You can download the spreadsheet and save it for a later date if you wish - thank you Yvonne for this!
Equilateral triangles are those where the internal angles are all 60 degrees and the three sides are all the same length. Trigonometry gives equations for working out height when you know length and vice versa. When designing quilts these equations can be used to determine the width of your finished quilt and the number of triangles you need to cut. After making a quilt that turned out smaller than expected I worked out an equation and kept in on hand in OneNote but Yvonnes spreadsheet now makes this so much easier! And there are options for adding pieces to square your quilt or cut away at the ends.
I find it easiest now when working with triangles to determine the height first as it is a really simple calculation in comparison to the width. For equilateral triangles the rule of thumb is to add 3/4" to the desired finished height of your triangle. A 6" cut triangle gave me a 5 1/4" height per row and approximately 58" tall quilt.
Changing to 7" triangles would have given me almost a 69" quilt and a more usable width of nearly 58".