About the Baby Age Calculator
Parents often count a baby's age in days, then weeks, then months. Check-ups, vaccines and milestones are all tied to age. This calculator takes the date of birth and the date to measure at, and gives the age in days, in weeks and days, and in completed calendar months and days. For a baby born early, it also gives a corrected age. Type the date of birth, the date to measure at and, optionally, the weeks born early and the baby age appears at once, with a few related figures underneath, and you can copy the result with one tap. The sums run in your browser, so nothing you enter is uploaded.
Updated October 2026
How to use the Baby Age Calculator
- 1Enter the date of birth, the date to measure at and, optionally, the weeks born early.
- 2Pick the units or options from the menus if the baby age tool offers them.
- 3Read the baby age in the large result box.
- 4Look at the extra rows for related figures.
- 5Press Copy result to keep the answer.
When you need the baby age
Health visitors and parents use ages in weeks for feeding and growth charts in the first months, and in months afterwards. Corrected age is used for babies born before 37 weeks, as their development follows the date they were due.
Doctors often use corrected age until a child is about two years old. If you are unsure of the number of weeks early, ask your health visitor or the hospital that cared for you.
How the baby age is worked out
Days = the number of days between the dates. Weeks = days ÷ 7. Months are counted on the calendar. Corrected age = age − the weeks born early. A baby born on 10 January 2026 is 90 days old on 10 April 2026: 12 weeks and 6 days, or exactly 3 months. A baby born 8 weeks early on 1 January and measured on 1 May is 17 weeks old by the calendar, but 9 weeks by corrected age.
Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.
- Days, weeks and months
- Corrected age
- Any two dates
Mistakes to avoid with baby age
The tool counts calendar months, so the number of days in a month depends on the dates. A baby born on the 31st has its monthly birthday on the last day of shorter months. This is a counting tool, not a developmental assessment.
On phone and computer
On a phone, type into the boxes one after another and read the baby age underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the baby age move. The baby age calculator works the same on both. Milestones are easier to plan with a few dates. Use Age Calculator for adult ages, Pregnancy Due Date Calculator before the birth, Days Between Dates for any gap, and Ovulation Calculator for family planning.
Features
- Use the due date for early babies
- Calendar months vary
- Ask your doctor about development
- Clear messages when the baby age inputs make no sense
- Copy the result with one tap
- Private: nothing is uploaded
Frequently asked questions
How do I calculate baby age?
Days = the number of days between the dates. Weeks = days ÷ 7. Months are counted on the calendar. Corrected age = age − the weeks born early. A baby born on 10 January 2026 is 90 days old on 10 April 2026: 12 weeks and 6 days, or exactly 3 months. A baby born 8 weeks early on 1 January and measured on 1 May is 17 weeks old by the calendar, but 9 weeks by corrected age.
What is the formula for baby age?
Days = the number of days between the dates. Weeks = days ÷ 7. Months are counted on the calendar. Corrected age = age − the weeks born early. The baby age calculator applies it exactly and rounds only the displayed answer to six significant figures.
What is corrected age?
The age counted from the baby's due date instead of the birth date. It is used for babies born early.
How many weeks is a 3-month-old baby?
About 12 or 13 weeks.
Until when should I use corrected age?
Usually until about two years, but follow your doctor's advice.
Is this baby age calculator free and private?
Yes. It is free, needs no account and runs in your browser, so the numbers you type are not uploaded or stored.