CALCULATE A BABY’S AGE
Calculator inputs and results
Enter a birth date to calculate a baby’s age in calendar months, weeks and days. If the baby was born early, add the due date to view corrected age; milestone ranges are general references, not developmental assessments.
How to use this calculator
What the result includes
- Age in months, weeks & days
- Corrected age for preemies
- First-year month-dot track
- General milestone windows
- Live first-birthday countdown
Why babies are counted in months 👶
In the first year, weeks and months matter more than years — development moves fast. "5 months" is counted in calendar months from the birth date (born March 12 → 5 months on August 12), while total weeks and days track the finer detail. This is also how checkup schedules are organized: 2 weeks, 2 months, 4 months, 6 months, 9 months, 12 months.
Corrected age for early arrivals 🍼
Babies born before 37 weeks are often measured against their due date rather than their birth date — the "corrected" age — because they had less time developing in utero. Pediatricians commonly use corrected age for developmental expectations until around age 2. This calculator shows the corrected figure when you flag an early arrival, as information; your pediatrician guides what applies for your baby.
The general first-year windows
| Skill | Typical range |
|---|---|
| Social smile 😊 | ~6–8 weeks |
| Rolling over 🔄 | ~3–5 months |
| Sitting with support 🪑 | ~4–6 months |
| Sitting alone | ~6–8 months |
| Crawling 🐛 | ~7–10 months |
| First words 🗣️ | ~9–14 months |
| Walking 🚶 | ~12–18 months |
Ranges are wide on purpose. A baby walking at 10 months and one walking at 17 are both completely normal. Milestone checklists are population curves, not race tracks — and any concern is exactly what pediatrician visits are for.
The month-by-month deep dive lives in Your Baby's First Year, and when the cake era begins in earnest, the Birthday Countdown keeps score. 🎂
Sources & limits
Corrected age is general orientation, while developmental milestones require individual clinical context.
Sources accessed: August 8, 2026.