Question 1

What is the difference between side and corner setting triangles?

Accepted Answer

SIDE triangles fill the long slanted edges of an on-point quilt (between the corners). They are cut as quarter-square triangles — a square cut on BOTH diagonals to make four triangles — so the straight of grain lands on the long outer edge and the quilt edge will not stretch. CORNER triangles fill the four corners and are cut as half-square triangles — a square cut on ONE diagonal to make two triangles — so the straight of grain lands on the two outer legs (the quilt edges). Using the wrong cut puts bias on the outside edge and the quilt will stretch.

Question 2

Why is the side-triangle square based on the block diagonal (√2)?

Accepted Answer

A block set on point presents its diagonal (finished block × √2) to the edge of the quilt. The side triangle's long edge must match that diagonal, so the square to cut = (finished block × √2) + 1¼″, rounded up to the next 1/8″. The 1¼″ covers the seam allowances for a quarter-square triangle.

Question 3

Why is the corner-triangle square (finished block ÷ √2) + ⅞″?

Accepted Answer

A corner triangle is a quarter of a block: its HYPOTENUSE is sewn to one side of the corner block (= finished block size) and its two legs — which become the quilt edges — measure finished block ÷ √2. So the square to cut = (finished block ÷ √2) + ⅞″, rounded up to the next 1/8″. The ⅞″ is the half-square-triangle seam allowance. Cut the square on ONE diagonal for two corner triangles, putting straight grain on both outer legs.

Question 4

How many triangles do I need?

Accepted Answer

In an on-point setting, "blocks across" and "blocks down" count the main diagonal grid; the layout also contains (across − 1) × (down − 1) offset blocks. Corner triangles are always 4 (one per corner), cut from 2 squares. Side triangles = 2 × (blocks across − 1) + 2 × (blocks down − 1), and each side-triangle square yields 4 triangles, so squares = ceil(side count ÷ 4). Examples: 3×3 → 13 blocks, 8 side + 4 corner = 12 triangles; 4×5 → 32 blocks, 14 side + 4 corner = 18 triangles; 5×5 → 41 blocks, 16 side + 4 corner = 20 triangles. Each case exactly fills the quilt rectangle, since a side triangle is half a block and a corner triangle is a quarter of a block.

Question 5

How is the yardage calculated?

Accepted Answer

Squares are laid out across the fabric width: squares across = floor(fabric width ÷ square size); rows = ceil(squares needed ÷ across); yards = (rows × square size) ÷ 36, rounded up to the next 1/8 yard.

Setting Triangle Calculator — On-Point Side & Corner Triangles

What this calculator determines

Related calculators

Examples

5×5 on-point, 6″ blocks, Side triangles

5×5 on-point, 6″ blocks, Corner triangles

4×6 on-point, 8″ blocks, Side triangles

Assumptions & Conventions

Frequently asked questions