How to Alter a Sewing Pattern for a Custom Fit

Alter the pattern before you cut, not the garment afterwards. Compare your bust, waist and hip measurements to the pattern’s chart, then adjust the tissue on the lengthen and shorten lines and at the bust, and make a muslin to check it before touching your real fabric.

Five alterations cover the large majority of fit problems:

• Lengthen or shorten at the printed lines on the bodice, sleeve or trouser leg. The most common alteration and the easiest.
• Full bust adjustment if you are larger than a B cup. Patterns are drafted for a B cup, so going up a whole size instead gives you a garment that is too wide at the shoulders and neck.
• Small bust adjustment for the opposite problem, where the bodice gapes at the armhole.
• Sway back adjustment if fabric pools horizontally just below your waist at the back.
• Broad shoulder adjustment if the shoulder seam pulls or sits inside your shoulder point.

Choose your size by high bust rather than full bust, then do a full bust adjustment. That single change fixes more fit problems than any other, because it keeps the shoulders and neckline correct while adding room only where you need it.

What You Need to Know First

I’ve been sewing for over twenty years, and if there’s one thing I wish someone had told me on day one, it’s this: no pattern fits perfectly straight out of the envelope. Not a single one. Patterns are drafted for a standardized body that, frankly, doesn’t exist in real life. We all have unique proportions, curves, and quirks that make us who we are, and our clothes should reflect that.

Changing a sleeve? See types of sleeves for which parts of a sleeve you can safely alter and which you cannot.

New to this topic? Start with our guide on How to Read a Sewing Pattern: Beginner’s Guide before diving in.

When I first started sewing, I’d follow the pattern exactly, sew the whole garment, try it on, and wonder why it pulled across my bust or gaped at my waist. I wasted fabric, time, and more than a little patience before I learned that pattern alteration is the bridge between a generic template and a garment that fits your body. Once I figured that out, everything changed.

In this article, I’ll walk you through the most common pattern alterations step by step. Whether you’re dealing with a full bust, a sway back, or shoulders that don’t match the pattern’s assumptions, you’ll find practical instructions here. Let’s get into it.

Why Sewing Patterns Need Altering

Commercial sewing patterns are designed using a set of standardized measurements. Pattern companies each have their own fit model, and that model’s proportions become the blueprint for every size in the range. The problem? Your body almost certainly differs from that fit model in at least a few ways.

Maybe your torso is longer than average. Maybe you carry more fullness through the bust than the pattern assumes for your hip size. Maybe your shoulders are broader or narrower than the standard. These differences are completely normal, and they’re exactly why alterations exist.

Before you alter anything, you need accurate body measurements. If you haven’t measured yourself recently, grab a friend and a flexible tape measure. I have a full guide on how to take body measurements for sewing that covers every measurement you’ll need.

You’ll also want to understand the pattern itself before you start cutting or slashing. If you’re newer to sewing, my article on how to read a sewing pattern will help you decode all those lines, symbols, and markings.

Ease and Why It Matters

One thing that trips people up is ease. Ease is the extra room built into a pattern beyond your exact body measurements. There are two types:

• Wearing ease allows you to move, breathe, and sit comfortably. Even a fitted garment includes some wearing ease.
• Design ease is extra fullness added for style. A boxy oversized top has a lot of design ease; a bodycon dress has almost none.

When comparing your measurements to the pattern’s finished measurements, keep ease in mind. You’re not trying to make the garment match your body exactly. You’re adjusting the pattern so the ease falls where it should.

Getting Started: Compare Your Measurements to the Pattern

1. Take your body measurements (bust, waist, hips, shoulder width, back length, front length, arm length).
2. Look at the pattern’s size chart on the envelope or instruction sheet. Choose your size based on the measurement closest to the garment’s primary fit area (bust for tops, waist or hips for bottoms).
3. Measure the actual pattern pieces. Subtract seam allowances, then compare those flat measurements to your body.
4. Note every place where your measurements differ from the pattern by more than half an inch.
5. Prioritize your alterations. Start with length adjustments, then move to width changes.

