01What this calculator tells you
When a baby is born prematurely, their age since birth (chronological age) overstates how far along their development should be. Adjusted age — also called corrected age — subtracts the weeks they were born early, so milestones are judged against the original due date. This calculator shows both figures side by side, plus exactly how many weeks premature the baby was.
Enter the birth date and either the gestational age at birth or the original due date, and the calculator returns the corrected age in weeks and months. For general growth tracking after correction, our BMI calculator covers older children and adults, but infant growth should always be plotted on paediatric charts by a clinician.
02How to read your result
The corrected age is the one to use when checking developmental milestones such as smiling, sitting, or crawling. The table shows how much correction different degrees of prematurity add, using the standard 40-week full-term reference against which the CDC developmental milestones are judged.
A baby born at 32 weeks has had eight fewer weeks to grow and develop than a baby born at term. Judging them by chronological age would make normal preterm development look like a delay. Correcting the age lines them up with where a full-term baby of the same developmental stage would be, which is why NICU teams and services such as the NHS and MedlinePlus use it.
- Developmental milestones (motor, social, language).
- Timing of assessments and screening.
- Introducing solids and other age-based steps, in discussion with your paediatrician.
03Related calculators
Working through a related project? Try our Is My Period Late Calculator, Body Shape Calculator, and Protein Intake Calculator.
01The corrected age formula
Corrected age is chronological age minus the number of weeks the baby arrived before their due date. Full term is defined as 40 weeks, so the weeks-early figure is 40 minus the gestational age at birth. If you know the original due date instead, the corrected age is just the time elapsed since that date.
Where:
- chronological age= the time elapsed between the birth date and today (or your chosen date).
- gestational age at birth= completed weeks of pregnancy at delivery, e.g. 32 weeks.
- original due date= the estimated 40-week due date from dating.
02Worked example
Take a baby born at 32 weeks gestation who is now 6 months old (chronological). Work it out step by step:
So this baby has a corrected age of about 4 months. You would expect them to reach milestones like a typical 4-month-old, not a 6-month-old — and by the time they pass roughly 24 months corrected, the CDC milestone checks use chronological age. Once your child is older, our BMR calculator can help estimate daily energy needs.