How we calculate

Every number on this site and in the Quiltwell app comes from the same few formulas, written out below with the assumptions we make and a worked example for a 90″ × 100″ quilt. Amounts always round up, so you never come home short.

Updated 2026-11-14. Spotted something that doesn't match your experience? Write to support@quiltwell.app and we will check it.

Backing

1. Add the overhang. 4″ on every side by default, because long-arm quilters need extra to clamp the backing on the frame (most ask for 4″–6″; for quilting on a home machine 2″–3″ is enough). 90″ × 100″ → 98″ × 108″.
2. Usable width. Quilting cotton is sold 42″–44″ wide. After trimming the selvages we count 40″ as usable. Each seam uses ½″ from both panels, so every extra panel adds 39″, not 40″. For 108″ wide backing we count 104″.
3. Count panels both ways. Seams running down the quilt (vertical): 3 panels × 108″ = 9 yd. Seams running across (horizontal): 3 panels × 98″ = 8 1/4 yd. We show the direction that needs less fabric — here horizontal, 8 1/4 yd — and the other one too, because some long-arm quilters prefer a direction.
4. 108″ wide backing: the backing fits on one width, so you buy its length: 3 yd.

Binding

1. Length. Perimeter + 12″ for joining the ends and turning four mitred corners: 2 × (90 + 100) + 12 = 392″.
2. Strips. Strips are cut across the fabric (40″ usable) and joined with diagonal seams. Each join uses about one strip width, so a 2½″ strip adds 37½″. 392 ÷ 37½ → 11 strips, always rounded up to a whole strip.
3. Fabric. Strips × strip width: 11 × 2½″ = 27 1/2″ → 7/8 yd.
4. Bias binding (curved edges): the side of the square for continuous bias is √(length × strip width) + strip width, rounded up to ⅛″.

Batting

Batting should be as big as the backing: the top plus 4″ on every side, so 98″ × 108″ for our example. We then pick the smallest standard package that covers it (king, 120″ × 120″) or the length to buy off a 96″ or 120″ roll. Package sizes we use: crib 45 × 60, throw 60 × 60, twin 72 × 90, full 81 × 96, queen 90 × 108, king 120 × 120. Brands differ by an inch or two, so check the label.

Quilt size for a bed

Width = mattress width + 2 × drop. Length = mattress length + drop + pillow tuck. Our charts use US mattress sizes (queen 60″ × 80″) and show several drops; a 12″ drop with a 10″ pillow tuck makes a queen quilt 84″ × 102″. UK and Australian beds are in centimetres on the size chart.

Half-square triangles, flying geese and setting triangles

• HST, 2 at a time: square = finished + ⅞″ (3″ finished → 3 7/8″). 4 at a time: finished × 1.414 + 1¼″ (5 1/2″). 8 at a time: finished × 2 + 1¾″ (7 3/4″). Trim each unit to finished + ½″.
• Quarter-square triangles: finished + 1¼″, two squares make two QSTs.
• Flying geese, no waste (4 at a time): one large square of 2 × finished height + 1¼″ and four small squares of finished height + ⅞″. Stitch and flip: a rectangle of (2 × height + ½″) × (height + ½″) and two squares of height + ½″.
• Setting triangles (blocks on point): side triangles from squares of block diagonal + 1¼″, cut twice on the diagonal; corner triangles from squares of block ÷ 1.414 + ⅞″, cut once. For 10″ blocks: 15 1/2″ and 8″.
• Every cut size rounds up to the next ⅛″, the smallest step on a quilting ruler.

Fat quarters and pre-cuts

• Fat quarters: the count is exact for a top made of same-size squares. Squares across = quilt width ÷ (cut size − ½″), rounded up, and the same down; squares per fat quarter = whole squares that fit in 18″ × 21″ (US) or 50 × 55 cm (UK), trying both ways round. Charts that divide areas ignore seam allowances and offcuts, so they come out short.
• Layer cakes and charm packs: the same square count with 9½″ or 4½″ finished squares, 42 to a pack.
• Jelly rolls: 2½″ strips (2″ finished) sewn end to end into rows; each strip gives about 40″ after the selvages, less ½″ per join; 40 strips to a roll.

Rounding and metric

• Yardage rounds up to the next ⅛ yard (4½″), the smallest cut most shops make.
• Metric amounts round up to the next 10 cm: our example backing is 7.5 m of 110 cm fabric.
• Cut sizes round up to the next ⅛″. Inputs accept fractions (60 1/2), decimals (60.5 or 60,5) and centimetres.

What the numbers don't include

• Shrinkage. Cotton can shrink a few percent when pre-washed. If you pre-wash, buy a little extra.
• Directional prints and matching. If a print must run one way or match across a seam, buy an extra pattern repeat per seam.
• Mistakes and straightening. The amounts are exact for the cuts described. Many quilters add a small margin for squaring up the fabric.
• Your long-arm quilter's rules. If they ask for more than 4″ per side, change the overhang in the calculator.

Who makes Quiltwell

Quiltwell is a small independent app studio. The same calculation code runs in the free calculators here and in the Quiltwell app for iPhone, iPad and Android, and is checked by automated tests against hand-worked examples like the ones above. No ads, no accounts, no tracking cookies.