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03 / Little triangles, big possibilities

Half-square triangle calculator

Less guessing, more patchwork. Choose a batch method and get starting squares, trim sizes and yardage for both colors.

Your quilt project

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Use your quilt top dimensions. This project follows you between calculators. All measurements are in inches.

Your quilt project is saved above. HST size and quantity are separate inputs for your pattern.

1

Build your batch

Extra is added to the starting square for 2- and 4-at-a-time; twice that extra is added for Magic 8. All starting cuts round up to ⅛ in. Trim to the unfinished size shown, whatever extra you choose.

Red: sew ¼ in · Blue dashed: cut

Schematic only, not to scale. Always sew with a ¼ in seam allowance. Make a test pair before cutting a whole project.

2

Your HST cutting plan

Starting squares · each color

4 in square

13 squares of A + 13 squares of B

Trim to · unfinished3 ½ inbefore sewing into the quilt
Finished in quilt3 inafter ¼ in seams on every side
Color A¼ yd
Color B¼ yd

0.25 yd EACH color, rounded up to ⅛ yd. Calculated fabric before purchase rounding: 0.2222 yd each.

Paired starting squares
13 pairs
HSTs produced
26 (1 spare)
Squares per WOF strip
10
Strips per color
2 × 4 in
Side offcut per full strip
0 in

Straight-row yardage layout. No credit for scraps, pre-cut squares or nested cutting.

Cut, stitch, press, trim

  1. From EACH color, cut 2 strips 4 in deep across 40 in usable WOF. Subcut 10 squares per full strip until you have 13 squares of color A and 13 of color B, each 4 × 4 in.
  2. Pair one A square with one B square, right sides together. Make 13 pairs.
  3. Mark one diagonal. Sew ¼ in on each side of that line, then cut on the diagonal. Each pair makes 2 HSTs.
  4. Press carefully and square each HST to 3 ½ × 3 ½ in UNFINISHED. Align your ruler’s 45° line with the seam before trimming.
  5. Use 25 HSTs. Spare HSTs: 1. A ¼ in assembly seam on all sides brings each unit to 3 in FINISHED.
QuiltCutCalc · quiltcutcalc.com

2-at-a-time HST cutting card

My quilt project · Quilt top 50 × 65 in

Buy ¼ yd (0.25 yd) EACH of A & B

13 squares per color at 4 in · 2 WOF strips per color · 40 in WOF · Extra cutting allowance ⅛ in

Starting 4 in → Trim 3 ½ in → Finished 3 in
Red: sew ¼ in · Blue dashed: cut
  1. From EACH color, cut 2 strips 4 in deep across 40 in usable WOF. Subcut 10 squares per full strip until you have 13 squares of color A and 13 of color B, each 4 × 4 in.
  2. Pair one A square with one B square, right sides together. Make 13 pairs.
  3. Mark one diagonal. Sew ¼ in on each side of that line, then cut on the diagonal. Each pair makes 2 HSTs.
  4. Press carefully and square each HST to 3 ½ × 3 ½ in UNFINISHED. Align your ruler’s 45° line with the seam before trimming.
  5. Use 25 HSTs. Spare HSTs: 1. A ¼ in assembly seam on all sides brings each unit to 3 in FINISHED.

Purchase sizes round up to ⅛ yard. Check fabric width and seam accuracy before cutting. Allow separately for shrinkage, print matching and mistakes.

What starting squares do I cut for 2, 4 or 8 HSTs?

For finished size f inches and ¼-inch seams, trim to f+½. Starting squares before rounding are f+⅞+extra (2), √2×(f+½)+½+extra (4), or 2×(f+⅞+extra) (8). Round cuts up to ⅛ inch. Pairs=ceil(target÷method); each color needs that many squares. Yardage uses whole WOF rows. Four-at-a-time leaves bias edges; test and trim before batching.

Extra is 0, ⅛ or ¼ inch; Magic 8 doubles it. WOF excludes selvages. Per-color yardage rounds up to ⅛ yard and gives no credit for scraps or nesting. Finished+1 for two-at-a-time and 2×(finished+1) for Magic 8 correspond to ⅛ inch extra, not every method or allowance choice.

  • Generations Quilt Patterns: conventional two-at-a-time allowance ↗
  • APQS: Magic 8 base formula (PDF) ↗
  • Sweet Red Poppy: four-at-a-time geometry and ¼-inch seams ↗
  • Suzy Quilts: padded cuts and trimming for each method ↗

Formula implementation and source pages checked 2026-10-14. This is a calculation and documentation check; sew a test sample before cutting a full batch.

The cutting allowance is explicit

For 2-at-a-time the starting base is finished + ⅞ inch; Magic 8 uses twice that base. Extra adds ⅛ or ¼ inch to the small starting size and twice that extra to the Magic 8 square. For 4-at-a-time it adds directly to the geometric starting square. Every starting cut rounds upward to ⅛ inch.

Batches round up; squares never split

Pairs = ceiling(target ÷ yield). A target of 25 HSTs needs 13 pairs for 2-at-a-time, 7 for 4-at-a-time or 4 for Magic 8. That produces 26, 28 or 32 units respectively. A spare unit is useful, but extra units do not increase your target quantity.

Yardage follows a real WOF layout

Squares per strip = floor(usable WOF ÷ starting size). Strips per color = ceiling(pairs ÷ squares per strip). Fabric length per color = strips × starting size. This uses complete crosswise rows, with no nesting or scrap credit. Make a test pair and confirm the trim size before cutting your entire batch.

A few helpful answers

Before you cut

What is the difference between finished, unfinished and starting size?+

Finished size is the visible HST size after it is sewn into the quilt. With quarter-inch assembly seams, unfinished trim size is finished plus half an inch. Starting size is the larger pair of fabric squares cut before diagonal piecing. These three measurements are not interchangeable.

Is finished size plus 1 inch always the exact starting size?+

No. For 2-at-a-time, finished plus 1 inch is a practical cut that includes room to square up when using the conventional seven-eighth-inch cutting allowance plus one eighth inch extra. Magic 8 doubles that padded size. Follow your pattern, sew a test pair and trim to the unfinished size.

What formula is used for 4-at-a-time HSTs?+

With quarter-inch perimeter seams, a starting square S produces an approximate unfinished side S divided by the square root of 2, minus the square root of 2 divided by 4. Inverting that gives S = square root of 2 × (finished + half an inch) + half an inch. Add the selected cutting buffer and round up to one eighth inch. Outer edges are on the bias and can stretch.

Why do I need the listed yardage for each color?+

Every pair contains one starting square from color A and one from color B. The number of pairs rounds up to meet the target batch yield. Each color then needs the full pair count, laid out as whole squares across usable WOF; the per-color amount must not be halved.

What if a starting square is wider than my fabric?+

The straight-row cutting layout cannot fit it. The calculator shows an error instead of inventing an area-based yardage estimate. Use wider fabric, reduce the HST size, or choose another method.

Method referencesSuzy Quilts: 2-at-a-time and Magic 8 tutorial ↗Sweet Red Poppy: 4-at-a-time geometry and cutting method ↗

Keep your project moving

01

Quilt backing

A backing that fits, seams included.

Let’s calculate
02

Quilt binding

Every strip and join, accounted for.

Let’s calculate
03

Half-square triangles

Two, four or eight at a time.

Let’s calculate
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Measure twice. Make something lovely.

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