How baby months are counted
Here's the confusion behind every "how many months is my baby" search: a calendar month isn't four weeks. It averages about 30.4 days, or roughly 4.35 weeks. So four weeks is a little under a month, and the gap grows as your baby gets older — by 16 weeks you're at about 3.7 calendar months, not a clean 4.
Many parents, apps and sleep charts use a shorthand "month" of exactly four weeks, which is why you'll see 16 weeks called "4 months". Neither is wrong — they're just two different rulers. This calculator shows both so you always know which one you're looking at.
Weeks to months chart
Common week counts in calendar months and the four-week shorthand:
| Weeks | Calendar months | Months + weeks | Four-week “months” |
|---|---|---|---|
| 4 weeks | 0.9 | 0m 4w | 1 |
| 6 weeks | 1.4 | 1m 2w | 1.5 |
| 8 weeks | 1.8 | 1m 4w | 2 |
| 10 weeks | 2.3 | 2m 1w | 2.5 |
| 12 weeks | 2.8 | 2m 3w | 3 |
| 16 weeks | 3.7 | 3m 3w | 4 |
| 20 weeks | 4.6 | 4m 3w | 5 |
| 24 weeks | 5.5 | 5m 2w | 6 |
| 26 weeks | 6 | 5m 4w | 6.5 |
| 30 weeks | 6.9 | 6m 4w | 7.5 |
| 36 weeks | 8.3 | 8m 1w | 9 |
| 40 weeks | 9.2 | 9m 1w | 10 |
| 44 weeks | 10.1 | 10m 1w | 11 |
| 48 weeks | 11 | 11m 0w | 12 |
| 52 weeks | 12 | 11m 4w | 13 |
Calendar months use 365.25 ÷ 12 ≈ 30.4 days per month; the four-week column is simply weeks ÷ 4.
Corrected age for premature babies & twins
PooPeeMilk is built twins-first, and twins often arrive early. If your baby was born before 37 weeks, milestones are usually tracked by corrected (adjusted) age: your baby's age minus the weeks they were born early. A baby born at 32 weeks who is 16 weeks old has a corrected age of about 8 weeks. Corrected age is conventionally used until around age 2, after which most children have caught up.
Baby age FAQs
How many months is my baby in weeks?
Divide weeks by about 4.35 to get calendar months, since a month averages 30.4 days, not 28. For example, 16 weeks is about 3.7 months. If you use the four-week shorthand instead (weeks ÷ 4), 16 weeks reads as 4 months — both are common, they're just different ways of counting.
Why is 16 weeks not exactly 4 months?
Because a calendar month is longer than four weeks. Four months on the calendar is about 17.4 weeks, so at 16 weeks your baby is roughly 3 months and 3 weeks — just shy of 4 months. The "16 weeks = 4 months" idea comes from counting each month as a flat four weeks.
What is corrected age?
Corrected (or adjusted) age is your baby's age minus the number of weeks they were born early, used for babies born before 37 weeks. It answers "how old would my baby be if they'd arrived on their due date?" and gives a fairer read on development and growth for preemies.
How long do you use corrected age?
Corrected age is typically used until around age 2. Most premature babies catch up to their birthday age by then, so after about 24 months their corrected and actual ages are treated the same.
Should I track my baby's age in weeks or months?
Weeks are handy in the early months, when a lot changes week to week and many guides (feeding, sleep, wake windows) are organised by week. Months become the more natural unit later on. This calculator shows both so you can use whichever a particular chart or pediatrician asks for.
PooPeeMilk tracks every feed, poo, pee and nap in one tap — and saves the milestones and memories worth keeping. Twins supported, no spreadsheet.
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