How to use
- Enter the date of birth. Today is filled in automatically; you can change it.
- For a premature baby, tick Born early and enter the weeks of pregnancy at birth.
- Read the age, day number, 100th day and first birthday.
Options explained
- 100th day
- The day of birth counts as day 1, so the 100th day is 99 days after birth — the date celebrated in many Asian cultures.
- Corrected age
- Actual age minus the weeks born early (40 − weeks at birth). Doctors use it for premature babies' growth and milestones, usually until age 2.
Worked examples
The day of birth counts as day 1, so the 100th day is 99 days later. A baby born on 1 March 2026 has the 100th day on 8 June 2026.
Corrected age subtracts the weeks the baby was early from the actual age:
weeks early = 40 − weeks of pregnancy at birth
corrected age = actual age − weeks earlyA baby born at 32 weeks was 8 weeks early. When that baby is 20 weeks old, the corrected age is 12 weeks, about 3 months. The tool shows the actual and the corrected age side by side so you can tell which one a milestone or growth chart refers to.
Frequently asked questions
Are vaccines given by actual or corrected age?
Usually by actual age; corrected age is for growth and development. Follow your clinic's advice.
How are months counted for babies born on the 31st?
In shorter months the last day of the month counts as the monthly birthday.
Is my data uploaded?
No. Calculations run in your browser.
Privacy
This tool runs entirely in your browser. Your files and text are processed on your device and are never uploaded to our servers.