Do you have a pattern with several half square triangles (HST)? Are you dreading the thought of cutting and then sewing all those small pieces? Wouldn’t it be great to do several at once?!
FOUR AT ONCE
First, the technique…then the math.
To do four at once, put 2 squares, right sides together, and sew a ¼” seam all around the outside. That’s right, not from point to point!

  

Then cut from point to point on the diagonals and voila, FOUR half square triangles ready to be pressed.
Be careful when pressing though because the outside edges are bias edges and can stretch out of shape.

    

Sounds great but what size do the two squares need to be?
If you need to end up with, say, four 3-1/2” half square triangle, you can’t use a 3-1/2” square to start.
Here’s where the math kicks in.
Take your final desired size, in this example 3-1/2”, and divide by 0.64. This gives a measurement of 5.47”. I’d recommend taking that size up to at least 5-1/2” or even 5-3/4”. Better to have to trim down a bit than to be a touch small.
So in this example, you would take two 5-3/4” squares, right sides together, sew a ¼” seam all around the outside, then cut point to point on the diagonals, press carefully, trim to 3-1/2”.
If you really dislike math, we have a chart below that will help!

Here’s a method that’s twice as nice! Making EIGHT half square triangles at once.
Slightly different math: take the final size you want, say 2-1/2”. Add 7/8” to get 3-3/8”. Then double that number to get 6-3/4”. That is the size of squares you’ll need. (chart below)
Draw a pencil line corner to corner along both diagonals (or use Diagonal Seam Tape). Sew ¼” on either side of the diagonal lines.

Cut through the centre of the squares in both directions. Then cut along both diagonal lines between the stitching. Done!

Chart for “FOUR at once”: HST SIZE (CUT SQUARE)
2-1/2″ (4″)
3″ (4-3/4″)
3-1/2″ (5-1/2″)
4″ (6-1/2′)
4-1/2″ (7-1/4″)

Chart for “EIGHT at once”: HST SIZE (CUT SQUARE)
2-1/2″ (7″)
3″ (8″)
3-1/2″ (9″)
4″ (10″)
4-1/2″ (11″)