I had an email question from a lovely young woman who has started designing swimwear in ready to wear sizing. She's been studying everything she can get her hands on and is genuinely a very talented person. With guidance she's been trying to establish her sizing intervals and size table. She settled on a 2" sizing interval because "that's what everyone knows" rather than on the performance of the textile and it's ability to span the interval. She's been sampling her range and finding people right in between sizes are having problems finding something comfortable or stable. I love that she's spending the time to test everything and properly observe the results .... she's going to go a long way! So I asked for her sizing table and to see a few patterns. Here's what she sent me.
Extra Large: 42-44"
2X Large: 46-48"
It's not a very American looking table, but she's not in America, she's in Africa! The problem however with her 2" interval should be fairly obvious ... it's not 2" it's 4". Really her medium should say 33-37". This is something I'm seeing more and more ... a 4" interval being sold or talked about as a 2" interval. But ignoring the misunderstanding about intervals, why was she having problems in the middle of the gap between sizes?
So I want to talk about sizing overlap. When we create a range of ready to wear sizes, most still do typically think in terms of small, medium and large rather than actual numbers that directly relate to a body measurement. I've never much liked that system because what one designer arbitrarily calls a medium and that of another aren't always the same. And as much as you might like to say "hey a medium is X inches and I'm that size in every designer", you'd just be wrong. These sort of terms are not standard and do not reflect an across the board actual measurement .... if anything they are tabulated as a range (as she did above) ... and this is where the problem just starts to compound itself.
Let's say we're making a one piece swimsuit ... to fit appropriately our pattern maker knows they have to make something with around 12% negative ease. Sounds fair ... but what part of that range is the actual measurement? Let's say your arbitrary Large is the 100-110cm bust range (a 4" range like above) ... I know that's big but we're demonstrating here. Which number does the pattern maker use? 100? 105? 110? The pattern isn't a range, its a fixed size. Let's say they choose 105cm ... at 105cm and 12% negative ease, the pattern's bust would be 92.4cm ....so 92.4cm has to spread for 100-11cm ... do the math .... if the client has a 100cm bust then the garment suddenly drops to 7.6% negative ease and at 110cm they'd be wearing a garment that's 16% negative ease. See the problem? You've designed a range so wide (10cm/4") that the negative ease varies 8.4% across each size.
This same situation exists even with specific numbered sizes relating to measurements. For example I typically do 4cm increments of bust ... eg; 84cm, 88cm, 92cm, etc and I label the sizes with that actual real number. I still have a range of 4cm between each size though ... that's called the sizing interval ... and it's different to sizing overlap.
When I design swimwear I have to choose a workable sizing interval. In stretch, that interval is not determined by an apparently arbitrary amount (eg USA's 2" interval for bust). It's instead determined by the range of negative ease which is acceptable.... ie; if I want 12% negative ease then I might accept a range of 10%-14% .... but I might say 6% to 18% is too much.
Let's try an example again ... a 100cm bust at 12% negative ease is 88cm on the pattern ... on an 88cm pattern a 98cm bust is 10.3% negative ease and a 102cm bust is 13.7% negative ease (just within 10-14%). If we size our 102cm bust up a size (the 104cm size) we now have the same customer wearing 10% a negative ease. So our 10-14% requirement is met nicely by that 4cm grading interval ... there is the tiniest overlap between sizes based on that requirement. If we found 9-15% worked well for our fabric and we used the same patterns then we could have a bigger acceptable size interval. If we wanted our negative ease to always be between 11-13% then we would find ourselves with no overlap ... or a hole between sizes that didn't fit a certain number of people as well as we wanted it to. Knowing that range of negative ease is essential for calculating the grading interval between sizes. Of course that interval (because it's based on percentages) will change as size increases and it will change with different weights of fabrics. But the most important thing to remember is that the finished garment should "feel" the same tension to your client irrelevant of the style or fabric.
What should be apparent very early on here is that 4-5cm between sizes is about as far as you can go on swimwear with 12% negative ease and the fairly safe stability range of 2% either side. If you're dealing with tie strings and such then of course there's more room for a larger size interval because the garment is adjustable. But 4" is going to be a massive interval and have no sizing overlap ... meaning around 25% of your customers won't find an acceptable fit based on your size table.
Size overlap is important ... you need a small area between sizes in which a customer who falls between the sizes can acceptably fit in both the slightly smaller and slightly larger sizes. You can only do that by testing your fabric and working out the minimum and maximum negative ease that'll be stable and comfortable .... and only then determine your sizing interval.
But wait, there's more. Whatever sizing interval you choose, you probably don't want a different one for each garment or fabric ... you want one table for everything in your brand. So don't skimp out on the overlap early on .... make sure you have enough overlap to account for variation in styles and fabrics, for now and in the future.