Baby age calculator
Enter your baby's date of birth and this tool gives their age in completed weeks, in completed months, and in total days, all at once. It shows both ways of converting weeks to months, because the two disagree by up to a week, and names which one health services actually use.
Baby age calculator
No sign-up · PrivateThe day your baby was born. The day of birth counts as day 0.
Leave blank for today. Fill it in if you are working out an age for a form, an appointment or a particular date.
Your baby is
as at 10 July 2026 — that is 0 days, or 0 days in total
In weeks: 0 days — 0 completed weeks
In calendar months (the method health services use): Not yet 1 month — 0 days old
In averaged months (total days ÷ 30.44): 0.0 months — the same age worked out the other way, because a month averages 4.345 weeks, not 4
In total days: 0 days
Date of birth: 10 July 2026
Worked out as at: 10 July 2026
Where this age sits in the NHS review schedule: Inside the window for the First weight check (10 July 2026 to 17 July 2026). Weighed at birth and again during the first week, mainly to check they are regaining their birthweight.
How often the NHS says a healthy baby should be weighed at this age: In the first 2 weeks your baby is weighed to check they are regaining their birthweight. That is a maximum, not a target.
Nothing you type leaves your device. The whole calculation runs in your browser.
How this is calculated
FormulaHow your baby's age is worked out
Age sounds like the one thing on this site that cannot be controversial. It is, and the argument is worth understanding, because it decides whether your baby is “3 months” or “13 weeks” on a day when both are true.
Step 1 — total days
total days = (as-at date − date of birth). The day of birth counts as day 0, so a baby born
yesterday is 1 day old. This is plain calendar arithmetic and no convention is involved.
Step 2 — completed weeks
completed weeks = floor(total days ÷ 7), with the remainder shown as days. A baby 7 days
old is 1 week; a baby 20 days old is 2 weeks and 6 days. Weeks are the unambiguous unit, which is why they are
used for everything clinical in the first months.
Step 3 — completed months, two different ways
This is where calculators disagree with each other, and the disagreement is real rather than a bug.
- Calendar months — count whole months from the same day number, then the leftover days. A baby born on 15 January is 1 calendar month old on 15 February, whatever February's length. This is the method this tool leads with, and the method used by health services when they invite you to a review.
- Averaged weeks —
months = total days ÷ 30.44, or equivalently weeks divided by 4.345. A month averages 4.345 weeks, not 4.
Both are shown. The gap between them reaches roughly a week by the first birthday, which is why “is my baby 3 months or 12 weeks” has no single answer: 12 weeks is 84 days, and 3 calendar months is 89 to 92 days depending on which months they are. Twelve weeks is not three months. The tool prints both rather than picking a winner.
Step 4 — what the age is used for
The tool maps the age onto the review points the NHS publishes: a weight in the first week, a review at 6 to 8 weeks, another at 9 to 12 months, and one at 2 to 2-and-a-half years. It also applies the NHS weighing frequency guidance — no more than monthly to 6 months, no more than every 2 months from 6 to 12 months, and no more than every 3 months over 1 — because a great deal of anxiety comes from weighing far more often than that.
What this tool deliberately does not do
It does not correct for prematurity. If your baby was born before 37 weeks, the age that matters for development and for growth charts is the corrected age, counted from the due date, and that is a separate calculation with its own rules about how long to keep applying it. The tool links you to it rather than guessing which age you wanted.
How old is my baby, exactly?
There is a stretch of early parenthood, somewhere between the first month and the first birthday, where you are asked your baby's age several times a day and never quite know which unit to answer in. The health visitor wants weeks. Your relatives want months. The soft play centre wants months. The clothing label wants months but means something else entirely by them. And the two units do not convert cleanly, which is the part almost nobody is told.
