Mary's Triangle Calculator

by Deborah Jones (

From any set of instructions for a quilt block containing Mary's Triangles, begin with the measurement for the small corner unit, referring to the following illustration:

You will make all other calculations for each set of two (2)Mary's Triangles units based on this measurement. For the examples, we use a 4" finished (4-1/2" raw) Mary's Triangle unit. In the examples, width = the short side measure; length = the long side measure.

For each TWO Mary's Triangle units:

  1. Cut 2 small squares for corner units at the measure prescribed for your block's desgin.

    Example: 2-1/2"   

  2. Cut 2 small rectangles that will form the small side traingles. Short side (width) measures the same as unit in step 1. Long side (length) is that measure plus 1 inch.

    Example: Width = 2-1/2" + 1" = 3-1/2"   

  3. Cut 1 large rectangle that will form the two large triangles, using the following formula:

    Width = Measurement from step 1 multiplied by 2 minus 1/2-inch seam allowance.
    Example: 2-1/2" X 2 - 1/2" = 4-1/2"

    Length = Width computed above for this large rectangle plus 1"
    Example: 4-1/2" + 1" = 5-1/2"

    Therefore, cut one rectangle at 4-1/2" X 5-1/2".