Baby Boy Shirt Size Chart | Sizes That Actually Fit

Baby boy shirt sizes depend on chest and height measurements, not age alone, so the right fit varies by brand.

For the full breakdown, see our best Baby Boy Shirts guide.

Most US parents shop for baby boy shirts by age label — 6M, 12M, 24M — and end up with a drawer of missized tops. The reliable way to pick a shirt that fits the first time is to measure your son’s chest and height, then compare those numbers to the specific brand’s chart. That single habit solves more sizing problems than any universal size guide, because no standard size table applies across all manufacturers.

How Baby Boy Shirt Sizes Work

US baby and toddler shirt sizes use age-based labels that map to body measurements. Infant sizes run from newborn (NB) through 24M, then crossover into toddler sizes like 2T, 3T, and 4T before moving to numeric boys’ sizes such as 4, 5, 6, and 7.

The catch is that the measurement behind each label changes by brand. Adidas maps its 0-3M shirts to a height of 20-22 inches, while the same label from another manufacturer might target a slightly different range. Under Armour’s boys’ tops chart lists chest, waist, hip, and height for sizes 2T through 7, giving you four data points to compare instead of guessing from age alone.

That is why the same 12M shirt from two brands can fit completely differently on the same child.

Measuring Your Son For A Shirt

Two measurements matter most for tops: chest and height. The Children’s Place tells shoppers to measure around the fullest part of the chest, and that advice applies to any brand’s chart.

• Chest: wrap a measuring tape around the fullest part of the chest, just under the armpits, keeping it level across the back.
• Height: measure from the top of the head to the floor with shoes off. For babies, lay them flat and measure along a straight line.
• Weight (optional): some charts include weight as a cross-check, especially for infant sizes where body shape varies a lot.

When the measurements fall between sizes, pick the larger size — shirts need room in the chest and shoulders, and a slightly roomy top beats one that pulls or restricts movement.

Brand Size Charts Compared

Brand charts differ in both the sizes they offer and the measurements they attach to each label. Below is a snapshot of how three popular brands structure their baby and boys’ shirt sizing.

Note that Under Armour’s boys’ range starts at 2T, while Adidas covers the full infant span. If you are shopping for a newborn or young infant, you need a brand that lists NB or 0-3M sizes; not every manufacturer does.

Common Sizing Mistakes To Avoid

The most frequent error is relying on age alone. Official charts list age as one field among several, and a 12-month-old can wear anything from 9M to 18M depending on their build.

Another common slip is mixing infant and toddler sizes. Infant sizes (NB, 0-3M, 6M, 24M) and toddler sizes (2T, 3T, 4T) are not interchangeable — a 24M shirt fits differently than a 2T, even though they sit next to each other on the rack.

Because chart ranges differ by brand and even by product line, always check the specific product page’s size guide before ordering, especially online. That is the only way to see the measurements for the exact shirt you are buying.