Diaper Size Calculator

How this is calculated

This tool matches your baby's weight against typical weight ranges for both the numbered US sizing system (Newborn, then 1 through 6) and the EU/international numbered system, which use somewhat different bands. The ranges are deliberately generous and overlapping between sizes, because two babies of the exact same weight can have quite different body shapes — a longer, leaner baby and a shorter, rounder one at the same weight often fit differently in the same size.

The figures used here reflect what's typical across several major brands' published charts, converted between pounds and kilograms, rather than being pulled from any single manufacturer's chart. That's intentional, since no two brands size identically.

The US and EU/international systems don't map onto each other exactly either — they were developed independently and use different weight bands, so the "EU size" shown alongside your US size result is an approximate equivalent, not a precise conversion. If you're shopping in a region that uses the other system, treat it as a helpful starting point for browsing rather than an exact match.

Common edge cases and things to know

Brand variation is the single biggest caveat here — a "size 3" in one brand can fit noticeably differently than a "size 3" in another, since each manufacturer designs its own cut and elastic placement. If you switch brands, expect to possibly size up or down even at the same weight.

Fit cues matter more than the number on the scale: a snug (not tight) waist and legs, no red marks or indentations after wear, and no gaps where the diaper could leak are the real test. Babies who are very long or very short for their weight, or going through a growth spurt, may need to size up temporarily before their weight "catches up" on a chart. Overnight and swim diapers are sometimes sized or cut differently from daytime diapers even within the same brand, and premature babies often need preemie-specific sizes smaller than a standard "Newborn."

Sample packs are a genuinely practical way to find the right fit before committing to a big box, since a size that fits perfectly in one brand may run slightly small or large in another. It's also normal to size up faster than you'd expect in the first few months, since weight gain is often fastest in that window — buying in bulk far ahead of a growth spurt can mean a package of diapers your baby outgrows before finishing it.

Sources

General newborn and baby care guidance is available from HealthyChildren.org (from the American Academy of Pediatrics) and Mayo Clinic.