Choosing the right starting size makes everything easier. If you’re unsure which size to cut, check out my guide on picking the right sewing pattern size before you go further.

Common Pattern Alterations: Step by Step

Below are the alterations I perform most often. I’ve arranged them from simplest to more involved, so if you’re new to pattern work, start at the top and build your confidence before tackling the trickier ones.

Lengthening or Shortening a Pattern

This is the most straightforward alteration and one nearly everyone needs. Most patterns include a “lengthen/shorten here” line printed right on the tissue. That’s your guide.

To shorten:

1. Draw a line perpendicular to the grainline at the lengthen/shorten mark.
2. Measure how much you need to remove. Fold a horizontal tuck in the pattern equal to half that amount (since the fold removes double its depth).
3. Tape the fold in place and re-draw any cutting lines or darts that were disrupted by the tuck.
4. True up the side seams so they flow smoothly.

To lengthen:

1. Cut the pattern apart along the lengthen/shorten line.
2. Place a strip of paper underneath. Spread the pieces apart by the amount you need to add.
3. Tape everything in place, keeping the grainline straight.
4. Redraw the side seams and any other lines that need to reconnect smoothly.

I find that I shorten almost every bodice pattern by about three-quarters of an inch. My torso is on the shorter side, and without that adjustment, the waistline of every top and dress sits too low. Once you know your personal adjustment, you’ll make it almost automatically.

Full Bust Adjustment (FBA)

If your bust measurement is larger than what the pattern expects for your size (based on your high bust or upper chest), you need a full bust adjustment. Without it, you’ll get pulling across the chest, the front hemline will hike up, and buttons will gap. I dealt with all of those problems for years before learning this technique.

1. Determine how much extra room you need. Subtract the pattern’s bust measurement (for your size) from your actual bust measurement. This difference is your total adjustment.
2. On the front bodice piece, draw a line from the bust point to the nearest side seam, parallel to the waistline.
3. Draw a second line from the bust point straight up to the shoulder seam, passing through or near the dart point if there is one.
4. Draw a third line from the bust point down to the hem, parallel to the grainline.
5. Cut along the line from the side seam to the bust point, and then from the bust point up to the shoulder. Leave a tiny hinge (about 1/8 inch) at the shoulder so the piece stays connected.
6. Cut from the bust point down to the hem. Leave a hinge at the hem.
7. Swing the side section open at the bust level until you’ve added the needed width (half the total adjustment, since you’re working on one side). The piece will fan open at the bust while staying hinged at the shoulder and hem.
8. Tape paper underneath to fill the gaps. The dart will have grown larger, which is correct.
9. True up the side seam and the waistline. You may need to add a small dart at the waist to take up the extra length that the slash created below the bust.

The FBA can feel intimidating the first time. But once you’ve done it two or three times, it becomes second nature. If your garment has darts and you need a refresher on sewing them, I cover that in my how to sew darts guide.

Small Bust Adjustment (SBA)

The SBA is essentially the reverse of an FBA. If your bust is smaller than what the pattern assumes for your chosen size, you need to remove width at the bust without affecting the shoulders or waist.

1. Calculate how much to remove: pattern bust measurement minus your bust measurement.
2. Use the same slash lines as the FBA (side seam to bust point, bust point to shoulder, bust point to hem).
3. Instead of swinging the piece open, overlap the sections at the bust by the needed amount.
4. The dart will become smaller. If it becomes very small (under half an inch), you can eliminate it entirely and ease in the tiny bit of remaining fullness.
5. True up all seam lines.

Sway Back Adjustment

A sway back means your lower back curves inward more than the pattern assumes. The telltale sign is excess fabric that bunches or sags just below your waist at the center back. I see this fitting issue constantly, and it’s one of the simplest fixes once you know what to look for.

