Between sizes in swimwear and not sure whether to go tighter or looser? You’re not alone – different brands, fabrics and styles can all fit differently, even when the size label is the same. This guide explains how to decide whether to size up or down in a swimsuit, plus smart tweaks if your top and bottom halves need different sizes.

Use this as your go‑to reference whenever you’re shopping swimwear online or trying on new suits at Splash.

1. First Step: Start With Your Measurements, Not Just Dress Size

Before you worry about being “between sizes”, make sure you know your current measurements. Swimwear is more fitted than regular clothing, so small changes in bust, underbust, waist and hip can move you between size columns on a chart.

For the most accurate starting point:

• Measure your bust at the fullest point and your underbust where a bra band sits.
• Measure your waist at the narrowest point and hips at the fullest point over bottom and thighs.
• Compare those numbers with each brand’s size chart and note where you sit between two sizes.

If your bust, waist and hips fall into different size rows, that’s totally normal – it just means using some fit strategies rather than forcing everything into one number.

2. General Rule: Swimwear Should Feel Snug, Not Painful

New swimwear is designed to feel firm when dry because it relaxes slightly in the water and with wear. The ideal fit is close enough that nothing gapes or sags, but not so tight that it digs in, leaves deep marks or restricts your movement.

As a rule of thumb when you’re between sizes:

• If a suit feels secure but slightly tight when you first put it on, it will usually relax to a comfortable fit.
• If it feels painful, pinching or hard to breathe/sit in, it’s too small – size up or change style.
• If it feels loose, baggy or easy to pull away from the body, it’s too big – try the smaller size.

3. When to Size Down in Swimwear

Sizing down is often the better choice if you want a secure, “locked‑in” feel and the fabric has good stretch. This is common in nylon/elastane or ribbed multi‑fit styles.

Consider going down a size if:

• The fabric is soft, stretchy and unstructured (for example, seamless or ribbed sets) and you’re between sizes on the chart.
• The band, straps or body feel loose or ride up when you lift your arms or move around.
• You can pull the waistband or underbust several centimetres away from your body without resistance.
• You’re mainly lounging or light swimming and want a more sculpted, body‑hugging look.

Sizing down works best for styles without underwire or rigid cups; the stretch in the fabric will do most of the shaping.

4. When to Size Up in Swimwear

Sizing up is usually the better choice for structured, less stretchy swimwear and for swimmers who prioritise comfort. This includes chlorine‑resistant polyester suits, underwire one‑pieces and styles with firm powermesh lining.

Consider going up a size if:

• You have a full bust and feel compressed, spilling over the cups, or the underwire digs in.
• The suit cuts in at shoulders, leg openings or underbust, leaving deep red marks after a short try‑on.
• You struggle to pull the suit over hips or shoulders or can’t comfortably sit or squat once it’s on.
• The fabric contains little elastane (for example, chlorine‑resistant polyester) and doesn’t give much when you move.

If you’re between sizes in a shaping or control suit, sizing up often looks smoother and more flattering than squeezing into the smaller size.

5. Between Band and Cup Sizes? Use “Sister Sizing”

For underwire bikinis, tankinis and one‑pieces that use bra sizing, you can use sister sizing to fine‑tune fit when you’re between sizes.

Sister sizes keep the same cup volume while changing the band:

• If you want more room around the ribs, go up a band, down a cup (for example 12D → 14C).
• If you want the band firmer, go down a band, up a cup (for example 14D → 12DD).

This helps if your usual bra size feels right in the cups but too tight or loose through the body in swimwear.

6. Mix & Match Sizes for Top and Bottom

Many people are one size on top and another on the bottom – that’s why mix‑and‑match sets exist. If you’re between sizes overall but clearly different top to bottom, don’t force your body into a single number.

Instead:

• Choose your top size based on bust and underbust measurements and the level of support you like.
• Choose your bottom size based on hip measurement and how much coverage you want.
• Use adjustable features – tie sides, drawcords, multi‑loop back closures – to fine‑tune the fit once you’ve chosen the base size.

On Splash you can easily pair different sizes and styles from our bikini tops and bikini bottoms to build a custom set.

7. Fabric Matters: Stretchy vs Firm Swimwear

The fabric blend tells you a lot about whether to size up or down:

• Nylon / elastane (Lycra) blends: very stretchy, soft and moulding. If you’re between sizes and like a firm fit, you can usually size down.
• Ribbed / seamless multi‑fit: designed to stretch over a size range; they often look tiny off the hanger but expand once on. Stick with the smaller end of your range unless you prefer a relaxed fit.
• Chlorine‑resistant polyester / PBT: tougher, less stretchy and often feel tighter when new. If you’re between sizes and swim laps, many swimmers prefer to stay true to size or go up for comfort.
• Shaping / powermesh suits: built‑in control panels give extra hold. These are meant to feel firm; if you feel restricted or can’t breathe easily, go up a size rather than down.

Always read the fabric details on the product page; it’s your best clue about how forgiving a suit will be.

8. Quick Fit Checklist When You’re Between Sizes

When you’ve tried both sizes, use this rapid checklist:

• Bust: Cups full but not overflowing? Underband firm but not painful?
• Shoulders: Straps sitting flat without digging or slipping?
• Tummy & torso: Can you sit, bend and take a deep breath comfortably?
• Bottom & legs: No severe digging, sagging or constant wedgies when you walk?
• Movement: After a quick squat, stretch and twist, does everything stay put?

If the smaller size passes all of these, it’s usually the better long‑term fit. If you’re fighting the suit or dreading wearing it all day, choose the larger size or a different cut.

9. Still Between Sizes? Choose Adjustability Over Perfection

If you are still torn between sizes, prioritise styles with built‑in adjustability:

• Tie‑back and multi‑loop bikini tops so you can tighten or loosen the band.
• Tie‑side or drawcord bottoms that can be loosened on fuller‑hip days.
• Straps with sliders or multi‑position hooks for fine strap length adjustments.
• Zip‑front or lace‑up fronts you can tweak depending on how secure you want to feel.

These details matter more than the number on the tag and will keep your swimsuit working as your body or comfort preferences shift.

10. When to Ask for Help

If you’re regularly between sizes and never quite comfortable, it’s worth getting a second opinion. At Splash Swimwear, our fit team helps customers every day who sit between sizes or have different top and bottom measurements.

Visit us in Darwin or contact us online with:

• Your usual bra size and clothing size.
• Your bust, underbust, waist and hip measurements (even rough numbers help).
• Whether you prefer a firm, athletic fit or a more relaxed holiday fit.

We can then recommend whether to size up or down in swimwear for specific brands and styles, and suggest adjustable options so you feel supported, secure and confident – even if you’re officially “between sizes”.