How to alter bikini bottoms to fit your figure perfectly

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An ideal swimsuit is a rare find that almost every woman has encountered. It often happens that the top fits perfectly, but the bottoms either cut uncomfortably into the hips or slip out upon the first dip in the water. Don't rush to put the item on a high shelf or spend your budget on a new bikini. You can tailor the swimsuit to your figure and give it a second life yourself at home.

Altering swimsuit bottoms is much simpler than it seems at first glance. Regardless of your goal—whether you want to take in the swimsuit briefs a size smaller, delicately increase their volume with stylish side inserts, or create a trendy back gathering—you only need basic sewing skills and a little patience. In this practical guide, we will step-by-step cover all the intricacies of working with elastic biflex, both on a sewing machine and by hand.

Preparation: Tools, Materials, and Working with Biflex

A quality swimsuit alteration starts not with scissors, but with proper preparation. Biflex is a highly elastic synthetic material that behaves capriciously under the foot of a standard sewing machine. To prevent seams from bursting under tension and to stop the fabric from bunching up, it is important to select the right arsenal of tools in advance. Even if you plan to tailor the bottoms by hand, knowing the specifics of working with elastic knitwear will save you from typical beginner mistakes. Let's review the basic material requirements that will help maintain the factory look of the item and ensure comfort when wearing it.

Choosing Needles, Threads, and Stitches for Elastic Fabric

Working with biflex requires the correct selection of tools, otherwise, the seam will burst upon the first tension of the swimsuit.

For work, use only special needles marked Stretch or Super Stretch (size No. 70–80). Their rounded tip gently separates the fibers of the elastic fabric without damaging them and prevents skipped stitches.

Thread selection is also critical for the durability of the item:

For the sewing machine needle, use strong 100% polyester that is resistant to chlorine and saltwater.
For overlock sergers, use textured threads that provide softness and maximum seam stretch.

If you are sewing on a home machine, forget about a straight stitch. Use elastic stitches:

Narrow zigzag stitch (width 1.5 mm, stitch length 2 mm) for trimming pieces.
Three-step zigzag for securely attaching elastic bands.
A special knit stitch for high-stress areas.

Features of Working with Crotch Liners and Seam Allowances

The crotch liner is a piece that requires special attention when altering a swimsuit. It ensures hygiene and comfort, so its seams must be as flat and soft as possible. When reducing or changing the shape of the bottoms, try not to affect the central part of the liner. If cutting is unavoidable, be sure to double it with a special swimsuit lining fabric or a second layer of biflex.

When working with elastic materials, seam allowances should be minimal—usually 5–7 mm. This is enough for finishing with an overlock or zigzag stitch. However, if you plan to sew a latex elastic band along the edges of the cuts, allow for an allowance equal to the width of the band plus 2–3 mm for folding (totaling about 8–10 mm).

Life hack: To prevent the fabric layers from shifting when sewing the liner to the main pieces, temporarily secure them with water-soluble pencil glue or thin pins, inserting them strictly perpendicular to the seam.

How to Take in Swimsuit Bottoms on the Sides (Machine and By Hand)

If the bottoms from your favorite swimsuit have become too large or were originally too loose, the most reliable way to restore a perfect fit is to take them in on the sides. Adjusting the side seams allows you to delicately reduce the volume of the item while maintaining the correct position of the liner and overall symmetry. Depending on your experience and available tools, this task can be solved in two ways.

For those who own sewing equipment, a quick machine method is suitable, guaranteeing factory-grade strength and elasticity in the stitching. If you don't have a machine handy, a careful hand stitch will accomplish the task just as well, allowing you to fit the fabric precisely to your figure. Let's examine both options in detail so you can choose the method of restoration that is most comfortable for you.

Step-by-Step Alteration on the Side Seams Using a Sewing Machine

For a quality reduction of the bottoms on a sewing machine, it is important to maintain the elasticity of the seams so that the threads do not break under the tension of the biflex.

Step-by-step procedure:

Fitting and Marking. Turn the bottoms inside out, put them on, and secure the excess fabric on the sides with tailor's pins. Remove the item and draw new stitching lines with chalk, strictly maintaining symmetry on both sides.
Cutting. Unpick the factory side seams. Leave new seam allowances from the marked lines (about 0.8–1 cm) and carefully cut away the excess biflex. If there is a swimsuit elastic band along the top edge, trim it to a similar size.
Equipment Setup. Install a Super Stretch needle (No. 75 or No. 90) in the sewing machine and thread elastic or textured threads. Select an elastic knit stitch or a narrow zigzag stitch (width 1–1.5 mm, stitch length 2–2.5 mm).
Sewing. Pin the pieces in place. Sew along the marked lines, gently pulling the elastic fabric with both hands in front of and behind the presser foot to keep the seam flexible.
Trimming Edges. Trim the excess allowances close to the seam and finish the edge with a wide zigzag or an imitation overlock stitch.