1. Put on your muslin (or pin the pattern pieces together and hold them up to your body). Pinch out the excess fabric at the center back, just below the waistline. Measure how much you’re removing.
2. On the back bodice pattern piece, draw a horizontal line about two inches above the waistline.
3. Fold a horizontal tuck at that line, tapering to nothing at the side seams. The tuck should equal half the amount you pinched out.
4. Tape the tuck in place and redraw the center back line and the waistline so they’re smooth.

This usually removes between a quarter inch and three-quarters of an inch. It’s a small change that makes a surprisingly big difference in how the back of a garment looks.

Broad Shoulder Adjustment

If your shoulders extend beyond the pattern’s shoulder line, seams will sit too far inward and the upper sleeve area will feel tight. Here’s how to add width at the shoulders without changing the neckline:

1. Measure from your neck point to your shoulder point on both sides. Compare to the pattern’s shoulder seam length (minus seam allowances).
2. Draw a vertical line on the front and back bodice pieces from the shoulder (about midway between the neck and the armhole) straight down to the hem. Draw a second line from that point along the shoulder to the armhole edge.
3. Cut along the shoulder line to the armhole and then down the vertical line, leaving a hinge at the hem.
4. Swing the shoulder/armhole section outward by the amount you need to add.
5. Tape paper behind the gap. Redraw the shoulder seam and armhole curve smoothly.
6. You’ll likely need to adjust the sleeve cap to match the new, slightly larger armhole. Measure both armholes (original and altered) and add the difference to the sleeve cap height.

For narrow shoulders, do the opposite: overlap instead of spreading, and reduce the sleeve cap accordingly.

Making a Muslin (Toile): Your Fitting Insurance

I can’t stress this enough: always make a muslin first, especially when you’re trying a new pattern or making significant alterations. A muslin is a test garment sewn from inexpensive fabric (muslin, old bedsheets, anything you don’t mind cutting up). Its only job is to check the fit before you cut into your good fabric.

1. Cut your altered pattern pieces from muslin or a similar weight fabric.
2. Sew the main seams with a long basting stitch. You don’t need to finish edges, insert zippers, or add facings. Just get the structure together.
3. Try it on. If possible, have someone help you evaluate the fit from behind.
4. Mark any areas that need further adjustment with a fabric pen directly on the muslin.
5. Take it off, measure the changes, and transfer those adjustments back to your paper pattern.
6. If you made big changes, sew a second muslin. Two muslins are cheaper than one ruined cut of expensive fabric.

When sewing your muslin together, a straight stitch with a longer stitch length works well since it’s easy to rip out. For a refresher on stitch options, see my guide to types of stitches on a sewing machine.

I keep a fitting journal where I note every alteration I make for each pattern. Over time, you’ll build a personal profile of adjustments. My notes say things like “Simplicity bodices: shorten 3/4 inch, FBA 1 inch, sway back 1/2 inch.” That saves me time every single project.

Comparison of Common Alterations

Tips for Successful Pattern Alterations

• Work on a copy. Trace your pattern onto Swedish tracing paper or lightweight interfacing before altering. Keep the original intact so you can use it again at a different size or for a different person.
• Use a clear ruler and a sharp pencil. Precision matters. A wobbly line on the pattern turns into a wobbly seam on the garment.
• Make one alteration at a time. If you change five things at once and the fit is off, you won’t know which change caused the problem.
• Always true your seams. After any slash-and-spread or tuck alteration, walk your seam lines to make sure adjoining pieces still match in length. Lay the front side seam against the back side seam, for example, and verify they’re the same length.
• Don’t skip the muslin. I know I already said this, but it bears repeating. Twenty minutes of basting a test garment can save you hours of frustration and an expensive piece of fabric.
• Record everything. Write your alterations on the pattern envelope or in a notebook. Future you will be grateful.

When to Combine Alterations

Most of us need more than one alteration per pattern. That’s perfectly fine, but the order matters. Here’s the sequence I follow:

1. Length adjustments first (lengthen or shorten the bodice, skirt, or sleeves).
2. Width adjustments second (FBA, SBA, adding or removing width at the hips).
3. Shape adjustments third (sway back, rounded back, forward shoulder).
4. Shoulder adjustments last (broad/narrow shoulders, sloped/square shoulders).

