Question 1

What is the difference between FINISHED, UNFINISHED, and CUT size?

Accepted Answer

FINISHED size is the size of the unit AFTER it is sewn into the quilt — what you see on the quilt top, no seam allowance. UNFINISHED (unit) size is the size before it is sewn in, including the ¼″ seam allowance on every edge, so unfinished = finished + ½″. CUT size is the size of the starting fabric squares you cut before sewing. Each construction method needs a different cut size to land on the same finished size.

Question 2

Why does the traditional method add ⅞″?

Accepted Answer

For the traditional (2-at-a-time) method you sew ¼″ on each side of a diagonal and cut on the diagonal. The ⅞″ added to the finished size breaks down as ½″ (two ¼″ seam allowances for the two legs that get sewn into the quilt) plus ⅜″ for the diagonal seam itself. So starting square = finished + ⅞″, and the unfinished HST = finished + ½″. This is the universally taught standard.

Question 3

Why does the 4-at-a-time method use a different formula and a bias warning?

Accepted Answer

The 4-at-a-time method sews around the perimeter of two stacked squares and cuts on BOTH diagonals. Geometrically each HST's unfinished size = starting square ÷ √2, so the exact starting square = (finished + ½″) × √2 (≈ × 1.414). The two straight legs of each HST end up on the bias of the fabric, so they stretch and distort easily — handle them gently and starch helps. Because of this, many quilters cut oversized (divide the unfinished size by 0.64) and trim down; that trim allowance is NOT added automatically here.

Question 4

Why does the 8-at-a-time method double the ⅞″ formula?

Accepted Answer

The 8-at-a-time (Magic 8) method cuts the large square into a 2×2 grid, then treats each sub-square like a traditional 2-at-a-time square. So the exact starting square = 2 × (finished + ⅞″), yielding 8 HSTs with no bias-edge problem. A popular variant adds 1″ instead of ⅞″ before doubling (2 × (finished + 1″)) to leave trim room — that extra ⅛″ per HST is a trim allowance and is NOT included by default.

Question 5

How is the yardage calculated?

Accepted Answer

Both fabrics need the same number of starting squares. Squares are laid out across the fabric width: squares across = floor(fabric width ÷ square size); rows = ceil(squares needed ÷ squares across); linear inches = rows × square size; yards = linear inches ÷ 36, rounded up to the next 1/8 yard. The yardage shown is PER FABRIC — you need this much of each of the two fabrics.

Half-Square Triangle (HST) Calculator — Cut Sizes & Yardage

What this calculator determines

Related calculators

Examples

20 HSTs, 3″ finished, Traditional

20 HSTs, 3″ finished, 4-at-a-time

20 HSTs, 3″ finished, 8-at-a-time

Assumptions & Conventions

Frequently asked questions