This method guarantees that the renewed side seams will withstand any beach activities and will not deform the fabric.

Hand Method for Taking in Swimsuit Bottoms Without a Sewing Machine

If you do not have a sewing machine, you can tailor the swimsuit by hand. The main thing is to maintain the elasticity of the seam so that the thread does not break when stretched. For this work, use a fine knit needle and a strong textured or polyester thread.

Step-by-step hand sewing process:

Fitting: Turn the bottoms inside out, put them on, and pin the excess fabric on the sides with tailor's pins.
Marking: Draw a temporary marking stitch with contrasting thread along the line of the pins and remove the bottoms.
Elastic Hand Stitch: Sew using a 'backstitch' (mimicking a machine stitch), but make the stitches very small (1–2 mm) and do not pull the thread too tight. An alternative is the 'whip stitch,' which stretches very well along with biflex.
Finishing Allowances: Cut away the excess fabric, leaving a seam allowance of about 5–7 mm. Finish the edges with a blanket stitch to prevent fraying, although high-quality biflex hardly frays.

This method allows you to neatly take in the bottoms in just half an hour, while preserving the aesthetic appearance of the item.

Methods for Increasing the Size of Swimsuit Bottoms

Sometimes a favorite swimsuit becomes tight in the hips, digs into the skin, or fits too snugly from the start, creating unsightly folds. Don't rush to discard the item or hide it on a distant shelf. It is quite possible to increase the size of your bikini bottoms at home, and you can do it in a way that makes the updated model look even more interesting and original than the factory original.

Techniques are used to expand tight bikini bottoms that not only add the missing centimeters but also serve as a stylish decorative element. Working with elastic biflex requires care, but with the right approach, you can delicately adjust the fit on the sides while preserving the garment's elasticity and comfort during movement.

Inserting Side Ties and Creating Drawstrings

This method is a real lifesaver if the bikini bottoms have become tight in the hips. It allows you not only to increase the size by 2–4 centimeters but also to make the fit adjustable.

Step-by-step process for creating drawstrings:

Preparation: Carefully unpick or cut the side seams of the bikini bottoms. Clean the edges of old threads.
Forming the drawstring: Fold the cut side edges of the front and back panels under toward the wrong side by about 1.5–2 cm. This distance is enough for the cord to pass through freely inside.
Sewing: Secure the fold with pins. Sew an elastic zigzag stitch or knit stitch on the sewing machine, leaving a 1.2–1.5 cm seam allowance from the folded edge. Be sure to use "super-stretch" needles so as not to damage the delicate biflex.
Creating the ties: Cut two strips from the leftover biflex or contrasting elastic fabric, each 3 cm wide and about 40–50 cm long. Fold them in half lengthwise, stitch them with a small zigzag, and turn them right side out.
Assembly: Thread the finished ties through the resulting drawstrings using a safety pin and tie them into a beautiful bow on the sides.

This method preserves the fabric's elasticity and instantly transforms the swimsuit's design.

Using Contrasting Biflex Inserts

Using contrasting biflex inserts is not only a practical way to increase the bikini bottoms by 1–2 sizes but also an excellent design technique. The contrasting side panels visually correct the silhouette, making the hips more elegant, and completely transform the look of the garment.

For this work, you will need:

A scrap of contrasting colored biflex (choose fabric that matches the original in density and stretchability);
Elastic waistband for the swimsuit;
"Super Stretch" needles and strong textured threads.

Step-by-step process for sewing in inserts:

Cutting: Carefully cut the side seams of the bikini bottoms. Determine the missing width. For example, if you need to add 6 cm to the total circumference, you will need two pieces 3 cm wide plus 1 cm on each side for seam allowances (total piece width — 5 cm).
Preparing the pieces: Cut two strips from the contrasting biflex. Their height must exactly match the height of the side cut of the bikini bottoms, including allowances for finishing the top and bottom edges.
Sewing: Pin the inserts to the front and back panels of the bikini bottoms with the right side facing in. Sew the pieces on the sewing machine using an elastic knit stitch or a narrow zigzag stitch.
Securing the waistband: To prevent the top edge of the bikini bottoms from stretching, sew a new elastic band along the top edge of the insert, carefully joining it to the factory waistband of the bikini bottoms, and secure it with an elastic stitch.

This method allows you to maintain the closed, classic cut of the bikini bottoms while making the fit as comfortable and personalized as possible.

Trendy Upcycling: How to Create a Back Gathering (Scrunch Butt Effect)

The striking gathering on the back panel of the bikini bottoms, known as scrunch butt, is one of the most popular trends in beach fashion. This simple upcycling trick works wonders: it visually rounds out the shape of the buttocks, makes the silhouette more toned, and turns even the simplest basic briefs into a ultra-fashionable designer piece. If your bikini bottoms already fit perfectly in width, adding this gathering will be a great way to refresh their design.

