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Setting triangles calculator

square in square block The applet below will help you calculate the size of the triangles 1 - 4 in the block shown to the left.
You have to know the length of the sides of the starting square marked 0 (finished size). Type this number into the box to the left, and click on the Results button. The length of the shorter right angled sides of triangles 1 - 4 will show to the right. This is also the finished size of the squares you will need to cut to make the half square triangles.
Remember to add seam allowance before cutting, see below.

step by step setting triangles


When typing decimals, use . (point) instead of , (comma) or the applet will not work.

Java applet courtesy of Ellinor Førde. If you found it useful, please

Your results will most often contain numbers with decimals. For those of you who work with inches, you will want to convert the decimals to fractions.
Use the table below. If you do not find the exact decimals in the table, use the closest one above your number - then your triangles will not be too small. (Too big - you can always trim afterwards).

Converting decimals to fractions

Decimals .0625 .125 .1875 .25 .3125 .375 .4375 .5 .5625 .625 .6875 .75 .8125 .875 .9375 1.000
*/16'' 1/16" 2/16" 3/16" 4/16" 5/16" 6/16" 7/16" 8/16" 9/16" 10/16" 11/16" 12/16" 13/16" 14/16" 15/16" 16/16"
*/8''   1/8"   2/8"   3/8"   4/8"   5/8"   6/8"   7/8"   8/8"
*/4''       1/4"       2/4"       3/4"       4/4"
*/2''               1/2"               2/2"

Adding Seam Allowance

 cut half square triangles The calculations above are for the finished size of the half square triangles.
Before you cut the square, you must add to the measurements of the square (which was calculated above) so that you allow for seam allowance for both the resulting triangles.
How much to add depends on what seam allowance you use when sewing.

If your seam allowance is 1/4'', then you must add 7/8'' to your measurements before you cut the square.

When using metric measurements:
Add 1,7 cm if you use a seam allowance of 0,5 cm
Add 2,56 cm if you use a seam allowance of 0,75 cm

More than four rounds of triangles in the block?

Look at the results you got in your calculation above. Can you see a "pattern" in the numbers?
Note that the square for cutting triangles no 2 are the same size you started out with (center square), and then triangles 4 are the double of no 2.
Likewise, triangles no 3 are twice the size of triangles no 1. Then we may expect that a 5th row of triangles will be double the size of no 3. The 6th row will be double the size of no 4.... ...and so on.

(Results with decimals will not always show the exact double because some decimals have to be rounded off during calculation)

Setting a block on point

block on point If you just want to set a block on point, you need only the first set of triangles.
This will also be the size of the corner triangles in a quilt with blocks on point.
In both cases, remember to add seam allowance.

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