Sister sizing is a really useful tool in bra-making. It’s the principle that some bra sizes share cup sizes, but have different band sizes, which basically means that you can create a new bra size, just by changing the length of your band. It’s a bit complicated to explain all in words so here is a bra chart:

This is the underwire sizing guide for full size range of the Black Beauty Bra. You can see that size 40B, 38C, 36D, 34DD, 32E, 30F & 28FF all use a size 40 underwire. They are all sister sizes- sizes that have the same cup, but different band lengths. That means that you can take any one of these sizes, and create one of its sister sizes by modifying the band length of the pattern.

I find that it’s easiest to think of sister sizes as sizes that all share an underwire size. So you can see from this chart that a size 34B, and a size 30 D are sister sizes, and a size 38DD and a 34F are sister sizes… etc

This also means that you can make sizes that are completely outside the range of the pattern, like shown in this chart:

You can see from this that if you had a size 40D pattern, you could make it into a size 42C pattern. Or if you had a 30G, you could modify it to be a 28GG. It really expands your size range.

It’s also really great if you fall between the two size ranges of my pattern. Say you measure to a 34DD, make up the pattern and realize that you want one cup size bigger – 34E, but that’s not available in your pattern! No worries, you can take the size 36DD, decrease the band size by one size and voila – 34E!

Then comes the question of how to make this alteration- how much do you remove or add from the back band for each bra size difference? Well I wanted to make that very simple for you, so I made a print out!

This print out is designed to work with my Black Beauty Bra pattern, and may not work for all bra patterns.

If you are decreasing your band size say from a 34DD to a 32E, you can print out the page here.

If you are increasing your band size, say from a 34DD to a 36D, you can print out the page here

Be sure to use 100% scaling when you print.

You can also watch my YouTube tutorial here

To Decrease Your Band Size

If you are decreasing your band size, you will take your back band piece, cut it in half (mark half by fold in your pattern piece in two) and mark a horizontal line to ensure your pattern pieces stay aligned. I call the two half ‘Center Back Band’ and ‘Outer Back Band’

Using the print-out, place your center back and along the indicated line, making sure that you line up your horizontal line too.

Then place your Outer Back band along one of the Band Size lines, overlapping your center back band, and keeping your pattern pieces aligned with the horizontal line. In this diagram I show decreasing by 2 band sizes (for example a 32 to a 28), but you should use whichever line works for your modification. Then you re-draw the top and bottom lines, keeping the side seam and hook and eye edge unchanged. Voila! Your new piece, and your new sister size!

To Increase Your Band Size

This alteration is very similar to the previous. You start by cutting your back band into two pieces and marking a horizontal line.

Then, using the print-out for increasing back band size, place your Center Back Band piece on the indicated line (aligning the horizontal lines) and place your Outer Back Band piece on the Band Line that suits the number of band size you wish to increase. In this example I show 2 sizes. Then you re-draw the top and bottom line of the pieces, keeping the side seam and hook and eye edge in-tact.

It’s really as simple as that!

I know that sister sizes really blew my mind when I first learned out them, and really opened up the possibilities of pattern sizing!

I’d love to know if you use my print-outs and if you find this tutorial helpful : )