Child age calculator: How old is my baby in months and days?

Ask a new parent how old their baby is and you will rarely hear “zero”. Babies are counted in days, then weeks, then months — “she’s 14 weeks”, “he’s seven and a half months” — long before birthdays enter the picture. If you have searched for a child age calculator to answer “how old is my baby, exactly?”, this guide explains why the counting works that way, and how to convert any birth date into weeks, months and days without mistakes.

Why babies are counted in weeks and months

There are practical reasons the small units survive. In the first year of life a baby changes fast, and “3 months old” carries far more information than “0 years old”. Parents also meet the small units everywhere in daily life:

• Appointments and paperwork: clinic visits, registrations and forms in the early months are scheduled and recorded using the baby’s age in weeks or months, so parents fall into the habit of tracking it that way.
• Clothing and product sizes: baby clothes, nappies and many products are labelled by age bands — newborn, 0–3 months, 3–6 months, 6–12 months — so a parent needs the month count every time they buy anything.
• Everyday conversation: weeks are the natural unit for the first couple of months, months take over until about age two, and only then do most people switch to years (often years plus “and a half”).

None of this needs a different kind of maths — a child age calculator uses the same calendar arithmetic as any age calculation. It just reports the answer in the smaller units parents actually use. Our Age Calculator does this on its stat cards, which show the same age as total months and total days alongside the years–months–days answer.

Worked example: born 3 March 2026

Say a baby was born on 3 March 2026. How old are they today, 20 October 2026, in each unit?

Age conversions for a birth date of 3 March 2026, counted against the real calendar to 20 October 2026.

The method: count whole months first (3 March to 3 October is exactly 7 months), then the leftover days (3 October to 20 October is 17 days). For weeks, count total days (231) and divide by seven — 231 ÷ 7 = 33 weeks on the nose. Two more examples show how the numbers behave. A baby born on 14 July 2026 is 98 days old today: exactly 14 weeks, or 3 months and 6 days. And a baby born on 25 December 2025 is 299 days old today: 42 weeks and 5 days, or 9 months and 25 days — already closing in on the first birthday, when the counting usually switches to “one” plus months.

The counting schedule most parents follow

The same baby, the same calendar — only the unit people naturally choose changes as the months pass.

Weeks or months? Avoiding the classic mix-up

The one real trap in baby-age counting is treating a month as four weeks. Four weeks is 28 days, but calendar months run 28 to 31 days, so the two counts drift apart quickly: a baby who is 6 weeks old (42 days) is not yet a month and a half in calendar terms — from 3 March, 42 days lands on 14 April, which is 1 month and 11 days. The safe rule is the one every baby age calculator follows: count weeks by dividing total days by seven, count months on the calendar by the date of the month (the 3rd to the 3rd), and never convert one into the other with a shortcut. This article is about counting only — for anything about your child’s health or development, your clinician is the right source, not a calculator.

One more example: a toddler, and a sibling gap

The same counting works past the first birthday. A child born on 22 January 2025 is today 1 year, 8 months and 28 days old — 20 completed months in total, or 636 days (that is 90 weeks and 6 days, if anyone still asks). Most parents drop the weeks by this stage and quote “20 months” or “one and two-thirds”, but the month figure is still the one that clothing sizes and nursery paperwork tend to want.

Sibling questions use identical arithmetic. If an older child was born on 10 November 2024 and a baby sibling on 3 March 2026, the gap between them is 1 year, 3 months and 21 days — 478 days exactly. Subtract the two birth dates, and the “age gap” people quote is simply the older child’s age on the day the younger one was born.