Quick answer: A month averages 4.35 weeks (30.44 days ÷ 7), so months and weeks never line up exactly. Four weeks is slightly less than one month.
How is 4.35 weeks per month calculated?
A year has about 365.25 days. Divide by 12 months to get 30.44 days per month, then divide by 7 days per week to get about 4.35 weeks per month.
Months to weeks and days table
| Months | Weeks (approx.) | Days (approx.) |
|---|---|---|
| 1 | 4.35 | 30 |
| 2 | 8.7 | 61 |
| 3 | 13 | 91 |
| 4 | 17.4 | 122 |
| 6 | 26 | 183 |
| 9 | 39 | 274 |
| 12 | 52 | 365 |
When should you count age in weeks, months or years?
- 0–12 weeks: count in weeks.
- 3–24 months: count in months.
- 2 years and up: count in years.
When is a baby exactly 1 month old?
A baby turns 1 month old on the same date the next month, so a baby born on the 10th turns 3 months on the 10th, three months later. If the date does not exist (for example, born on January 31), most people use the last day of the month.
Key takeaways
- An average month is 30.44 days or 4.35 weeks.
- Calendar months range from 28 to 31 days.
- Weeks are more precise for newborns, months for older babies.
Frequently asked questions
How many weeks are in 3 months?
Three months is about 13 weeks.
How many days are in a month for baby age?
An average month is about 30.44 days, but calendar months range from 28 to 31 days.
Why do doctors use weeks for newborns?
Babies change quickly in the early weeks, so weeks give a more precise picture of growth and development than months.
Written by the Baby Age Calculator editorial team. Last updated October 28, 2026. General information only, not medical advice. See our medical disclaimer.