15 Inch Quilt Block: Cut Sizes, Grids and How Many Blocks per Quilt
A 15 inch finished quilt block is cut 15 1/2 x 15 1/2 inches, because every edge carries a 1/4 inch seam allowance. It divides cleanly into 5 ruler-friendly grids, including a four patch of 7 1/2 inch units cut at 8 inches. A queen quilt needs 36 blocks with no sashing.
A 15 inch finished quilt block is cut 15 1/2 x 15 1/2 inches square, because each of its four edges carries a 1/4 inch seam allowance. Quilt block sizes are always given as the finished size, the part you still see once the block is sewn into the quilt, so a 15 inch block is pieced and trimmed to 15 1/2 inches and then loses 1/4 inch on every side to the seams that join it to its neighbors.
What size do you cut the pieces for a 15 inch block?
Most traditional blocks are built on a grid. Divide 15 inches by the number of grid units to get each unit's finished size, then add 1/2 inch for squares or 7/8 inch for two-at-a-time half square triangles. Grids marked "awkward" land between the eighth-inch marks on the ruler, and the quilt block size calculator handles any grid this chart does not list.
| Grid | Unit finishes at | Cut squares | Cut HST squares | Ruler friendly |
|---|---|---|---|---|
| Four patch (2 x 2) | 7 1/2 in | 8 in | 8 3/8 in | Yes |
| Nine patch (3 x 3) | 5 in | 5 1/2 in | 5 7/8 in | Yes |
| 4 x 4 grid | 3 3/4 in | 4 1/4 in | 4 5/8 in | Yes |
| 5 x 5 grid | 3 in | 3 1/2 in | 3 7/8 in | Yes |
| 6 x 6 grid | 2 1/2 in | 3 in | 3 3/8 in | Yes |
The table above shows every grid a 15 inch block divides into: 5 of the five are ruler friendly, the four patch grid gives 7 1/2 inch units cut at 8 inches, and a nine patch gives 5 inch units cut at 5 1/2 inches.
Those units are where this block meets the other cutting charts. Its 7 1/2 inch four patch unit is outside the charted HST sizes, so take the cut size from the HST column above. Drop to a nine patch and the units become 5 inch half square triangles. A sawtooth star at this size would need geese finishing at 3.75 inches, which is not one of the charted flying geese sizes.
How many 15 inch blocks do you need for each quilt size?
Blocks across and down are rounded to the nearest whole block, so the finished top lands close to the typical quilt size. Sashing is the strip of fabric between blocks, and with 2 inch finished sashing you need fewer blocks for the same quilt; the sashing and borders calculator sizes those strips. To check a count against your own layout, use the blocks needed calculator.
| Quilt | Typical size | No sashing | Top size | With 2 in sashing | Top size |
|---|---|---|---|---|---|
| Baby | 40 x 40 | 3 x 3 = 9 | 45 x 45 | 2 x 2 = 4 | 32 x 32 |
| Crib | 36 x 52 | 2 x 3 = 6 | 30 x 45 | 2 x 3 = 6 | 32 x 49 |
| Lap | 45 x 60 | 3 x 4 = 12 | 45 x 60 | 3 x 4 = 12 | 49 x 66 |
| Throw | 50 x 65 | 3 x 4 = 12 | 45 x 60 | 3 x 4 = 12 | 49 x 66 |
| Twin | 68 x 90 | 5 x 6 = 30 | 75 x 90 | 4 x 5 = 20 | 66 x 83 |
| Twin XL | 68 x 95 | 5 x 6 = 30 | 75 x 90 | 4 x 6 = 24 | 66 x 100 |
| Full / Double | 83 x 90 | 6 x 6 = 36 | 90 x 90 | 5 x 5 = 25 | 83 x 83 |
| Queen | 90 x 95 | 6 x 6 = 36 | 90 x 90 | 5 x 6 = 30 | 83 x 100 |
| King | 106 x 95 | 7 x 6 = 42 | 105 x 90 | 6 x 6 = 36 | 100 x 100 |
| California King | 102 x 99 | 7 x 7 = 49 | 105 x 105 | 6 x 6 = 36 | 100 x 100 |
The table above turns a 15 inch block into a block count for 10 standard quilt sizes: a 50 x 65 inch throw takes 12 blocks and a 90 x 95 inch queen takes 36. Once those blocks become a top, the quilt top yardage calculator converts the count into fabric, and a queen still needs queen quilt backing yardage plus enough queen quilt binding to cover its 370 inch perimeter.
Block size drives the workload more than any other choice: a queen takes 36 blocks at 15 inches. A 14 inch block would need 42. A 16 inch block would need 36. A 10 inch block would need 90.
Make a test block first
Sew one block and measure it before cutting fabric for 36. If it comes out 1/8 inch small, your seam allowance is slightly wide, and that error would add up to inches across a quilt. A scant 1/4 inch seam usually fixes it.
Tools for cutting and squaring 15 inch blocks
Olfa 60mm Ergonomic Squeeze-Handle Rotary Cutter RTY-3/DX
Under $50Cuts several layers of fabric at once, which speeds up cutting dozens of identical squares.
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Creative Grids 12-1/2 x 24-1/2 Rectangle Quilt Ruler
Under $100Long grippy ruler for cutting strips to the exact width before subcutting squares.
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Olfa 17 x 17 Rotating Cutting Mat RM-17S
Under $250A rotating mat lets you trim all four sides of a block without moving the fabric.
Check pricePrices change often, so confirm on Amazon. As an Amazon Associate we earn from qualifying purchases.
Frequently asked questions
What is the unfinished size of a 15 inch quilt block?
A 15 inch block is the finished size, measured after it is sewn into the quilt. Before that it measures 15 1/2 x 15 1/2 inches, because each edge carries a 1/4 inch seam allowance that disappears into the seams. Trim every block to exactly 15 1/2 inches with a square ruler before joining them, so the rows line up.
How many 15 inch blocks do I need for a queen quilt?
A 90 x 95 inch queen quilt takes 6 blocks across and 6 down, 36 blocks in total, for a finished top of 90 x 90 inches. Add 2 inch sashing between the blocks and the count drops to 30. A border reduces it further, so settle the layout before you piece every block.
What size squares do I cut for a 15 inch nine patch?
Each of the nine squares finishes at 5 inches, so cut them 5 1/2 x 5 1/2 inches. Sew three squares into each of three rows with 1/4 inch seams, press the seams in alternating directions so they nest, then join the rows. The finished block should measure 15 1/2 inches before it goes into the quilt.
What size half square triangles go in a 15 inch block?
In a four patch layout each half square triangle finishes at 7 1/2 inches, cut from 8 3/8 inch squares with the two-at-a-time method, or 8 1/2 inch squares if you would rather trim them down. In a nine patch layout each HST finishes at 5 inches, cut from 5 7/8 inch squares.
How much fabric do I need for 15 inch nine patch blocks?
For a throw of 12 blocks, a two-color nine patch with five dark and four light squares per block needs 1 3/8 yards of dark and 1 1/8 yards of light fabric, cutting 5 1/2 inch squares from 42 inch strips. A queen of 36 blocks needs 4 and 3 1/4 yards.
Is 15 inches a good quilt block size?
Most quilt patterns use blocks between 6 and 12 inches, so 15 inches counts as large and fast: a queen quilt needs only 36 blocks, and big prints get room to show. The trade-off is that intricate piecing repeats fewer times across the quilt. Large blocks also make long, heavy rows, where a machine with generous throat space is easier to work with.
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.
