One Piece Swimsuit

What You’re Designing

A leotard or swimsuit is a close-fitting stretch garment that must stretch with your body, stay in place during movement, and fit smoothly without sagging or pulling.

Before You Start: Fabric & Measurements

Fabric Choice

Choose fabrics with good stretch in both directions:

• Lycra / Spandex blends
• Swim knit
• Stretch velvet / velour
• Performance knits

Stretch Test (Do This First)

1. Stretch the fabric in both directions
2. If 4 inches stretches to 5 inches, you have 25% stretch

You want good stretch lengthwise and crosswise. This determines how much the pattern should be reduced and directly affects comfort and fit.

Body Measurement

Measure your total torso length: start at the shoulder, go down the front, through the legs, and up the back. If wrong — too short pulls and rides up; too long sags and feels loose.

Understanding Stretch & Negative Ease

This is the most important concept in swimwear drafting — and the one most beginners skip. Getting it wrong is the #1 cause of poor fit.

What Is Negative Ease?

In regular garments, ease is extra room added so the garment is larger than the body. In stretch garments, you use negative ease — the pattern is intentionally smaller than the body, and the fabric’s stretch makes up the difference. The garment hugs the body because it’s always trying to return to its smaller, relaxed state.

How to Calculate Negative Ease

• 25% stretch fabric: reduce pattern measurements by 15–18%
• 50% stretch fabric: reduce pattern measurements by 20–25%
• 75%+ stretch fabric (power stretch): reduce pattern measurements by 25–30%

Apply Reduction Selectively

• Width measurements (bust, waist, hip): apply full negative ease reduction
• Length measurements (torso, side seam): apply a smaller reduction (about half) — lengthwise stretch is usually less than crosswise stretch
• Crotch width: reduce minimally or not at all — too much reduction here causes discomfort and pulling

Part 1: Front Pattern

Step 1: Start with Your Bodice Block

Use your fitted bodice front as the base.

Step 2: Adjust Waist Length

Take your shoulder-to-waist length and subtract ~3 inches (7.6 cm). Stretch fabric replaces that missing length.

Step 3: Waist Width

Use your waist measurement and reduce slightly — fabric stretch will compensate.

Step 4: Bust Area Options

• Option A: Keep Bust Dart — best for larger bust; provides shaping
• Option B: Convert to Gathers — same dart width turned into soft fullness
• Option C: Remove Dart — drop underarm slightly (~1 inch / 2.5 cm) and redraw a straight side seam; best for athletic wear and smaller bust

Step 5: Add Hip Shaping

Use your hip measurement and blend from waist to hip for a smooth fit over the hips.

Step 6: Choose Leg Opening Height

Measure 5–7 inches (13–18 cm) below the waist:

• High-cut: longer legs look
• Low-cut: more coverage

Step 7: Set Center Front Length

Using your total body measurement, measure down from the neckline to ensure correct vertical fit.

Step 8: Draw Crotch & Leg Curve

Crotch width ~1¾ inches (4.4 cm), curving upward slightly then into the leg.

• Lower curve: more coverage
• Higher curve: less coverage

Part 2: Back Pattern

Step 1: Start with Back Block

Use your bodice back as the base.

Step 2: Match Side Seam Length

The back side seam must match the front side seam exactly — adjust for dart removal if needed. This is critical for sewing accuracy.

Step 3: Waist & Hip

Same process as the front, with slightly smaller hip shaping if needed.

Step 4: Back Length

Use your full body measurement and mark from the back neckline down.

Step 5: Back Crotch Curve

The back crotch is slightly wider (~1¾ inches / 4.4 cm) and curves outward more than the front to fit over the buttocks. Front curve = flatter; back curve = fuller.

Back Style Variations

Racerback

Narrows the back straps and brings them toward the center, creating a Y-shape that allows full shoulder mobility. The most common back style for athletic swimwear and leotards.

1. Mark the new strap width — a typical racerback strap is 1–1.5 inches (2.5–3.8 cm) wide at the narrowest point
2. Draw the new armhole curve — from the strap edge, draw a deep, sweeping curve down toward the underarm; no sharp angles
3. Lower the back neckline (optional) — lower the center back neckline by 1–3 inches (2.5–7.6 cm) for a more open look
4. True the strap — walk the front and back strap edges together to confirm they match in length
5. Elastic application — apply elastic around the entire new armhole/racerback opening; stretch the elastic slightly more than usual as you sew to prevent the edge from flipping outward

Low Back

A design statement — but it removes the structural support of a traditional back, so construction details matter more.

1. Lower the center back — mark how low you want the back to go and draw a new center back line
2. Reshape the side seam — taper smoothly from the underarm down to the new back height; a gentle curve looks more intentional and fits better
3. Add a back strap or tie — very low backs often need a thin strap or tie across the mid-back to prevent the suit from gaping during movement
4. Reinforce the neckline — when the back is low, the front straps carry more tension; interface or line the front straps to prevent stretching out of shape

Part 3: Sleeve Design (Optional)

Step 1: Lower the Sleeve Cap

Drop the cap height ~2 inches (5 cm).

