Free Tools
Corrected Age Calculator
Adjust a preterm baby's age to their due date — the age milestones and growth charts are read against.
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How this works
Enter the day your baby was born and the gestational age at delivery — the weeks and days written on the discharge summary, like 32+3. The calculator subtracts the time your baby missed in the womb and shows the corrected age next to the chronological one, along with the correction in days and the due-date equivalent it counts from. Nothing is uploaded; the numbers update as you change the fields, and your inputs live in the URL so a result can be bookmarked or sent to a partner.
Correction applies only to babies born before 37 weeks and 0 days. At 37 weeks or later there is nothing to correct, and the tool says so plainly instead of quietly subtracting days. It also stops at 24 months of chronological age: the AAP says that during the first 2 years, using corrected age gives a better idea of when a baby should reach common developmental goals, and how long your own clinician keeps correcting is a good question for the next visit.
Corrected age is the age that milestones and WHO growth percentiles should be read against — it is how pediatricians track a preterm baby, and it is not a diagnosis of delay. Vaccinations are the exception. Immunisation schedules run on chronological age, and babies born preterm are vaccinated on the same calendar as babies born at term, so never correct a vaccine date.
Twins born on the same day at the same gestational age share one corrected age, which is why the twins view shows two identical cards on purpose; if your babies have different recorded gestational ages, run each one separately. Multiples make this common — the NHS defines premature labour as labour before the 37th week and notes that twins and triplets are often born prematurely. Before your baby reaches their original due date the corrected age reads 0 days, which is the standard convention rather than a delay.
The math behind it
The formula is the one the AAP describes: corrected age = chronological age − (40 weeks − gestational age at birth). This calculator works in days rather than whole weeks, so the odd days on your discharge summary are not thrown away. Gestational age at birth in days = weeks × 7 + days. Correction = 280 − that number, floored at 0. Due-date equivalent = birth date + correction. Corrected age = the number of days between the due-date equivalent and the date you are calculating for.
A worked example. A baby born on 1 June 2026 at 32+3 was 32 × 7 + 3 = 227 days of gestation, so the correction is 280 − 227 = 53 days and the due-date equivalent is 24 July 2026. On 5 September 2026 that baby is 96 days old chronologically and 96 − 53 = 43 days corrected — six weeks and one day. The same baby at 40+0 would have had no correction at all, and at 37+0 the tool reports no correction because 37 weeks is term.
Corrected age is floored at 0 days. If you calculate for a date before the due-date equivalent — common while a very preterm baby is still in hospital — the answer is 0 days rather than a negative number, because developmentally the baby has not reached term yet. The due-date equivalent shown here is simply birth date + correction; it is an age reference point, not a re-dating of the pregnancy, and it does not replace the due date your provider recorded.
Two gates decide whether the correction is used at all. Born at 37+0 or later: no correction, and chronological age is the age everyone uses. Past 24 months chronological: providers stop correcting, because by then the two ages have converged; the tool shows both numbers side by side for reference and labels them. Gestational age is accepted from 22 weeks 0 days to 42 weeks 6 days, and anything outside that range is rejected with a message rather than silently computed.
This mirrors the ProbaBaby app, which stores gestational weeks at birth for each baby and looks up WHO growth percentiles on corrected age in days — a measurement taken on day 23 is scored against day 23 rather than rounded to a month — and applies the same 37-week and 24-month gates. In the app, milestones and growth use corrected age while vaccination schedules stay chronological; the widgets and the Apple Watch also show chronological age.
Frequently asked questions
Corrected age — also called adjusted age — is your baby's age counted from their original due date instead of their birthday. The AAP's method is to begin with your baby's actual age in weeks and subtract the number of weeks they were born early. It is used for babies born before 37 weeks, so that milestones and growth charts compare them with babies who had the same amount of development, not the same number of birthdays.
Take 40 weeks (280 days), subtract the gestational age at birth, and subtract that difference from your baby's age today. A baby born at 32 weeks and 3 days was 227 days of gestation, so the correction is 53 days: at 96 days old they are 43 days — about 6 weeks — corrected. Working in days rather than whole weeks keeps the odd days from the discharge summary in the answer.
This calculator, like the ProbaBaby app, corrects until 24 months of chronological age and then stops. The AAP says that during the first 2 years, corrected age gives a better idea of when a baby should reach common developmental goals. Some clinicians keep correcting longer for very preterm babies, so ask yours how long they will use it — after that point the two ages have usually converged anyway.
Only if they were born before 37 weeks — which is common, since the NHS notes that twins and triplets are often born prematurely. Twins born on the same day at the same gestational age have exactly the same corrected age, so the twins view shows two identical cards on purpose. If your babies have different recorded gestational ages, calculate each one separately.
No. Immunisation schedules run on chronological age, and babies born preterm are vaccinated at the same chronological age as babies born at term rather than being delayed to their corrected age. Correct the age you use for milestones and growth charts; never correct a vaccine date. If you are unsure about your baby's schedule, ask your pediatrician or health visitor.
Because corrected age is floored at 0. Before the due-date equivalent a preterm baby has not yet reached the point a term baby starts from, so the honest answer is zero rather than a negative number. It is a counting convention, not a statement about your baby. Chronological age keeps counting from birth the whole time, and it is shown alongside.
No. Correcting is standard practice, not a delay diagnosis — it is how pediatricians read milestones and growth for babies born early, and it exists so that a preterm baby is not judged against development they have not had time for yet. If you have a specific worry about how your baby is developing, corrected age is the number to bring to the conversation, not a reason to skip it.
It is on the discharge summary or the red book / baby health record, usually written as weeks and days, like 34+2. If you only have whole weeks, enter the weeks and leave days at 0 — the answer moves by at most six days. If you have no record at all, your pediatrician's notes will have it, and it is worth asking for because growth charts and milestone windows depend on it for the first two years.
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