Baby Age in Months & 100-Day Calculator
From the date of birth, see how many months and days old your baby is, which day it is, the dates of day 50, day 100, day 200 and the first birthday, and the corrected age for babies born early.
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Baby Age in Months & 100-Day Calculator: complete guide
Walk through the steps and examples, or jump straight to the topic you need.
At a glance
- Months
- A month is complete when the same date as the birth date comes around (Korean Civil Act Article 160). If the month has no such date, it completes on the 1st of the next month.
- Days
- Days since birth = as-of date − birth date, and day N = days since birth + 1 (the birth day is day 1).
- Milestones
- Day 50 = birth date + 49 days, day 100 = + 99, day 200 = + 199, day 300 = + 299, day 1000 = + 999, and the first birthday is the first anniversary of birth.
- Corrected age
- If you enter the due date and the baby was born before 37 weeks 0 days, the corrected months and weeks are counted as as-of date − due date.
- Scope
- This is a date calculation, not an assessment of growth or development. Dates you enter aren’t saved or sent.
How to use Baby Age in Months & 100-Day Calculator
Choose the date of birth
Type a date in the date of birth field or pick it from the calendar. The result appears as soon as you choose.
Change the as-of date if needed
The as-of date starts at today. If you need the age on a specific day, such as a vaccination or checkup, enter that date.
Enter the due date for a baby born early
If you enter the due date (optional), the gestational age at birth and the corrected age are shown too. Babies born at 37 weeks 0 days or later get no separate calculation.
Check the age and milestone days
Check the age in months, the day count and the age in weeks, plus the dates and D-days of day 50, day 100, day 200, day 300, the first birthday and day 1000, and copy them with Copy result.
What it’s good for
A month is complete when the same date comes around
A baby born on September 12 is 1 month old from October 12 and 2 months old from November 12. Months follow the calendar date rather than blocks of 30 days (Korean Civil Act Article 160). If the month has no such date, as for a baby born on January 31, the month completes on the 1st of the next month.
Day 100 counts the birth day as day 1
Because the birth day is day 1, day 100 is the birth date + 99 days, day 50 is + 49 days and day 200 is + 199 days. That is why day N is one more than the days since birth (as-of date − birth date).
Corrected age for babies born early
If your baby was born before 37 weeks 0 days, the tool also shows the corrected age, which subtracts the days born early from the actual age. Ask your baby’s doctor how long to use the corrected age.
Worked examples
A baby 48 days after birth
Date of birth 2026-09-12
As of 2026-10-301 mo 18 d · day 49 · 6 wk 6 d
48 days since birthDay 50, day 100 and day 200
Date of birth 2026-09-12Day 50 2026-10-31 · day 100 2026-12-20
Day 200 2027-03-30 · first birthday 2027-09-12A baby born at 34 weeks 0 days
Date of birth 2026-09-12 · due date 2026-10-24
As of 2026-12-203 mo 8 d
Corrected age 1 mo 26 d (8 wk 1 d)Ways to count age and days
Swipe the table sideways to see the rest.
| Format | Calculation | When to use |
|---|---|---|
| N months old | 1 month each time the same date comes around | When reading vaccination, checkup or weaning schedules |
| Day N | As-of date − birth date + 1 | When counting milestone days such as day 50, day 100 and day 200 |
| Corrected age | As-of date − due date | When reading development guides for babies born before 37 weeks |
When the result isn’t what you expected
"The date of birth is after the as-of date."
You entered a future date of birth or moved the as-of date before the birth. Check both dates.
"The due date is more than 20 weeks after the birth date."
Check that you didn’t enter a date in a different year. Due dates up to 20 weeks after the birth date are accepted.
Day 100 differs by one day from another calculator
Adding 100 days to the birth date gives a date one day later. This tool counts the birth day as day 1.
Key terms
- Day N
- The day count with the birth day as day 1. Day 100 is the hundredth day.
- Corrected age
- The actual age minus the days the baby was born before the due date.
- Full term
- Pregnancy from 37 weeks 0 days onward.
Baby Age in Months & 100-Day Calculator FAQ
Isn’t day 100 the birth date plus 100 days?
No. Because the birth day counts as day 1, the birth date + 99 days is day 100. For a baby born on September 12, 2026, day 100 is December 20.
Is the date of birth I enter saved?
It isn’t saved or sent. Everything is calculated in this browser.
When is a baby born on January 31 one month old?
February has no 31st, so the month ends on the last day of February, and the baby is counted as 1 month old from March 1.
Is the first birthday day 365?
No. It is the first anniversary of birth. If a leap day falls in between, it is day 366.