4 Inch Half Square Triangles: Cut Sizes for 2, 4 and 8 at a Time
For a 4 inch finished half square triangle, cut two 4 7/8 inch squares and use the two-at-a-time method, or cut 5 inch squares and trim. Before it is sewn into a block, the HST should measure 4 1/2 x 4 1/2 inches. Eight at a time uses 9 3/4 inch squares.
A half square triangle, or HST, is a square made of two right triangles sewn together along the diagonal. To finish at 4 inches, cut two 4 7/8 inch squares and sew them the two-at-a-time way. It is the most common unit in quilting, used in pinwheels, sawtooth stars, bear paws and chevrons, and every cut size for 4 inch units is below.
What size do you cut squares for 4 inch HSTs by each method?
"Exact" sizes give the finished HST with no trimming, if your seams are perfect. "Trim" sizes run a little larger so you can square each unit to 4 1/2 inches afterward, which most quilters prefer. For a size this chart does not list, use the half square triangle calculator.
| Method | Exact cut | Cut to trim | HSTs per pair | Yards of each fabric for 100 |
|---|---|---|---|---|
| Two at a time (draw a diagonal, sew both sides) | 4 7/8 in | 5 in | 2 | 1 yd |
| Four at a time (sew around the square) | 7 in | 7 1/4 in | 4 | 1 1/8 yd |
| Eight at a time (magic eight) | 9 3/4 in | 10 in | 8 | 1 1/8 yd |
The table above compares the three ways to cut 4 inch HSTs: two at a time needs 4 7/8 inch squares, while the magic eight method gets eight units from a pair of 9 3/4 inch squares and uses 1 1/8 yards of each fabric for 100 of them.
Trim to the unfinished size
Every 4 inch HST should be squared to 4 1/2 inches. Line up the 45 degree line of a square ruler with the seam, trim two sides, rotate, and trim the other two.
Which blocks use 4 inch half square triangles?
HSTs fit block grids exactly, so multiply the finished HST size by the number of units across the block. Sizes that have their own cutting chart are linked in the table.
| Grid | Finished block | Example blocks |
|---|---|---|
| 2 x 2 | 8 in | Pinwheel, broken dishes |
| 3 x 3 | 12 in | Shoo fly, churn dash variation |
| 4 x 4 | 16 in | Sawtooth star, flying geese star, Ohio star variation |
| 5 x 5 | 20 in | Bear paw variation, 25 patch stars |
| 6 x 6 | 24 in | Large star medallions, pieced medallions |
Of those five grids, 5 land on a standard block size with its own chart, starting with a 2 x 2 grid that finishes at 8 inches. The same 4 inch height also gives 4 x 8 inch flying geese, the unit that turns a plain HST block into a sawtooth star.
Can you cut 4 inch HSTs from precut fabric?
2 of the three common precut squares are large enough to make 4 inch HSTs two at a time, which needs 4 7/8 inch squares.
| Precut | Two at a time | Eight at a time |
|---|---|---|
| Mini charm (2 1/2 in) | Too small | Too small |
| Charm pack (5 in) | Yes | Too small |
| Layer cake (10 in) | Yes | Yes |
If 4 inch units are not the right scale for your block, the neighbouring charts do the same math one step up or down. 3 1/2 inch HSTs cut from 4 3/8 inch squares put more piecing into the same space. 4 1/2 inch HSTs cut from 5 3/8 inch squares cover the quilt faster with fewer seams.
Tools for accurate 4 inch HSTs
Bloc Loc Half-Square Triangle Ruler Set #2
Under $100A slotted ruler that locks onto the seam so every HST trims to size with sharp points.
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Olfa 17 x 17 Rotating Cutting Mat RM-17S
Under $250Turn the mat, not the HST, when trimming all four sides of dozens of units.
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Olfa 45mm Quick-Change Rotary Cutter RTY-2/C
Under $25A precise 45 mm cutter for trimming small units cleanly.
Check pricePrices change often, so confirm on Amazon. As an Amazon Associate we earn from qualifying purchases.
Frequently asked questions
What size square do I cut for a 4 inch half square triangle?
For the two-at-a-time method, add 7/8 inch to the finished size and cut 4 7/8 inch squares. Many quilters add a full inch instead, cutting 5 inch squares and then trimming each unit back to 4 1/2 inches for sharp points. Trimming wastes a little fabric but forgives a slightly wobbly seam.
What size is a 4 inch HST before it is sewn into a block?
The unfinished size is 4 1/2 x 4 1/2 inches, which is the 4 inch finished size plus a 1/4 inch seam allowance on each side. Square every unit to 4 1/2 inches with a small square ruler, lining the ruler's diagonal up with the seam, before you sew it to anything else.
How do I make eight 4 inch HSTs at once?
Cut two 9 3/4 inch squares, or 10 inches if you plan to trim. Put them right sides together, draw both diagonals, and sew 1/4 inch out from each side of both lines. Cut vertically and horizontally through the center, then along both drawn lines, and press open. That gives eight HSTs to trim to 4 1/2 inches.
Can I make 4 inch HSTs from a charm pack?
Yes. A 5 inch charm square is big enough for two-at-a-time HSTs that finish at 4 inches, since you only need 4 7/8 inch squares. Pair a light charm with a dark one, sew on both sides of the diagonal, and cut them apart. A 42 piece charm pack makes up to 42 HSTs that way when it is paired with 21 contrasting squares.
How much fabric do I need for 100 4 inch HSTs?
Two at a time with trimming takes 50 squares of each of two fabrics cut at 5 inches, which is 1 yards of each from 42 inch cotton. Eight at a time cuts faster: 13 squares of each fabric at 10 inches, or 1 1/8 yards of each.
How we research: we compare manufacturer specifications, owner manuals, published standards and recurring themes in verified owner reviews. We do not test products in person, and this is researched guidance, not professional advice. Follow your machine manual and pattern, and take care with rotary cutters, hot irons and needles. As an Amazon Associate we earn from qualifying purchases.
