# Baby Age Calculator: Age in Days, Weeks and Months _Your baby's exact age from the birth date, the next milestone days, and what is typical for sleep and teeth at that age._ - Source: https://proba.baby/en/tools/baby-age-calculator - Language: English - Last verified: v1.0.4 · 7 Sep 2026 - Last updated: 2026-10-11 ## How it works Enter your baby's date of birth. The calculator shows the age in the form people actually say it — days for the first two weeks, weeks and days up to eight weeks, then months and days — together with the total in days, in weeks and days, and in calendar months and days. Change the second date to see the age on a check-up, a photo day or a birthday. Below the age it lists what is coming up: 1 month, 100 days, the half birthday, the first birthday, 500 days, 18 months, 1,000 days and the second birthday, each with its date and how many days away it is. It also gives the date of the next CDC milestone checklist age, so you know when to look at what most babies do by then. For babies under two it adds a snapshot of the age: the wake window and nap count from the ProbaBaby app's sleep bands, and how many teeth are typical by the ADA eruption chart. If your baby was born before 37 weeks, milestones and growth are read by corrected age instead; the corrected age calculator works that out. Nothing you enter is uploaded. The dates live in the page address, so you can bookmark the result or send it to a partner, and it updates every day you open it. ## The math Days old = the number of calendar days from the birth date to the chosen date; the day of birth is day 0. Weeks = days ÷ 7, rounded down, with the leftover days shown beside them. Months are calendar months, not 30-day blocks. A month is complete on the same day of the next month, or on that month's last day when it is shorter: a baby born on 31 January is 1 month old on 28 February (29 in a leap year), and a baby born on 29 February turns 12 months on 28 February the next year. The leftover days are counted from that point. Worked example. Born 14 March 2026, on 11 October 2026: 211 days old, 30 weeks and 1 day, and 6 months and 27 days — 6 months were completed on 14 September, and 27 days have passed since. Milestone days are the same arithmetic forwards: 100 days old on 22 June 2026, the first birthday on 14 March 2027, 1,000 days old on 8 December 2028. This is the ProbaBaby app's own age engine, checked against the app's output for named cases and for 471 birth-date and day pairs around leap-year Februaries and month ends. The sleep snapshot uses the app's nine sleep bands and the teeth line the app's tooth chart, both read at the age in days. ## FAQ **How do I work out my baby's age in months?** Count whole calendar months from the birth date: a month is complete on the same date of the next month. Born on 14 March, your baby is 6 months old on 14 September, and on 11 October is 6 months and 27 days. When the next month is shorter, the month completes on its last day — born on 31 January, a baby is 1 month old on 28 February. **How many weeks old is a 3-month-old baby?** About 13 weeks. Three average months are 91.3 days, which is 13.0 weeks; by the calendar it is 89 to 92 days depending on which months they are. The tables below convert every week up to 52 and every month up to 24. **Why do apps and doctors sometimes give a different age?** Because there are two conventions. Some count weeks and convert with an average month of 30.4 days; others count calendar months from the birth date, as this calculator does. The two can be a few days apart. For a baby born early, clinicians may also use the corrected age for milestones and growth. **Should I use corrected age for my premature baby?** For milestones and growth charts, yes, until 24 months if your baby was born before 37 weeks — that is how paediatricians track a baby born early, and it is not a sign of delay. Vaccinations are the exception: they follow the age from birth. Our corrected age calculator works out both. **When is my baby 100 days old?** On the birth date plus 100 days, counting the day of birth as day 0. Born on 14 March 2026, a baby is 100 days old on 22 June 2026. The calculator lists the date, and the 500 and 1,000 day marks, under the age. **What are the CDC milestone checklist ages?** 2, 4, 6, 9, 12, 15, 18, 24, 30 and 36 months, among others up to 5 years. Each checklist lists what about 75% or more of children do by that age, a level the CDC chose so that even one missing item is worth acting on: if your baby is not doing one or more of them, or has lost a skill, talk to your paediatrician rather than wait and see. **Does the age change if I calculate it in another time zone?** No. The calculator works in whole calendar days, using the date on your device, so the age only changes at midnight where you are. If your baby was born in another time zone, enter the birth date on the birth certificate. ## Sources - CDC — Learn the Signs. Act Early.: CDC's Developmental Milestones — Milestone checklists from 2 months to 5 years; if a child loses skills or misses milestones, act early: https://www.cdc.gov/act-early/milestones/index.html - CDC — Key Points about CDC's Developmental Milestone Checklists — Milestones placed where about 75% or more of children would be expected to show them: https://www.cdc.gov/act-early/milestones/key-points.html - AAP HealthyChildren.org — Corrected Age for Preemies — Corrected age for developmental goals through the first 2 years: https://www.healthychildren.org/English/ages-stages/baby/preemie/Pages/Corrected-Age-For-Preemies.aspx - WHO — Multicentre Growth Reference Study (Child Growth Standards) — Growth standards read at the child's age, birth to 5 years: https://www.who.int/tools/child-growth-standards/who-multicentre-growth-reference-study - American Dental Association (MouthHealthy) — Eruption Charts: primary teeth — The tooth windows behind the teeth line: https://www.mouthhealthy.org/all-topics-a-z/eruption-charts - World Health Organization — 2019 guidelines on physical activity, sedentary behaviour and sleep for children under 5 — Sleep per day by age, behind the sleep line: https://www.who.int/news/item/24-04-2019-to-grow-up-healthy-children-need-to-sit-less-and-play-more --- ProbaBaby (listed on the App Store as “Pregnancy & Baby Tracker - PB”) is a free iPhone, iPad and Apple Watch app for pregnancy, newborn and postpartum tracking, built for twins, in 32 in-app languages, with an optional AI baby-face prediction that computes Mendelian genetics (Punnett squares) from the traits you enter before any image is generated. Tracking, tools, PDF reports and family sharing are free; only AI predictions use credits — 5 starter credits cover your first Scientific prediction — and no subscription is required. Requires iOS 26 or later. Not available on Android.