You do not need complex patterns to create this effect. You just need an elastic band, suitable threads, and a little patience. Let's look at how to properly prepare the fabric for this delicate alteration so that the finished item looks professional and does not deform while wearing it.

Marking and Technology for Back Seam Gathering

The striking relief of the scrunch butt visually tightens the silhouette and makes the bikini bottoms fit more anatomically. To create the gathering, you will need a thin latex elastic band (3–5 mm wide) and tailor's chalk for marking.

Marking: Lay the bikini bottoms flat, folded in half along the back. Draw a vertical line with chalk strictly down the center of the back panel—from the top edge of the waistband to the beginning of the crotch seam.
Preparing the elastic: Measure a section of the latex elastic band. Its length should be approximately 50–60% of the length of the marked vertical line. This proportion will ensure a beautiful yet comfortable gathering without excessive pressure.
Fixing: Pin one end of the elastic band to the top edge of the bikini bottoms on the wrong side, and the other to the crotch seam.
Sewing: Set the sewing machine to an elastic zigzag stitch. Start sewing, gently pulling the latex band with both hands until it reaches the length of the fabric. The biflex itself should not be stretched during this process.

Once you finish sewing, the elastic band will contract, forming a uniform and aesthetic gathering in the center.

Secrets to Seam Strength When Stretching Fabric

To ensure the striking gathering does not pop open during the first squat or active swimming, you must observe three golden rules of strength:

Correct Threads. Ordinary sewing threads will not withstand the extreme tension of biflex. Use exclusively textured polyester threads (thread them into the sewing machine bobbin). They have a unique ability to stretch along with the material.
Elastic Stitch. Set the narrow zigzag stitch (width 1–1.5 mm, stitch length 2 mm) or a special three-step zigzag. A straight stitch is strictly forbidden here—it will burst on the first try.
Secure Anchors. At the beginning and end of the gathering, make reinforced anchors (preferably by hand with several stitches through the edge), sewing the swimsuit elastic through completely. This prevents the elastic band from slipping out of the seam under strong fabric tension.

Replacing and Attaching Elastic for a Perfect Fit

Even if you have perfectly adjusted the side seams and created a trendy gathering, the swimsuit will not fit flawlessly without a quality elastic band. Over time, the original elastic in the bikini bottoms stretches from chlorinated water and sun, causing the bottoms to slip or bunch unattractively. Replacing this element is the final and most important touch in updating the bikini bottoms, restoring their original elasticity and guaranteeing a secure fit on the body during movement.

For working with biflex, regular lingerie elastic will not suffice—you need a special latex or silicone tape that is resistant to salt and chlorine. Properly sewn elastic not only holds the fabric but also shapes a beautiful silhouette without constricting the skin. Let's examine how to technically integrate a new elastic band into the edges of the swimsuit so that it looks like a high-quality factory product.

How to properly sew a new elastic band for a swimsuit with a zigzag stitch

Use a special latex or silicone elastic for swimsuits—it does not degrade from chlorine or saltwater. Regular woven lingerie elastic will quickly stretch out and become unusable.

Step-by-step sewing process:

Preparation and fixing. Cut the elastic to be 10–15% shorter than the length of the swimsuit edge being processed. Divide the edge and the elastic into four equal parts, mark them with pins, and align the marks. This ensures even fit distribution.
First stitch. Apply the elastic to the wrong side of the edge, flush against the seam. Sew it with a zigzag stitch (width 3 mm, stitch length 2–2.5 mm), gently pulling the elastic (but not the fabric!) to the length of the biflex. Use a Super Stretch needle.
Folding and finishing stitch. Fold the edge with the elastic to the wrong side. Sew a second zigzag stitch (or a triple zigzag) on the right side, carefully overlapping the inner seam.

This method will preserve the elasticity of the biflex and prevent the swimsuit from deforming when wet.

Fitting and final tension adjustment

The final fitting is a crucial stage that determines how comfortable you will feel at the beach. Put on the swimsuit and assess the elastic tension while moving: squat, take a few steps.

Checklist for a perfect fit:

The elastic does not dig into the skin. If folds or creases form on the body, the tension is excessive.
The fabric fits snugly against the body. If the biflex lifts away from the skin when bending or squatting (especially on the buttocks or in the groin area), the elastic needs to be tightened.
The seams retain elasticity. Try stretching the edge by hand—the threads should not crack or break.

If the tension requires adjustment, gently support the zigzag stitch near the side seam, free the ends of the latex tape, and adjust their length. Shorten the elastic by 1–1.5 cm for a tighter fit or, conversely, slightly loosen the tension. After adjustment, secure the ends of the elastic with a tight stitch overlap and finalize the seam.