Quilt Block Calculator

Planning Your Quilt Block Layout

Every quilt starts with a block layout — how many blocks across, how many down, and what size each block finishes at. These three numbers determine the finished quilt dimensions and drive every other calculation: fabric yardage, backing size, binding length, and border requirements.

The key distinction in quilting is between finished and unfinished block size. A finished block measures from seam to seam after it has been sewn into the quilt top. An unfinished block — sometimes called the cut block — includes the quarter-inch SA on all four sides. A 12-inch finished block is cut at 12.5 inches. Enter the finished size in this calculator; the seam allowance is added automatically.

To plan from a target quilt size, work backwards. A twin bed is approximately 68 by 86 inches. Without sashing, 12-inch blocks need 6 across and 7 down (72 by 84 inches) — close but not exact. Adding 2-inch sashing between blocks changes the maths: five 12-inch blocks across with four 2-inch sashing strips gives 68 inches, and seven blocks down with six sashing strips gives 96 inches. The quilt sizes chart lists standard bed dimensions to help you target the right grid.

How Block Fabric Requirements Add Up

Once you know the total block count, the calculator works out how many blocks fit across a single WOF row. Quilting cotton gives you 42 inches of usable width after removing selvages from a standard 44 to 45 inch bolt. A 12.5-inch cut block fits three times across 42 inches (3 times 12.5 equals 37.5 inches), leaving 4.5 inches of waste per row.

The fabric yardage is calculated in rows, not individual blocks. Each WOF row is one cut-block height tall. For 35 blocks at 3 per row, you need 12 rows — 12 times 12.5 inches equals 150 inches of fabric, or about 4.17 yards. The calculator rounds this up to the nearest eighth yard for your purchase amount.

This per-row approach matches how you would actually cut at the table: rotary-cut a 12.5-inch strip across the full WOF, then sub-cut that strip into individual blocks. Understanding this sequence helps you plan your cutting order and estimate waste.

Adding Sashing to a Block Layout

Sashing — the strips of fabric between blocks — changes both the quilt dimensions and the fabric requirements. Even narrow sashing adds to the finished size. A 5 by 7 grid of 12-inch blocks without sashing makes a 60 by 84 inch quilt. Add 2-inch sashing and the same grid becomes 68 by 96 inches, eight inches wider and twelve inches longer.

This calculator treats sashing fabric as a separate requirement from block fabric. You will see two purchase amounts: one for blocks and one for sashing. This separation is practical because most quilters choose a different fabric (often a solid or tone-on-tone print) for sashing than for blocks.

The sashing calculation accounts for two strip types.

• Vertical sashing strips sit between blocks within each row. Their length matches the cut block size (finished block plus seam allowances). For a 5 by 7 grid, that is 7 rows times 4 gaps per row, giving 28 vertical strips.
• Horizontal sashing strips span the full width of the quilt between rows. They must be pieced from WOF cuts if the quilt is wider than 42 inches. For a 5 by 7 grid, that is 6 horizontal strips.

If you want cornerstones at the sashing intersections, the sashing calculator handles that geometry separately, including cornerstone fabric from a contrasting material.

Matching Blocks to Standard Bed Sizes

Most quilters design for a specific bed. The block count, block size, and sashing width together must produce a quilt that fits. Standard US mattress tops are approximately these dimensions, with finished quilt sizes including typical bed drop.

• Crib: 28 by 52 inch mattress (finished quilt roughly 36 by 52 inches with borders)
• Twin: 38 by 75 inch mattress (finished quilt roughly 68 by 86 inches with drop)
• Full or Double: 54 by 75 inch mattress (finished quilt roughly 78 by 86 inches)
• Queen: 60 by 80 inch mattress (finished quilt roughly 86 by 92 inches)
• King: 76 by 80 inch mattress (finished quilt roughly 100 by 92 inches)

The finished quilt sizes above include bed drop — the overhang on sides and foot. A quilt that sits on top of the mattress without drop uses the mattress dimensions plus borders. A quilt with generous drop adds 12 to 15 inches on each side and the foot.

When your block grid does not match the bed size exactly, borders can bridge the gap. A quilt centre of 60 by 72 inches with a 4-inch border becomes 68 by 80 inches — close to a twin. Plan the borders alongside the blocks to avoid ending up with a size that fits no bed.

Limitations of the Block Fabric Estimate

This calculator assumes all blocks use the same fabric — a single-colour estimate. Most quilt patterns use multiple fabrics per block: a nine-patch uses at least two colours, a log cabin uses five or more. Divide the total block count by the number of blocks per colour, then run the calculator for each colour separately.

For example, a 30-block quilt using two alternating colours needs 15 blocks of each. Run the calculator twice — once for 15 blocks in colour A, once for 15 in colour B — to get separate yardage estimates. The general fabric yardage calculator can help with non-standard piece shapes like triangles or rectangles that do not fit a square block model.

The calculator also does not account for fussy cutting — selectively cutting motifs from printed fabric. Fussy cutting increases waste because you skip sections of fabric to align the print. Add 25 to 50 percent extra fabric when fussy cutting. For simpler quilt construction without individual blocks, the strip piecing calculator handles strip-based designs, and the backing calculator estimates backing fabric for the finished quilt top.