HOW DO I?
Figure the size of HST (Half Square Triangles) needed to make SquareinaSquare blocks with a uniquely sized center square
Triangle Calculators will figure the dimensions for the corner triangles needed for any size center square to make a SquareinaSquare block. While this example shows how to use the Triangle Designer to make HST, the Triangle calculators can be very helpful when figuring measurements for “funky” triangles cut from rectangles. 
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This square measures 6 1/2, which is its CUT size. Therefore, consider the seam allowance (1/4" each side) and subtract it (1/2" ) from the overall size (6 1/2") to find the FINISHED size of 6". If you prefer to work with cut size when using the Yardage Calculator make this small adjustment. 
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Choose half square triangles for this project to put the straight of grain on the outside of the block.
Quarter square triangles have stretchy, bias edges on the outside of the block. In contrast, Half square triangles avoid stretching because they place the straight grain on the outer edges. Learn more about the significance of fabric grain as you plan projects.
The calculator determines the size of the square you need to get your half square triangles. Follow these steps:
In this example, the calculator figures that we need to cut a 5 1/8" square. What will be the block's finished size? 
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Use the Yardage Calculator to figure the fabric needed for the Corner Triangle with a 4 1/4" finished size (or set the seam allowance to 0" and enter the cut size of 5 1/8") and the number of triangles needed (not the number of squares to cut!)
The QuiltSandwich app figures yardage and shows in a cutting diagram how to cut the number of strips to subcut into squares and then subcut into triangles. Quick Calc is set at quarterinch increments, just tap ‘Switch Calc’ in the upper left of the Yardage Calculator to go to Super Calc where any measurement and changes to allowances can be input. A good resource to find decimal values for fractions is Conversions in Notions. It has tables with decimal values for many unusual fractions like 1/8" (which is .125) 
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