A months old calculator that works by date of birth — exact age in months and days, weeks-to-months conversion, and corrected age for premature babies.
Your age in months is the number of completed calendar months since your date of birth, plus the days left over. It is not total days divided by 30.44 — calendar months have different lengths, so dividing gives an approximation that drifts by several days over a year.
Worked example: someone born on 20 November 2024, checked on 5 September 2026. The provisional count is (2026 − 2024) × 12 + (September − November) = 24 − 2 = 22 months. The 20th of September has not arrived yet, so drop one month to 21. Counting from 20 August to 5 September gives 16 days. The answer is 21 months and 16 days — that is 1 year 9 months, or 654 days.
Some months are too short to contain every birth date. When that happens, the monthly anniversary is clamped to the last day of the month. A baby born on 31 January 2026 is exactly 1 month old on 28 February 2026, and 1 month and 2 days old on 2 March. The calculator above applies this rule, which is the same convention used by standard date libraries, so the day count never comes out negative.
Working by date of birth is the only way to get an exact answer. A birth year or even a birth month is not enough: two babies born in the same month can be four weeks apart, which is a meaningful gap on any infant growth chart.
This calculator counts from the date of birth to today, which is what a paediatric appointment form usually wants. If you need the age in months on a different day — a school cut-off, a past check-up, an assessment scheduled next month — the same three-step method works with that date in place of today. The age on date calculator does it for you, and the days alive calculator gives the total days for the same span.
Infant age is quoted in weeks for the first couple of months and in months after that, and the two units do not line up. An average calendar month is 30.44 days, not 28, so four weeks is slightly less than one month and the gap widens as the baby grows. By twelve months a baby is 52 weeks old, not 48.
| Weeks | Days | Approx. months | Typically used for |
|---|---|---|---|
| 4 weeks | 28 days | 0.9 months | Newborn weight checks |
| 8 weeks | 56 days | 1.8 months | First immunisations in many schedules |
| 12 weeks | 84 days | 2.8 months | Second immunisation round |
| 16 weeks | 112 days | 3.7 months | Third immunisation round |
| 20 weeks | 140 days | 4.6 months | Pre-weaning review |
| 24 weeks | 168 days | 5.5 months | Approaching solids |
| 26 weeks | 182 days | 6.0 months | Six-month review |
| 39 weeks | 273 days | 9.0 months | Nine-month development check |
| 52 weeks | 364 days | 12.0 months | First birthday |
These conversions are averages. For the exact figure, count real calendar months from the date of birth — that is what the calculator above returns, and what a clinician records. The baby age chart lays the same conversions out week by week.
A baby turns six months old on the same day of the month, six months after birth. A baby born on 15 March 2025 is 6 months old on 15 September 2025. Where the birth day does not exist in the target month — a birth on the 31st, for instance — the milestone lands on the last day of that month instead.
| Milestone | Born 15 March 2025 reaches it on | Why it matters |
|---|---|---|
| 2 months | 15 May 2025 | Early immunisations, first growth review |
| 4 months | 15 July 2025 | Rolling over, four-month check |
| 6 months | 15 September 2025 | Introducing solids, sitting up |
| 9 months | 15 December 2025 | Development review, crawling |
| 12 months | 15 March 2026 | First birthday, walking and first words |
| 18 months | 15 September 2026 | Toddler review, vocabulary check |
| 24 months | 15 March 2027 | Two-year review; age usually quoted in years from here |
Plenty of parents mark each monthly birthday — the "monthiversary" — through the first year, and the arithmetic is the same one the milestone table uses: the same day number, one month later. Enter the date of birth above and the calculator tells you exactly how many complete months have passed and how many days remain until the next one. For the annual version, the birthday countdown counts down to the next full birthday.
A baby born preterm has two ages, and mixing them up makes normal development look delayed. The chronological age is the plain time since birth. The corrected age, also called adjusted age, is counted from the original due date instead:
Corrected age = chronological age − (40 weeks − gestational age at birth)
Tick "Born premature? Also show corrected age" in the calculator above and enter the gestational age at birth in weeks and days. The tool works out the original due date, then reports both ages side by side.
A baby born on 1 March 2025 at 32 weeks arrived 8 weeks early, so the original due date was 26 April 2025. On 1 September 2025 that baby is 6 months old chronologically but the corrected age is 4 months and 6 days. A health visitor comparing the baby against six-month milestones would see a delay that is not there; comparing against four-month milestones gives the accurate picture.
Clinicians typically correct for prematurity until around age 2, by which point most children born moderately preterm have caught up and the two figures matter less. Babies born extremely preterm are sometimes corrected for longer. Our corrected age calculator handles the same adjustment as a standalone tool.
This section explains how ages are counted, not what they mean clinically. Gestational age, growth and developmental milestones should always be discussed with your paediatrician, health visitor or midwife.
During infancy and early childhood, monthly age tracking is standard medical practice. Pediatricians, developmental specialists, and standardized assessment tools all use age in months rather than years because development happens so rapidly during this period.
Note: Developmental ranges are general guidelines. Always consult a pediatrician for individual assessment.