Seam Allowance & Fabric Calculator
Add seam allowance to your finished dimensions and find the total fabric length you need to cut a set of panels — including a waste margin.
How it works
Cut size = finished + 2 × seam · Panels per row = floor(bolt width ÷ cut width) · Rows = ceil(panels ÷ per row) · Length = rows × cut height × (1 + waste)
Every seam eats fabric equal to the allowance on both pieces being joined, so a panel must be cut larger than its finished size by twice the seam allowance on each axis. Laying panels out across the bolt width tells you how many fit side by side; the remaining panels stack in rows down the length. Multiplying rows by the cut height gives the raw yardage, and a waste margin covers pattern matching, shrinkage and cutting errors. Buying to the nearest quarter-yard above this figure is the safe habit.
Worked example
A 20 × 30 in finished panel with a 5/8 in (0.625) seam allowance is cut at 21.25 × 31.25 in. A 44 in bolt fits floor(44 ÷ 21.25) = 2 panels across, so 4 panels take 2 rows. That is 2 × 31.25 = 62.5 in, and with 15% waste about 71.9 in — roughly 2.0 yd.
Frequently asked questions
What seam allowance should I use?
Home-sewing and garment patterns usually use 5/8 in (0.625 in). Quilting uses 1/4 in (0.25 in), and many knit patterns use 3/8 in (0.375 in). Enter the value your pattern specifies, since it is applied to every edge on both dimensions.
Why add a waste allowance?
Fabric shrinks when prewashed, prints need matching across seams, and cutting is never perfect. A 10–20% margin keeps you from coming up short mid-project, which usually means an off-dye-lot replacement.
Does bolt width really matter?
Yes — how many panels fit across the usable width sets how many rows you need down the length, which is what you actually pay for. A wider bolt often means fewer rows and less total yardage for the same panels.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.