Weeks are the honest unit early on
For the first few months, weeks are what health services use, and there is a reason beyond habit. A week is always seven days. A month is 28, 29, 30 or 31, so the same baby ages at a different rate depending on which month they are passing through. When a difference of a few days genuinely matters — feeding, weight gain, the timing of a check — the unit that does not wobble is the one worth using.
This is why you will notice professionals switching units on you. Newborn feeding and weight are discussed in days and weeks. Development and vaccinations move to months. Clothing and nursery places move to months and then years. Nobody is being inconsistent; each unit is being used where it is least misleading.
The 12 weeks versus 3 months problem
Twelve weeks is 84 days. Three calendar months, depending on where in the year you are standing, is 89 to 92 days. So a baby who is 12 weeks old is not yet 3 months old, and will not be for another week or so. Most online converters quietly pick one method and present it as the answer. The gap widens as you go: by the first birthday, 52 weeks is 364 days and 12 calendar months is 365 or 366.
The practical consequence is small but real. If you are told to start something “at 3 months” and you count 12 weeks, you are about a week early. If a growth chart is plotted by month and you plot by week, the point lands in the wrong place. Neither is a disaster; both are avoidable by knowing which convention you are in.
What age is used for, and when it stops mattering
Age drives the routine review schedule. The NHS offers a weight check in the first week, a thorough physical examination and review at 6 to 8 weeks, a further review at 9 to 12 months covering language, learning, safety, diet and behaviour, and another at 2 to 2-and-a-half years. Those are offers, not deadlines, and being a few weeks either side of them is ordinary.
How often a baby should actually be weighed
The NHS is unusually specific here, and the specificity is protective rather than restrictive. If your baby is gaining weight and neither you nor your health visitor has concerns, they should be weighed no more than once a month up to 6 months, no more than once every 2 months from 6 to 12 months, and no more than once every 3 months after the first birthday. More frequent weighing does not produce more information; it produces more noise, because normal short-term fluctuation gets read as a trend.
This is worth saying plainly because the scales are the single biggest source of avoidable worry in the first year. A baby who has just fed, or has not just had a nappy change, weighs measurably more than the same baby an hour later. Weekly weighing turns that into a jagged line that looks like a problem and is not.
Premature babies count differently
If your baby was born before 37 weeks, there are two ages: the actual age from the birthday, and the corrected age counted from the due date. For development milestones and for plotting growth, the corrected age is the one that means something, because a baby born eight weeks early has had eight fewer weeks of growing. A tool that hands you a flat chronological age and says nothing about this is setting up a false alarm.
Corrected age is not applied forever. Guidance sets an age at which the two are allowed to converge, and it differs for growth charts and for developmental expectations. That is its own calculation and this tool points you at it rather than silently guessing.
Why clothing sizes make no sense
A clothing label reading “3-6 months” is a manufacturer's estimate of the age at which an average baby fits a garment of a particular length, and manufacturers do not agree with each other. Babies of identical age routinely wear sizes two bands apart, and that is not a sign of anything. Age bands on clothes carry no clinical meaning at all; the only thing that matters is whether the garment fits, and for sleepwear, that it fits well enough not to ride up.
What the number does not tell you
Age sets expectations, and expectations are where the anxiety lives. Babies acquire skills across wide windows, and the width of those windows is the normal finding rather than the exception. Two babies born the same day can be months apart on any given skill and both be entirely typical. Age tells you which review is due and roughly what to look out for. It does not tell you whether your baby is ahead or behind, because for most of the first two years those words do not describe anything measurable.
If you are worried about your baby's development, the age in weeks is not the thing to check. Speak to your health visitor or GP, who can look at the whole picture rather than a single date subtraction.
Sources
- Your baby's health and development reviews — NHS, accessed
- Your baby's weight and height — NHS, accessed
- WHO Child Growth Standards — World Health Organization, accessed
- UK-WHO growth charts, 0-4 years — Royal College of Paediatrics and Child Health, accessed
- Ages and stages: baby — American Academy of Pediatrics (healthychildren.org), accessed