Baby Age Calculator

1. Why babies are measured in weeks, then months

For roughly the first three months, a baby's age is quoted in weeks; after that it shifts to months, and somewhere around two years it becomes years and months like everyone else's. This is not arbitrary. In early infancy, a fortnight is a large fraction of a lifetime and a genuinely large developmental difference, so weeks are the smallest unit that stays informative without being noise.

Weeks are also unambiguous. A week is always exactly seven days, whereas “two months old” could mean 59, 60, 61 or 62 days depending on which months they were. That ambiguity is why this calculator shows age in all three units at once — completed days, weeks plus leftover days, and completed calendar months plus leftover days — rather than converting between them with a fixed multiplier.

2. Corrected age for a baby born preterm

A baby born before 40 weeks of gestation has two ages that both matter. Chronological age is time since birth. Corrected age — also called adjusted or postmenstrual-derived age — is chronological age minus the number of weeks the baby was born early. It answers the question “where would this baby be if the pregnancy had gone to term?”, which is the fairer basis for comparing development.

Enter a gestational age below 40 weeks above and the calculator adds the corrected figure. The arithmetic is deliberately simple and transparent:

Corrected age is conventionally used until about two years, and up to three for babies born extremely preterm or where a team is still following development closely. It is not used indefinitely — at some point the gap stops being meaningful and the child is simply their own age. This calculator stops reporting a corrected figure after three years for that reason. When the transition happens for a particular baby is a clinical judgement, not a formula.

3. Which age to use for milestones, feeding and vaccinations

The two ages are used for different things, and mixing them up is the most common source of unnecessary worry:

• Developmental milestones — use corrected age for a preterm baby. Rolling, sitting, babbling and walking track brain maturity, which follows the due date more closely than the birth date.
• Growth charts — preterm babies are usually plotted on a preterm reference (such as Fenton or INTERGROWTH-21st) until term-equivalent, then on the standard WHO or national chart using corrected age for the first one to two years.
• Immunisations — routine schedules are normally given according to chronological age, not corrected age, so a preterm baby is vaccinated on the same timetable as a term baby of the same birth date. Confirm the schedule with the clinic holding the child's record; national programmes differ in detail.
• Introducing solids — guidance is generally given around six months, and for preterm babies it is individualised. This is a conversation with a health professional rather than a calculation.

This tool computes ages. It does not give medical advice, and nothing here replaces the judgement of the paediatrician, neonatal team or health visitor who knows the child.

4. Developmental stage labels and what they do not mean

The stage shown alongside the age uses the conventional descriptive bands: neonate for the first 28 days, infant to the first birthday, toddler to three, preschooler to five, then school age. These labels are terminology, not assessment — they describe how old the child is, not how they are doing.

The first 28 days matter enough to have their own word because neonatal physiology is genuinely different: feeding patterns, jaundice risk, weight loss and regain, and temperature regulation all belong to that window. After it, the pace of change slows enough that months become the useful unit.

For a preterm baby the stage shown follows chronological age, since that is what the terminology conventionally tracks. Read it alongside the corrected age rather than instead of it.

5. Getting the dates right

Two small things cause most of the errors people run into:

• Gestational age is weeks plus days. A birth recorded as 32+4 means 32 weeks and 4 days. This calculator takes whole weeks, so enter 32 — the extra days shift corrected age by less than a week and are usually recorded separately anyway.
• The day of birth is day zero. A baby born today is 0 days old, not 1 day old, and turns 1 week old on the seventh day after birth. That convention is used consistently here.

Everything runs locally in your browser: no birth date, due date or gestational age is transmitted, logged or stored. Once your child is past the weeks-and-months stage, the child age calculator keeps the age-in-months figure but leads with years for school forms, and before the birth the gestational age calculator works in the weeks-plus-days notation maternity records use. When you want the full picture — totals, birthday countdown, the weekday they were born — the advanced age calculator covers it, and the chronological age calculator is the one to use for assessment paperwork.