Use the finished HST size
Enter the size of the half-square triangle unit after it's sewn into the quilt, not the larger unfinished square you'll hold in your hand. The calculator uses the two-at-a-time method: two starting squares make two HST units. For an odd number of units, it rounds up to the next complete pair.
Start large enough to trim
For a 3-inch finished HST with ¼-inch seams, cut two 4-inch starting squares. Pair the fabrics, sew on both sides of the diagonal and cut the pair apart. Press the units, then trim each one to 3½ inches square. That remaining ½ inch is the seam allowance around the finished unit.
Count starting squares by fabric
If you need five HSTs, make three pairs from six starting squares. You'll get six HST units, including one spare. When each pair uses two different fabrics, cut three starting squares from each fabric. This plan doesn't assign colors or combine fabric requirements across several different HST sizes.
Sew one pair before cutting the rest
Check your seam width, pressing and trimming on a sample pair. The extra fabric is there to make squaring up easier, but the sewn result still depends on your technique. Other HST methods use different starting sizes and yields; use this plan only for the two-at-a-time oversize method.