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Baby Age Calculator

Age in Weeks, Months and Corrected Age

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How the Baby Age Calculator works

Enter your baby's birth date and this baby age calculator shows exactly how old they are in months and days, weeks and days, and total days. Add the original due date for a baby born early and it also works out corrected age, which is what health visitors use to check development.

How the baby age calculator works

Baby age is counted in weeks for the first few months and then in months, so the calculator shows both. Months are counted by calendar date, so a baby born on the 14th turns another month old on the 14th of each month. For premature babies, corrected age takes off the weeks they were born early. The formula is corrected age = actual age − (due date − birth date). Use corrected age for milestones, growth charts and weaning until about age two.

First example. A baby born on 14 March 2026 is 6 months 17 days old on 1 October 2026, which is 28 weeks 5 days or 201 days. Born 4 weeks early at 36 weeks, the corrected age is 5 months 20 days.

Second example. A baby born on 1 January 2026 is exactly 9 months old on 1 October 2026, which is 39 weeks or 273 days.

How many weeks are in each month of a baby's life? About 4.35 on average, so a 3-month-old is roughly 13 weeks and a 6-month-old about 26 weeks.

Leave the "age on" date blank to use today, or set a future date to see how old your baby will be at a holiday, a jab appointment or a first birthday party.

Baby age in weeks converted to days and months

Weeks Days Months (approx.) Days until 1,000 days old
2 14 0.5 986
4 28 0.9 972
6 42 1.4 958
8 56 1.8 944
10 70 2.3 930
12 84 2.8 916
14 98 3.2 902
16 112 3.7 888
18 126 4.1 874
20 140 4.6 860
22 154 5.1 846
24 168 5.5 832
26 182 6.0 818
28 196 6.4 804
30 210 6.9 790
32 224 7.4 776
34 238 7.8 762
36 252 8.3 748
38 266 8.7 734
40 280 9.2 720
42 294 9.7 706
44 308 10.1 692
46 322 10.6 678
48 336 11.0 664
50 350 11.5 650
52 364 12.0 636

FAQ

How do I work out my baby's age in weeks?

Count the days since birth and divide by 7. The calculator does this for you and shows the leftover days, so a baby of 45 days is 6 weeks and 3 days old, which matches how health visitors record it.

What is corrected age?

Corrected or adjusted age is a premature baby's age counted from the due date rather than the birth date. A baby born 8 weeks early who is 6 months old has a corrected age of about 4 months.

When should I stop using corrected age?

Most health services use corrected age until about 2 years old. By then most premature babies have caught up, though very early babies may be followed for longer by their neonatal team.

Should vaccinations use corrected age?

No. Immunisations follow actual age from the birth date, even for premature babies. Corrected age is only used for development checks, growth charts and deciding when to start solid foods.

What is the 1,000 days milestone?

The first 1,000 days, from conception to age two, are often called the most important for growth and brain development. Counting from birth, 1,000 days old falls at about 2 years and 9 months.

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