How to Lengthen or Shorten a One Piece Swimsuit Sewing Pattern

One of the most common adjustments in a one piece swimsuit is lengthening or shortening the torso to fit your unique body!

Typically ready-to-wear swimsuits do not work well for those with long torsos, and even ones labeled for long torsos do not provide enough length. This is super common. Sewing your own swimwear is by far the best way to overcome this common issue, and the adjustment is easier than you think. Let’s get started!

In this example we will be showing the Edgewater Avenue Poppy One Piece, but this method can apply to any one piece sewing pattern. You can also use this method to lengthen/shorten a bikini top or bottom- the same principles will apply.

Do you need to lengthen/shorten the pattern?

First we need to gather some measurements. Take out a soft tape measure and measure your vertical trunk. Begin at mid shoulder, bring the tape down through your legs and then back up to mid shoulder. Your tape measure is likely too short, so you may need to do two measurements and add them together. When measuring, try to keep the tape flush with your skin, as this is where the swimsuit will be sitting.

Compare your vertical trunk measurement with the “torso” measurement on the size chart for the size you plan to make. If your torso measurement lands in a different size than for your bust/waist/hip (or if it’s completely off the size chart), it’s a good idea to make a length adjustment to achieve the intended fit.

How to determine the amount to lengthen/shorten by

Here is where you can either get very simple or very complex, depending on how much time you really want to spend doing this.

Easy Method

Say you’re making an XS, for which the torso length is 54-56” (it is safe to split the difference and use 55”). Your torso measures 60”, so it is 5” longer than the pattern’s torso. You can lengthen the pattern by a total of 5” and it will get you where you need to be.

If you come up with a negative number when subtracting, that means you will be shortening.

However, this doesn’t take into account vertical negative ease so it isn’t as precise as the next method. Spoiler alert: when I use the accurate method with these same measurements, the difference is just ¼”. Do with that what you will… It is more accurate but the easy method is fine in most cases. If you want to understand the mechanics behind it all, keep reading!

Accurate method

Some one piece patterns have vertical negative ease, which means that the length of the one piece is actually shorter than your torso. It is meant to stretch vertically to fit your torso. This is the same concept as horizontal negative ease, in which case the suit stretches around/horizontally to fit your circumference.

Not all one pieces have vertical negative ease however, in fact, many have zero! You will typically see vertical negative ease being anywhere from 0%-8%. Meaning, the length of the one piece is smaller than your body length measurement by the determined percentage.

With this in mind, if your torso measures 60”, due to negative ease you may not necessarily want to draft for a 60” torso unless you plan to have 0% vertical negative ease. If you plan to have, say, 5% vertical negative ease, you’re essentially drafting for a 57” torso (5% of 60” = 3”, take that away from 60” ((or the quick and dirty way is 60” x 95%))).

We can apply this same logic to the size XS torso measurement- in a sense, it is not drafted as a 55” torso, instead it comes out to a 52.25” torso when you consider the negative ease.

So now if you take the 57” adjusted torso length you plan to draft for, and subtract the adjusted size XS 52.25”, instead of a 5” difference we actually get 4.75”.

All that for just ¼”, yes… That’s why I said it’s ok to skip the rest of this section in most cases! For many home sewers, ¼” difference is within a reasonable margin of error in the cutting and sewing process, especially since it gets divided between front and back panels, so it’s actually a ⅛” difference on each. I know I’ve accidentally shaved off more than that before.

How much vertical negative ease should there be?

Also note, 5% vertical negative ease is just an example. The Poppy One Piece is right around 5%, but most of my one piece patterns have less than that. The amount of negative ease is largely up to personal preference, but if you go past around 7-8% it may be uncomfortable. I think 3% is a good amount of vertical negative ease to start with!

Coming up with the final adjustment number

Now coming back to our adjustment. Find the total length difference between your torso measurement and the pattern’s torso measurement, using either the easy method or the accurate method. Say we came up with 5” length difference. Divide that number by two, since we will make the adjustment evenly on the front and the back panels- 2.5”. Now we know we will adjust the front by 2.5” and the back by 2.5”, for a grand total of 5”.

*Keep in mind, you may decide you do not want to divide the adjustment perfectly in half! If, say, you know you have more tummy on the front, you may want to do something like 65% of the length adjustment in the front, and the remaining 35% on the back. This is especially important to consider for extended sizes!

Pattern adjusting

Begin by printing out the pattern for your chosen size. If you’re blending sizes at the bust/waist/hips, do that first before making the torso adjustment- we’ll be able to smoothly blend the side seams again when we’re done.

Let’s start with the front panel. We need to make horizontal “slash lines” where we will either spread the pattern to lengthen it, or overlap the pattern to shorten it. You can make just one slash line right around the waistline, but it is better to make more than that so the length will be more evenly distributed instead of just having a longer waist.

If you prefer to keep things simple, you can absolutely just do one slash line at the waist, especially if your adjustment is around 1” or less. \

I like to make one at the approximate waist, one along the leg line, and one at the approximate bust line. If you’re adjusting a darted piece, you can place the slash line above or below the dart to avoid having to mess with it.

This is also an area where you can be super technical or not. Slash lines in different places will make that particular area “grow”. For instance, if you have a large bust, you need the extra length at the actual bust, so it would make sense to put your slash lines at or near the bust line.

Maybe you don’t have a large bust, so instead you choose to put a slash line at the shoulder straps instead of at the bust. This will lengthen the straps but it won’t lengthen the bust area.

Now cut the pattern along your slash lines. If you’re lengthening the one piece, get out some spare paper and tape for the next step.

Let’s continue with the 5” total length adjustment. We will need to add 2.5” length to the front and 2.5” to the back. Since we have 3 slash lines, we will divide that into 3, so 0.83” per slash line.

*side note, that awful measurement is exactly why I prefer to work in centimeters & millimeters. 2.1cm is so much easier to measure out than 0.83”)

Since we are lengthening, spread out each pattern section line by 0.83” each and place a piece of paper under the areas where we spread. If we were shortening, we would overlap the pattern pieces. Tape the spread out pattern to the paper below.

The last step is blending our seams back out. Start from the original pattern seams and then smooth out as needed to connect the new lengths added. Wait to cut out the new pattern!

We will repeat this same process with the back piece. Again you are welcome to do 1, 2 or even more than 3 slash lines. Here we will do 3- approximately at the hips, the waist, and somewhere at or just below the straps.

Again, spread or overlap your pattern as needed and tape the new pattern to a piece of paper. Since we determined we’re doing 2.5” length increase on the back piece as well, we’ll again spread our slash lines each by 0.83”.

Blend the lines once more.

Check your side seams to ensure the front and back lengths are the same, called “truing”. You can do this by “walking the pattern” pieces at the side seam.

Then, cut your new pattern pieces and you’re all done! You can follow the tutorial as usual. It wouldn’t hurt to hold up the new paper pattern pieces to your body to see how it compares. If it’s looking way too long or too short, you can go back and fix that now. Also make sure you like where, say, the bust is hitting or where the neckline is now. Just remember the paper pattern has seam allowance in it.

I know this tutorial got into the nitty gritty but I hope you could pick out the pieces you needed to help make an accurate and swift adjustment to your pattern!

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