How many weeks is 3 months old?

About 13 weeks, not 12. Three calendar months average 91 days and 12 weeks is only 84, so a baby hits 3 months roughly a week after they hit 12 weeks. Health visitors and growth charts use months, wake window guidance usually uses weeks, which is where the confusion comes from.

How many days is 3 months?

Between 89 and 92 depending on which months they fall in. There is no single answer, because calendar months are 28 to 31 days. The tool above counts the actual days from the date you enter.

How many days is 4 months?

Between 120 and 123. Again it depends on the months involved, which is why counting from the date of birth is more reliable than multiplying.

What is corrected age and when do I use it?

Corrected age subtracts the weeks a baby was born early from their calendar age. A baby born eight weeks early at six months old has a corrected age of about four months. Use it for sleep, feeding and developmental expectations through at least the first year, and for wake windows in particular.

Do twins use the same age?

They share a date of birth, so calendar age is identical. If they were premature, which is common with twins, corrected age is also the same. What often differs is where each baby sits developmentally, so treat the number as shared and the expectations as individual.

Why does my baby's age in months not match the week count?

Because months and weeks are different units and neither divides into the other cleanly. Twelve weeks is not three months, and 26 weeks is not quite six. The tool shows both so you can use whichever the guidance in front of you is written in.

Baby age in weeks and months (and corrected age)

Baby age converter

Date of birth

Weeks early (optional)

Worked out in your browser. Nothing you enter is sent anywhere.

Months and weeks are not the same unit

Before birth, months are defined as blocks of weeks. After birth, they go back to being calendar months, and the two conventions disagree. A baby born on 3 March is three months old on 3 June, which is 92 days, or 13 weeks and one day. Twelve weeks would have been 26 May.

That week is enough to matter when you are reading advice. Growth charts, vaccinations and health visitor reviews are scheduled in months. Wake windows, sleep schedules and feeding guidance are usually written in weeks. Using the wrong one puts you roughly a week out at three months and further out later.

Roughly, month by month

Age	In weeks	In days
1 month	about 4 weeks	28–31 days per month, 29–31 days total
2 months	about 9 weeks	28–31 days per month, 60–62 days total
3 months	about 13 weeks	28–31 days per month, 90–92 days total
4 months	about 17 weeks	28–31 days per month, 121–123 days total
5 months	about 22 weeks	28–31 days per month, 151–153 days total
6 months	about 26 weeks	28–31 days per month, 182–184 days total
7 months	about 30 weeks	28–31 days per month, 212–214 days total
8 months	about 35 weeks	28–31 days per month, 243–245 days total
9 months	about 39 weeks	28–31 days per month, 273–275 days total
10 months	about 43 weeks	28–31 days per month, 303–305 days total
11 months	about 48 weeks	28–31 days per month, 334–336 days total
12 months	about 52 weeks	28–31 days per month, 364–366 days total

Those are averages. The tool above uses the real dates, which is what you want for anything that matters.

Corrected age, and why it matters more with twins

If a baby was born early, corrected age is their age minus the weeks they arrived early. A baby born eight weeks premature has a corrected age of four months when their calendar age is six.

This is not a technicality for twins. Multiple pregnancies frequently deliver before 37 weeks, so a large share of twins have a corrected age that differs from their birthday age for the whole first year. Wake windows, nap counts and milestone expectations should all follow the corrected number. The corrected age calculator does this on its own if that is all you need.

Where the app fits

Two babies means two ages to convert, two sets of milestones and two growth curves, usually while holding at least one of them. Multibaby keeps each baby on its own track and shows both side by side, so the arithmetic stops being something you do from memory.

Quick answer

Baby age in months follows the calendar, not four-week blocks: a baby born on the 3rd turns 3 months old on the 3rd, three months later. That is why 3 months is about 13 weeks rather than 12, and why there is no fixed number of days in a month. Enter a date of birth below for the exact age in months, weeks and days, and add a prematurity offset to get corrected age.

How to use it

Enter the date of birth. Set your baby's date of birth. Everything is worked out in your browser and nothing is sent anywhere.
Read the age three ways. You get age in months and days, the equivalent in weeks, and the total number of days.
Add weeks early for corrected age. If your baby was premature, enter how many weeks early they arrived. Corrected age is the one to use for sleep, feeding and milestone guidance.

Frequently asked questions

How many weeks is 3 months old?: About 13 weeks, not 12. Three calendar months average 91 days and 12 weeks is only 84, so a baby hits 3 months roughly a week after they hit 12 weeks. Health visitors and growth charts use months, wake window guidance usually uses weeks, which is where the confusion comes from.
How many days is 3 months?: Between 89 and 92 depending on which months they fall in. There is no single answer, because calendar months are 28 to 31 days. The tool above counts the actual days from the date you enter.
How many days is 4 months?: Between 120 and 123. Again it depends on the months involved, which is why counting from the date of birth is more reliable than multiplying.
What is corrected age and when do I use it?: Corrected age subtracts the weeks a baby was born early from their calendar age. A baby born eight weeks early at six months old has a corrected age of about four months. Use it for sleep, feeding and developmental expectations through at least the first year, and for wake windows in particular.
Do twins use the same age?: They share a date of birth, so calendar age is identical. If they were premature, which is common with twins, corrected age is also the same. What often differs is where each baby sits developmentally, so treat the number as shared and the expectations as individual.
Why does my baby's age in months not match the week count?: Because months and weeks are different units and neither divides into the other cleanly. Twelve weeks is not three months, and 26 weeks is not quite six. The tool shows both so you can use whichever the guidance in front of you is written in.