Precut math: charm packs, jelly rolls, and layer cakes

The quick answer — what each precut finishes at once it’s sewn:

Precut	Cut size	Finishes at	Typical count per pack
Mini charm	2 1/2“	2“	42
Charm pack	5“	4 1/2“	42
Layer cake	10“	9 1/2“	42
Jelly roll strip	2 1/2“ × WOF	2“ × ~41 1/2“	40 strips
Fat quarter	18“ × 21“	(yardage, not a unit)	8–12

Every calculation below is the same move: subtract 1/2“ for seam allowance, then divide the quilt into the finished units.

How many charm packs for a quilt?

A charm square finishes 4 1/2“. A quilt W“ × L“ set edge to edge takes:

columns = W ÷ 4.5, rows = L ÷ 4.5 (round to whole numbers)

Quilt	Grid	Squares	Charm packs (42)
Baby, 36 × 45	8 × 10	80	2
Lap, 54 × 63	12 × 14	168	4
Twin, 63 × 90	14 × 20	280	7

Sashing between squares stretches a pack much further — 2“ sashing turns a 4 1/2“ grid into a 6 1/2“ one, and that twin drops from 7 packs to about 4.

Jelly roll math

One strip is 2 1/2“ × ~42“. The two workhorse numbers:

• Subcutting: a strip yields ⌊42 ÷ cut length⌋ pieces — sixteen 2 1/2“ squares, eight 5“ bricks, four 10“ rectangles.
• Strip piecing: sew strips lengthwise and each adds 2“ of finished height. A “jelly roll race” quilt from one 40-strip roll comes out roughly 50“ × 64“: the roll’s ~1,600 running inches, folded into 50“-wide rows, gives 32 rows × 2“.

Jelly rolls also make binding: one strip binds about 40 running inches, so a 240“-perimeter lap quilt takes 6 strips.

Layer cake math

10“ squares finishing 9 1/2“ cover ground fast: a 6 × 7 grid (42 squares — exactly one cake) makes a 57“ × 66 1/2“ lap quilt. Layer cakes are also the efficient source for big half-square triangles: two 10“ squares, the two-at-a-time method, yields two HSTs finishing about 8 3/4“.

The honest caveats

• Don’t prewash precuts — they’re cut to size before shrinkage and the edges fray badly.
• Pinked edges: measure from the outer points; that’s the true cut size.
• “~42 per pack” varies by manufacturer; check the sleeve before the math depends on all 42.

The free precut calculator runs these numbers for any quilt size and precut type — squares needed, packs to buy, and the leftover count.

Working specifically in fat quarters? The fat quarter yardage chart covers what one is worth in yards, how many make a yard, and the squares you get from one at every cut size.