Preemie Clothing Size Finder & Weight Converter
Premature baby clothing is sized by weight, not age. Enter your baby's current weight in grams or pounds below and we'll tell you the right size category with a direct link to shop. Both fields update together so you can convert between grams and pounds at the same time.
Preemie size finder & weight converter
Type in grams or lbs — both fields update together
Grams (g)
Pounds (lbs)
Enter your baby's weight above to see their size.
Sizing is a guide only and may vary between garments. Always check individual product size guides and speak to your NICU team if unsure.
Premature Baby Corrected Age Calculator
Corrected age, sometimes called adjusted age, is calculated from your baby's original due date rather than their date of birth. It is used to track development milestones for premature babies, usually until around two years of age. Enter your baby's birth date and the number of weeks gestation at birth to calculate their corrected and chronological age.
Corrected age calculator
Calculate your baby's corrected and chronological age
Enter your baby's birth date and gestation to calculate their corrected age.
Corrected age is calculated from your baby's original due date (40 weeks). This tool is for guidance only — always discuss your baby's growth and development with your healthcare team.
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About These Tools
These free premature baby calculators were created by Simply Preemie to help parents of premature babies answer two of the most common practical questions in those early weeks and months. Rachael, who founded Simply Preemie after her son Theo was born at 28 weeks weighing just 600g, knows first-hand how overwhelming those first decisions can feel.
Preemie clothing size finder
Premature baby clothing in the UK is sized by weight rather than age, covering babies from 1lb up to 8lb across three main categories: micro preemie (1lb–3lb), early baby (3lb–5lb), and tiny baby (5lb–8lb). Our size finder takes your baby's current weight in grams or pounds and instantly tells you the right size category, with a direct link to shop that range. Both weight fields update together so you can convert between grams and pounds at the same time — useful if your NICU team uses grams but clothing labels use pounds.
Always use your baby's current weight when choosing a size, not their age or due date. Babies commonly lose up to 10% of their birth weight in the first days after birth, so check their most recent weight with your care team before ordering. For more detail on sizing, visit our premature baby clothing size guide.
Premature baby corrected age calculator
Corrected age, also known as adjusted age, is the age your baby would be if they had been born on their original due date. It is calculated by subtracting the number of weeks premature from your baby's chronological age. For example, a baby who is 16 weeks old but was born 8 weeks early has a corrected age of 8 weeks. Corrected age is used by healthcare professionals to track growth and development milestones for premature babies, typically until around two years of age.
Our corrected age calculator takes your baby's date of birth and gestational age at birth and instantly shows their corrected age, chronological age, and how many weeks early they were born. For more on growth and development timelines, read our guide on when premature babies catch up in size.
Both tools are provided for general guidance only and do not constitute medical advice. Always follow the guidance of your neonatal care team or healthcare professional regarding your baby's growth, development, and care.
This page is part of our premature baby knowledge hub. You can also read Theo's story to learn more about the experience behind Simply Preemie.