Baby Swimwear Sizing: The Complete Guide for Ages 6–36m

How to measure your baby, read the size chart, and choose with confidence.

The short answer: find your baby's height and weight in our size guide page, and if they fall between two sizes, go up. The right size means your baby is comfortable, free to move, and fully protected from the sun all day.

How to Measure Your Baby

Age labels on baby clothing can be misleading — a 12-month label at one brand can mean something very different at another. Height and weight are always the most reliable guide. Here is how to measure in two quick steps:

1. Height: 

2. Weight: 

Once you have both measurements, use the chart above. If your baby's height and weight fall in different size rows, always go with the larger size — a swimsuit that is slightly generous is comfortable to wear; one that is too small is not.

What a Good Fit Looks Like

A well-fitting MIKOU swimsuit sits close to the body and gives full coverage — but your baby should be completely free to move. Arms and legs can kick and reach without any restriction. The fabric has elastic in it, which gives just enough room for movement while keeping everything comfortably in place.

A good fit is not a tight fit. If the fabric is pulling, leaving marks on the skin, or your baby seems uncomfortable, go up a size. If it is bunching or sagging around the torso, try a size down. The goal is a smooth, secure fit that stays put in and out of the water.

The elastic in our Italian recycled fabric means a swimsuit that looks close-fitting on the hanger sits comfortably on your baby — it moves with them, not against them.

Between Two Sizes? Here's How to Decide

Your baby is 78 cm tall but weighs 10.5 kg — right on the border between 12M and 18M. Here is how to choose:

Prioritise height

The swimsuit needs to cover the full torso. If it is too short in the body, the lower back and tummy will be exposed to the sun. A little extra room at the shoulders is easy to live with; uncovered skin is not.

Comfort is the deciding factor

If in any doubt, size up. A swimsuit that is slightly generous still provides full protection — and your baby will be happier wearing it for a full day at the beach or pool.

Quick rule: when height and weight point to different sizes, always choose the larger one.

Buying for the Season

If you are buying in spring for a summer holiday, it is worth thinking ahead. Babies in the 6–24M range typically grow around 1–2 cm per month — roughly one MIKOU size every 3–4 months.

If your baby is at the lower end of a size range in April, that size should still fit comfortably through July. If they are already at the top of a size range, buying one size up means the suit will last the whole season without needing to reorder mid-trip.