Halloween costumes run differently than everyday baby clothes, and ordering the wrong size usually isn't discovered until the night before trick-or-treating. Here's how to size a baby Halloween costume correctly the first time, so you're not stuck re-ordering or safety-pinning a collar at 6pm on October 31st.
Go by Current Measurements, Not Current Age
Costume size charts are usually built around age ranges, but babies vary widely in height and weight at the same age. Measure your baby's chest, length, and weight the week you order, and match those numbers to the brand's actual size chart rather than assuming "9 months" means 9 months old. If your baby falls between two sizes, sizing up is almost always the safer call for a one-night costume.
Leave Room for What's Underneath
Unlike a regular onesie, a costume is often worn over a base layer on a cool October night. If you plan to layer underneath for warmth, size up slightly so the costume doesn't pull tight across the chest or restrict arm movement once that extra layer is on.
Check Snap and Zipper Placement
A costume with a full snap panel down the legs is far easier to size correctly than one that only opens at the neck, since you can still get a quick diaper change without forcing a too-small opening. Before buying, check whether the listing shows the closure type — a snap panel is more forgiving if your estimate is slightly off.
Do a Trial Run Before the Big Night
Try the costume on a few days ahead, not the morning of. This gives you time to exchange for a different size if needed, and lets your baby get a short practice wear-in so the fabric and fit feel familiar rather than new and itchy on Halloween itself.
Looking for easy, comfortable everyday basics to pair under or after the costume? Browse our organic cotton baby clothes collection for soft layers that work all season.