Step 2: Choose Length

• Short sleeve: basic cut
• Long sleeve: cut slightly shorter than arm length; stretch fabric will extend

Part 4: Seam Allowances & Finishing

• Most seams: ~⅜ inch (1 cm)
• Crotch seam: ~⅝ inch (1.5 cm)

Elastic Areas

Apply ⅜ inch (1 cm) elastic to legs, neckline, and armholes.

Elastic Application Options

• Sew-On Elastic: measure opening, subtract ~2 inches (5 cm), sew elastic stretched to fabric, then fold inside and stitch. Creates a snug, secure fit.
• Encased Elastic: fold fabric edge, stitch a casing, and insert elastic. Softer, less sporty finish.

Lining Options

• Full lining: the entire garment is lined with a second layer of swimwear fabric or tricot. Best for sheer or light-colored fabrics. The two layers are treated as one during construction.
• Partial lining (crotch and bust only): the most common approach. Line only the areas that need opacity or hygiene protection.
• Built-in shelf bra: a partial lining that includes a band of elastic at the bottom for support. Draft it as a separate piece covering the bust area, add a fold-over elastic band at the lower edge, and attach it to the inside of the front bodice at the neckline and side seams.
• Tricot lining: lightweight, slippery knit fabric that’s comfortable against the skin, dries quickly, and doesn’t add bulk. Cut it slightly smaller than the outer fabric (apply additional negative ease) so it doesn’t cause the outer layer to sag.

Part 5: Crotch Lining (Recommended)

1. Trace the crotch area (~3 inches / 7.5 cm long) and cut on fold
2. Sandwich between front and back, stitch all layers, then fold to inside for a clean, professional finish

Part 6: Construction Order

1. Sew center seams (if any)
2. Sew side seams
3. Sew shoulders
4. Attach sleeves (if used)
5. Sew crotch seam
6. Add elastic to all openings

Stitching Techniques for Swimwear

Regular straight stitching will pop when the fabric stretches — swimwear requires specific stitches to hold up under tension, chlorine, and repeated wear.

• Serger / overlock stitch: the standard for swimwear seams. A 4-thread overlock is strong, stretchy, and trims the seam allowance in one pass. Use woolly nylon thread in the loopers for extra stretch and softness against the skin.
• Lightning bolt stitch: excellent for swimwear seams when a serger isn’t available. It stretches significantly without breaking.
• Zigzag stitch: use a wide, short zigzag (width 2.5–3, length 1.5–2) for seams that need to stretch. Not as strong as a serger but workable for low-stress seams.
• Twin needle: creates two parallel rows of stitching on the right side with a zigzag on the wrong side. Ideal for hemming and topstitching. Gives a professional, store-bought look to necklines and leg openings.
• Fold-over elastic (FOE): folds over the raw edge and is stitched in place, finishing and elasticating the edge in one step. Popular for leotards and dancewear. Choose FOE that is at least 50% stretch.

Fitting Across Different Body Types

• Full seat / high hip: add width to the back hip curve and blend smoothly to the waist and leg opening; if the suit pulls across the seat, also lengthen the back crotch curve slightly
• Flat seat: reduce the back crotch curve and take in the back side seam below the hip — too much fabric in the back creates sagging and bunching
• Long torso: add length at the waist (between the bust and hip) on both front and back; do not add length at the shoulder or crotch
• Short torso: remove length at the waist on both front and back; if the suit still feels too long, also shorten slightly at the shoulder
• Full bust: keep the bust dart (Option A) or convert to gathers (Option B); do not remove the dart for a full bust — the suit will gap at the chest and pull across the bust
• Narrow shoulders: reduce the strap width and move the strap position inward toward the neck; true the front and back strap edges after adjustment
• Wide shoulders: widen the strap and move it outward toward the armhole; be careful not to extend past the natural shoulder point or the strap will fall off the shoulder

Design Variations to Try

• Necklines: scoop, V-neck, or high neck
• Back styles: low back, racerback, or open back
• Leg cuts: high cut, moderate, or full coverage
• Design lines: princess seams, color blocking, or panels

Common Mistakes (and Fixes)

• Suit rides up: not enough body length
• Saggy fit: fabric not stretchy enough, or negative ease not applied correctly
• Tight at crotch: increase crotch length or width
• Elastic too tight: reduce stretch when sewing elastic
• Seams popping: using a straight stitch instead of a stretch stitch or serger
• Fabric runs or ladders: using a sharp needle instead of a ballpoint or stretch needle

The Big Picture

Once you understand this, you can design swimsuits, dancewear, athletic wear, bodysuits, and gymnastics leotards.