When I was teaching full-time one thing I taught was how to draft patterns for clothing. It was the rare student who liked this topic. I think it was because most students didn't have an ability to visualise what they were making. This is a skill that isn't really taught in school. You might get a bit of it in art but nowhere is it actually 'taught' in the curriculum. You only have to look at all the Apps available on line that do this for you. They range from doing your make-up to designing houses and interior design and a whole range of other tasks that you have to 'visualise'. Some people are born with this ability but for the majority it has to be learnt. The other thing that most students found difficult was the math.
This brings me to the box bags I have been experimenting with. I found a pattern on the Bernina site "We all Sew". It is a great pattern and just fine for a knitting bag. Like a lot of things it was designed to sell the designers fabric and I wanted to use my own. So, I added some handles and did just that.
Then I got to thinking about making it in other sizes. Now there is a simple mathematical formula for working this out. For the length you add your desired finished length to your desired height plus 2 times your seam allowance. For your width use 2 times your desired width plus 2 times your desired height plus 2 times the seam allowance. That gives you the size of the fabric you need to start with.
EVERYONE JUST SWITCHED OFF, DIDN'T THEY?
I thought back to my classroom days and saw all these blank faces, with one or two exceptions.
The internet is a wonderful thing so I went looking for an App or a blog post that would do this for you. And I found a blog that had an automatic calculator on it's site. It was a blog written by Sarah Kirsten. She has a "Pouch Pattern Calculator" that you just fill in the measurements and it works it out for you.