What Size Baby Clothes to Buy: A Parent’s Seasonal Guide

Sizing baby clothes can be tricky—babies grow in spurts and brands fit differently. This guide answers what size baby clothes to buy when you plan ahead by season, with practical tips for tall babies, diaper room differences, and how to avoid overbuying.

Why Sizing Ahead Is Hard (But Worth Planning)

Growth isn’t linear—some babies jump from one size to the next in weeks, others linger in one size for months. Brands also cut differently. Still, planning with a light hand saves money and stress—just avoid deep stock-ups until you’re closer to the season.

Buying Ahead by Season: General Rules

Rule #1: Buy a Little, Not a Lot

Unless it’s a major clearance, start with a small basket and top up later. You can always add, but you can’t make “too small” bigger.

Rule #2: Size Up for Seasonal Pieces

For seasonal outfits you plan months in advance (swimwear, summer sets, jackets), choose one size up—slightly loose is workable, too tight is a waste.

Rule #3: Core Basics Are Year-Round

Short/long-sleeve bodysuits, lightweight cardigans, leggings, zip sleepers—these layer across seasons and are safer to buy ahead.

What Size Baby Clothes to Buy: Decision Guide

If Your Baby Is Tall (70th percentile+ height)

Favor the larger size for the target season. For late spring–early summer, if your baby is in 9M now with room, consider 12M for early summer and 18M for late summer.

If Your Baby Is Average

Buy true-to-age for the next 1–2 months of wear; size up for items you’ll use 3+ months later.

If Your Baby Is Petite

You can often stick to labeled age. Add one size up only for pieces you want to last all season or for brands known to run small.

Fit Details: Tall Babies, 24M vs 2T, Diaper Room

Tall Babies and Bodysuits

Bodysuits often run short in torso length. Consider shirts + shorts/leggings sooner, or choose brands with longer torso cuts; two-way-zip sleepers also accommodate length better.

24M vs 2T: What’s the Difference?

24M is “baby” sizing with more diaper room and shorter proportions; 2T is “toddler” sizing—slimmer rise, longer legs/arms. If your child is close to potty training or tall/slim, 2T may fit better; if still in diapers and needs room, 24M is safer.

Brand Variation