Age Calculator vs Gestational Age Calculator
The age calculator and the gestational age calculator both answer “how old is someone?” — but they count from different starts and report in different units. The age calculator counts completed years, months, and days from a date of birth to any as-of date, the way a birthday, school cutoff, or sports-eligibility rule expects. The gestational age calculator counts pregnancy age in completed weeks and days from a dating start — last menstrual period, conception date, or ultrasound — the way prenatal care expects. Both are calendar-aware date arithmetic on date-only Gregorian inputs, and the choice between them comes down to whether the start date is a birth or a pregnancy dating event.
What each calculator does
The age calculator takes a birth date and an as-of date, verifies the birth date is not after the as-of date, and counts completed years, remaining months, and remaining days. It borrows across real month lengths when day differences go negative and treats February 28 as the anniversary of a February 29 birthday in non-leap years. It also reports total months, total weeks, total days, the weekday of birth, and the countdown to the next birthday. The primary result is a years-months-days reading such as 35 years, 0 months, 28 days.
The gestational age calculator takes a dating basis and a calculate-as-of date. In last-menstrual-period mode it counts calendar days from the LMP date; in conception mode it subtracts 14 days from the conception date to create a dating start; in ultrasound mode it converts the scan’s weeks and days to days and adds elapsed days since the scan. The result is completed weeks plus days — for example, 63 days from January 1 to March 5, 2026 is 9 weeks, 0 days — along with an estimated due date 280 days after the dating start, an estimated conception date 14 days after it, trimester, milestone dates, and suggested appointment timing. The calculator rejects future dating inputs and ages above 42 weeks.
Side-by-side comparison
| Feature | Age calculator | Gestational age calculator |
|---|---|---|
| Start point | Date of birth | Dating start (LMP, conception, or ultrasound) |
| Primary output | Years, months, days | Completed weeks and days |
| Extra outputs | Total months/weeks/days, weekday born, next birthday | Due date, conception date, trimester, milestones |
| Unit convention | Calendar years and months | Weeks and days, capped at 42 weeks |
| Input rule | Birth date must not be after as-of date | Dating input must not be after as-of date |
| Leap-day handling | Feb 28 anniversary convention | Calendar-day arithmetic |
When to use which
Use the age calculator when the start is a birth and the question is ordinary calendar age: birthday planning, school cutoffs, sports eligibility, records, benefits, and age-in-days milestones. Use the gestational age calculator when the start is a pregnancy dating event and the question is prenatal timing: how far along a pregnancy is, when the due date falls, and when common milestones and appointments land. After a birth, the two scales diverge — a full-term newborn is about 40 weeks by gestational dating and zero days by calendar age — and for premature infants, clinicians may track development by a corrected age derived from the dating start rather than the birthday. Neither tool decides which definition a school, league, insurer, or health professional uses; both pages are educational date arithmetic, and pregnancy dates should be confirmed with a qualified healthcare professional.
Informational note: This page is an educational comparison, not medical or eligibility advice. Organizations define age cutoffs and effective dates by their own rules, and pregnancy dating should use the best obstetric estimate confirmed with a clinician.