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Baby age calculator (with adjusted age for preemies)

Enter your baby's birth date. If they arrived early, add the due date too, and you will see their adjusted age.

By the Swaddle team · Reviewed October 25, 2026

Baby age calculator

Age

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Enter a birth date.

How baby age is counted

  • Chronological age is the time since birth. It is what you would put on a birthday cake.
  • Adjusted age (also called corrected age) is chronological age minus the number of weeks your baby was born early. Full term is counted as 40 weeks of pregnancy.

Months are counted by calendar date: a baby born on March 10 turns 1 month old on April 10. In weeks, a month is about 4.3 weeks.

Adjusted age example

Mila was born at 32 weeks, 8 weeks before her due date. Today she is 6 months old. Her adjusted age is 6 months minus 8 weeks, about 4 months. When her pediatrician checks milestones like rolling, they compare her with 4 month olds, not 6 month olds.

Why it matters for sleep

Sleep develops with the brain, so babies born early often reach sleep milestones closer to their adjusted age. That includes longer night stretches, the move to fewer naps and the 4 month sleep change. If your baby was born early, try the wake windows and schedules for their adjusted age first.

Questions parents ask

How do I calculate my baby’s age in weeks?

Count the days since birth and divide by 7. A baby born 45 days ago is 6 weeks and 3 days old. The calculator does this for you.

What is adjusted age?

Adjusted (or corrected) age is a premature baby’s age counted from their due date instead of their birth date. A baby born 8 weeks early who is 6 months old has an adjusted age of about 4 months.

Until what age should I use adjusted age?

Pediatricians commonly use adjusted age for growth and development until about age 2 (2 years from birth). Ask your pediatrician what they use for your baby.

Should I use adjusted age for sleep schedules?

Many families find adjusted age a better guide for wake windows and sleep milestones like the 4 month change, since sleep development follows brain maturity. Swaddle’s nap forecast uses corrected age too.

Sources

  1. American Academy of Pediatrics: Corrected Age For Preemies (healthychildren.org)
  2. Engle WA, AAP Committee on Fetus and Newborn. Age terminology during the perinatal period. Pediatrics 2004;114(5):1362-1364

This page is general information from the sources above, written by the Swaddle team and last reviewed October 25, 2026. It is not medical advice and it is not a substitute for your pediatrician. Every baby is different. If something feels wrong, call your doctor. In an emergency, call 911 (US) or your local emergency number. How we write these pages.

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