Baby Age Calculator

Written byOleh Soloninupdated 25 September 2026

Date of birth

Measured on

Leave empty to use today.

Weeks at birth

Result

Age18 months and 21 days
Whole weeks81
Days since birth570
Corrected ageborn at term — no correction needed
Next month milestone2026-10-15
Days until it9

This result is informational only and is not professional advice.

Paediatrics counts in weeks, the calendar in months

Works out a baby's age in months, weeks and days, and gives a corrected age for babies born before term.

In the first year a baby's age is counted in weeks and in months, and the two scales do not line up: a month is not four weeks but about 4.35. By the first birthday the gap is more than four weeks — 52 weeks against 48 counting 'four to a month' — which shows when you read developmental charts.

Premature babies also have a corrected age: the weeks they arrived before 40 are taken off their actual age. A baby born at 32 weeks is 9 weeks old in corrected terms at four months — about two months — and development is judged by that figure.

How to use

Enter the date of birth.
Leave the 'on' date empty to count to today, or enter another date.
If the baby was born before 37 weeks, enter the gestational age at birth — the calculator will work out the corrected age.
The result gives the age in months and days, in whole weeks and in total days since birth, and the date of the next month-birthday.

Months and weeks over the first eighteen months

Three months is 13 weeks, six months 26, a year 52. Feeding, vaccination and development charts are often written in weeks, so the conversion is useful from the first months.

Born on 15 March 2025; each row is on the same date of the month in question. Every row comes from the calculator on this page.
Age	Full weeks	Days
1 month	4	31
3 months	13	92
6 months	26	184
9 months	39	275
12 months	52	365
18 months	78	549

Corrected age for a premature baby

The weeks short of 40 come off the actual age. At four months — 17 full weeks — a baby born at 32 weeks is developmentally a nine-week-old.

Age on 15 July 2025 for a birth on 15 March 2025. Every row comes from the calculator on this page.
Born at, weeks	Actual age	Corrected age
40	17 weeks	no correction needed
36	17 weeks	13 weeks
34	17 weeks	11 weeks
32	17 weeks	9 weeks
28	17 weeks	5 weeks

What this calculator shows

Age in months and days, in whole weeks, and in days since birth
The corrected age, if the baby was born before 37 weeks
The date of the next month milestone and the days until it

What to keep in mind

Months are counted by the calendar: from a date to the same date next month, not in 30-day blocks.
Corrected age is conventionally used until two years of age.

FAQs

How many weeks are in six months?

26 full weeks — 184 days from 15 March to 15 September. A calendar month averages 4.35 weeks, so six months is not 24 weeks.

How many weeks old is a one-year-old?

52 full weeks — 365 days. Counting four weeks to a month would give 48, a month short.

What is corrected age?

Age counted from the due date rather than the birth date. The weeks of prematurity come off the actual age: born at 32 weeks means minus eight weeks.

Until what age is corrected age used?

For judging development, conventionally until two. Vaccinations follow the actual age — that is for the paediatrician to decide.

When is a baby considered full term?

A baby born before 37 completed weeks is premature, and only then does the calculator correct the age. Full term proper is 39 weeks to 40 weeks 6 days; 37 and 38 weeks count as early term, but no correction is applied to them.

Why do clinics use months and charts use weeks?

Months suit appointments and vaccination schedules; weeks are more precise for development in the first year, when change is fast.

Worked example

Six months from birth

Input: Born 2025-03-15, measured on 2025-09-15

Output: 6 months exactly — 184 days and 26 whole weeks

Note: Six months is 26 weeks, not the 24 you would get counting four weeks to a month. Which is why it pays to use the unit the guidance itself uses: vaccination schedules mix weeks, months and years.

Related tools

Pregnancy Weight Gain Calculator

Compares actual weight gain against the IOM recommendations for your gestational week and your pre-pregnancy body mass index.

Age Calculator

Work out an exact age from a date of birth, including leap years. Also shows total days lived and the next birthday.

Useful links

All tools in Health calculators for weight, lab values and training