This order prevents earlier changes from distorting later ones. Length changes are the most foundational, so they go first. Shoulder adjustments are the most localized, so they go last.

My Personal Experience with Pattern Alteration

When I made my first “real” garment, a simple A-line skirt, back in the early 2000s, I picked my pattern size based on ready-to-wear sizing. That was mistake number one. The skirt was tight at the hips and too long, and the waistband sat about an inch below my actual waist. I was so discouraged I almost gave up.

A friend who’d been sewing since the 1970s sat me down and said, “Donna, the pattern isn’t wrong and you aren’t wrong. You just need to introduce yourselves properly.” She showed me how to measure my body, compare those numbers to the pattern, and make adjustments before cutting. That afternoon, she taught me how to do a basic lengthen/shorten alteration and a simple hip adjustment.

I made the skirt again with those changes, and it fit beautifully. I still remember the feeling of putting it on and having it sit exactly where it should. That was the moment sewing stopped being a frustrating hobby and started being something I genuinely loved.

Over the years, I’ve developed a standard set of alterations I apply to almost every pattern: shorten the bodice, FBA about one inch, sway back half an inch. Knowing my body’s tendencies means I spend less time fitting and more time sewing, which is the whole point.

Frequently Asked Questions

Do I need to alter every sewing pattern I use?

Almost always, yes. Even if you find a pattern company whose sizing runs close to your measurements, you’ll likely need at least minor adjustments. The good news is that once you learn your standard alterations, applying them becomes quick and routine. Many sewers develop a personal “recipe” of tweaks that they apply to every bodice or every pair of pants without even thinking about it.

Can I alter a pattern if it doesn’t have a “lengthen/shorten” line printed on it?

Absolutely. That printed line is just a suggestion for the best place to make the change. If your pattern doesn’t have one, find a spot between the armhole and the waist (for a bodice) or between the hip curve and the hem (for a skirt) that’s relatively free of darts, pockets, or other design details. Draw your own line perpendicular to the grainline and proceed the same way.

What’s the difference between a full bust adjustment and just going up a size?

Going up a size adds width everywhere: shoulders, waist, hips, and bust. If you only need extra room at the bust, sizing up will make the rest of the garment too big. An FBA adds width only where you need it, at the bust, while keeping the shoulders, neckline, and waist unchanged. It’s a targeted fix rather than a blanket change, and the fit difference is dramatic.

How do I know if I need a sway back adjustment?

Try on your muslin and look at yourself from the side in a mirror. If the fabric below the waistline at center back is baggy, wrinkled, or drooping, you likely have a sway back. Another clue: the back hemline of your tops tends to dip lower than the front. Have someone pinch out the excess fabric horizontally at the center back, about two inches below the waist seam. The amount they pinch out is roughly the amount you need to remove from the pattern.

Should I alter the pattern or adjust the garment after sewing?

Alter the pattern whenever possible. Adjusting a sewn garment is harder and often results in compromises. Seams can only be let out if there’s enough seam allowance, and taking in a seam at one point can create puckering or distortion at another. Pattern alteration addresses the fit issue at its source. That said, minor tweaks after sewing (taking in a side seam slightly, or adjusting a hemline) are perfectly fine and normal. Just don’t rely on post-construction fixes as your primary fitting method.

Final Thoughts

Pattern alteration isn’t a sign that something is wrong with your body or with the pattern. It’s simply the step that turns a mass-produced template into something made for you. Every experienced sewer I know alters their patterns, and every one of them will tell you the same thing: once you get comfortable with these techniques, your sewing satisfaction goes way up.

Start with the easy changes, like lengthening and shortening, and build up to the more involved ones as you gain confidence. Make your muslins, keep notes, and give yourself grace when things don’t work perfectly the first time. That’s how we all learn.

Happy sewing, friends. Your perfect fit is closer than you think.