Nappy / Diaper Size Calculator
Nappies are sized by your baby's weight, not age. Enter the weight for the right size and the typical range.
Find the nappy size
How to measure
Go by weight, not age
Babies grow at different rates — the weight on the pack is the reliable guide.
Watch for the fit signs
Red marks mean too small; gaps at the legs or leaks mean it's time to size up.
Sizes overlap
Weight ranges overlap between sizes, so two sizes can both fit during a growth spurt — choose by leaks and comfort.
Night-time
Many parents size up at night for extra absorbency.
Frequently asked questions
What nappy size for a 7 kg baby?
Around Size 3 (typically 5–13 kg), though Size 2 may still fit — go by leaks and comfort.
Should I choose nappies by age or weight?
By weight. Babies of the same age vary widely, so weight gives a far more reliable fit.
How this is calculated
Nappy sizes are defined by weight, and the ranges overlap between sizes. We map the entered weight to the common size bands (Newborn, then Sizes 1–6). Brands vary slightly, so always confirm the fit by checking for leaks, gaps and red marks rather than the number alone.
Sources: Common nappy/diaper weight bands across major brands, 2026.