Half Square Triangle (Two at a Time) Math

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Last week we talked about how to make half square triangle (HST) blocks one or two at a time. But how do you know what size square you need to cut?

Figuring out how much you need is often a struggle, but fortunately this is one of the easier formulas!

First, it’s important to understand the difference between finished and unfinished block sizes. Finished is referring to the measurement of the block after it’s sewn into the quilt. Unfinished is after the components of the block have been sewn together but before it is sewn into the quilt.

It’s all very confusing but it’s important to know.

For this calculation we start with the measurement of the finished block size and add ⅞”. Easy peasy right?

Let’s use an example. Say I’m making a quilt with a finished block size of 4″ (because I am!). That means I need to cut out squares that are at least 4 ⅞”.

I say at least because that will give you exactly the right size if you’re perfect and exact at sewing. I like to give myself a little wiggle room so I tend to add at least ⅛” but you can do more if you like. All you do is trim the block down to size. No one will ever know! I personally started with 5″ squares.

Don’t want to do the math? Lucky for you I’ve put together this handy printable with several common sizes already mathed out for you! I also included the formula if you have a size that’s not listed.

Happy sewing!

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