut lengthCut length = finished drop + top/bottom allowances

Pattern repeat can increase this cut length further.

Repeat roundingAdjusted length = ceil(raw length ÷ repeat) × repeat

Rounds upward to the next whole repeat.

Upholstery and Cushion Formulas

Cushion cut dimensionCut size = finished size + 2 × seam allowance

Apply to each sewn edge dimension.

Dining-seat wrapCut size = seat size + 2 × wrap allowance

Allows fabric to continue around the seat and reach the underside fastening area.

Boxing heightCut height = finished thickness + 2 × seam allowance

Used for loose box-cushion side strips.

Cutting extraAdjusted amount = base amount × (1 + extra %)

Apply the percentage before purchase rounding.

Purchase Rounding

Fabric is often purchased in fixed increments. To round upward instead of to the nearest increment:

Rounded purchase = ceil(exact amount ÷ increment) × increment

For example, 2.13 yards rounded upward to the next quarter yard becomes 2.25 yards.

Use the Matching Calculator for Layout Constraints

Formulas explain the math, but many projects cannot be solved by area alone. Fabric width, whole-strip counts, full-width drops, directional prints, repeat matching and seam placement can force a larger purchase. Use the relevant Fabric Formula calculator when those constraints matter.