A few weeks ago I mentioned I used Algebra to figure out the dimensions of the bags, totes, baskets, and boxes I design.

Today I’m demonstrating the calculations I use.

But first a little background….

Last month I said I was making a bag for my daughter-in-law’s school’s fall festival auction using these Premier Prints fabrics…

But I really messed up the pocket on the bag plus ran out of the gray thread that I was using. I was working on a very short timeframe so rather than getting the ripper out and then driving to Jo-ann’s which is 35 minutes away to get more thread, I pulled some other Premier Prints fabrics out of my stash and made this bag instead:

Then I put a little tutorial together that explained how I made the boxed corners for the bottom which gives the bag that nice structure.

And my daughter-in-law just texted me that the bag went for $50 at the auction!

I hate leaving anything unfinished before I move onto another project. The orange and gray fabric was sitting there nicely cut out and lined in fleece, it just needed a little TLC and I had already picked up more thread. So I decided to fix the pocket and create a new tote.

And here’s where the calculations come in….

I keep an Excel Spreadsheet on my computer with formulas in it just to figure out–quickly and easily–how to change the sizes on bags, totes, baskets and boxes.

My three variables are height (h), width (w), and depth (d).

I use 1/2″ seams on all my totes, bags, and boxes.

I want this tote to be 12 1/2″ in height (h), 13″ in width (w), and have a 4″ depth (d).

So I need a piece of fabric cut 35″ x 15 1/2″. And how did I know that?

To get the width, the calculation is ((2 * w) + (2 * d) + 1)

The 1 in the above formula is for the 1/2″ seam on the sides of the bag.

The desired width is 13″ and the depth is 4″ so I just fill in the variables.

(2 * 13) + (2 * 4) + 1 = 35

The height formula is (h + (1/2 * d) + 1)

Again, the 1 is for the seams on the top and bottom.

The height is 12 1/2″ and the depth is 4″

(12.5 + (.5 * 4) + 1) = 15.5

One last calculation allows me to figure out how to box the corners.

(d/2)

My depth on this bag is 4″

4/2 = 2″

Confusing? Not really. It’s easy-peasy. Especially if you put the formulas into Excel. But, I always liked Algebra–except those word problems.

And here’s the inside of the bag.

I don’t have any plans for this bag yet. Perhaps it will go into into my inventory so next time I get a last minute text that I need to make a bag–I’m ahead of the game!

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My brain is hurting so early ! (LOL – kidding)

Thanks for the formula! That’s what I always loved about algebra, because with formulas, you only have to plug in numbers. Getting ready, now, to calculate the measurements for a small purse.

I keep the formula in a spreadsheet and turn to that every time I design a new bag, tote, or box!

Thank you for this. I’m reteaching to myself to sew. I was injuring my brain trying to figure out how to calculate how to box corners for some boxes I’m going to make. I’m going to create a spreadsheet right now!

Glad you can use it!

HEy this is really useful thank you, would it be the same formula to use if I wanted to make a square box closed at the top with a zipper? So material forming a complete cube…. maths really wasn’t my strongest subject!

I think it would be. I haven’t made one with a zipper on top so I can’t really advise you on that one. I always liked math but I have this formula in an excel spreadsheet and just plug in the numbers whenever I need to make a new size!

Thanks a bunch for this information! Making 2 different sizes of fabric baskets for my daughters new boot bench. Was trying to calculate fabric needed for her to order. She wants me to make 3 of each, wish me luck!!

Hope the fabric baskets worked out, Jan! I have more sewing tutorials on my other blog, DesignsbyKTGreen.com. Thanks for reading and commenting.

Nice formula. You haven’t calculated / included the Handle fabric Consumption. But really useful.

Can you Teach me more in Calculating Fabric Consumption for Woven Cotton Madeups like Bags, Pillow Cases Etc…

Ramesh Kumar

India

I’m not sure what you need, Ramesh. I have many more tutorials on my other website, DesignsbyKTGreen.com. Including bags, pillowcases, etc. Thanks for reading and commenting.

Dear Spreadsheet Queen, There’s nothing I love more than an excuse to make a useful spreadsheet such as this. I’m so glad you provided the formulas for all three calculations required. I had been trying to work them out, via a bit of trial and error with fabric scraps, but was doing it in multiple steps versus one nice, beautiful, succinct formula such as yours. Brilliant work! Many, many thanks!!

So the next challenge is to reverse calculate the multiple dimension possibilities of a bag one can make with a fabric rectangle of known dimensions, like all those scraps in my closet! I’ll work on that later…..;-)

Dear Carol,

Thank you for reading my blog…and commenting!

I love numbers. Always did. Loved Algebra even though I was too lazy to study and get good grades. Still loved it. Love sewing even more so being able to use a little algebra in my favorite hobby is even more fun!

So happy you like my little formula and understand it. When I explained it to one person she gave me the “deer in the headlights” look! I just think it makes my sewing life a little easier!

I have been moving my tutorials over to my other blog DesignsbyKTGreen.com which is dedicated to sewing and other handiwork.

Kathy