Age Calculator
Age Calculator works out Age in years and Extra months from What to work out, Date of birth and Age at this date. The default figures on this page give age in years of 20 years; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
5 steps
- Born 2000-01-01 (a Saturday); measuring on 2020-01-01.
- Whole years first: 2000-01-01 + 20 years lands on or before 2020-01-01.
- Then whole months and days: + 0 month(s) + 0 day(s) reaches 2020-01-01.
- Elapsed days = 7,305, i.e. 1043 weeks and 4 day(s).
- Next birthday 2021-01-01 (Friday), 366 day(s) away; turning 21.
The same age in every unittable
Its 9 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
5 sources
What the Age Calculator calculates
Age in completed years, months and days on any date, with the total days lived, the weekday of birth, the next birthday and the gap between two birth dates; it also runs backwards to recover a date of birth from an age, breaks an age down into every time unit, and corrects an infant age for prematurity. Age counts completed calendar months and days, so a 29 February birth attains the next year of age on 1 March in common years.
How to use this Age Calculator
Enter What to work out, Date of birth, Age at this date and Second date of birth (optional). Age in years updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Age in years, Extra months and Extra days.
The method follows ISO 8601-1:2019, Date and time - Representations for information interchange, Howard Hinnant, chrono-Compatible Low-Level Date Algorithms, US Social Security Administration, Program Operations Manual System GN 00302.400 (age computation), Squires J. and Bricker D., Ages & Stages Questionnaires, Third Edition (ASQ-3) User’s Guide, Brookes Publishing and American College of Obstetricians and Gynecologists, Committee Opinion 700: Methods for Estimating the Due Date.
Born 2000-01-01, age on 2020-01-01 example
With Date of birth: 2000-01-01, Age at this date: 2020-01-01, the calculator returns Age in years: 20, Extra months: 0, Extra days: 0. This example is checked against an independent source in the test suite.
What you can compare
Change one input at a time to see how it affects Age in years, Extra months and Extra days. The inputs you can vary are What to work out, Date of birth, Age at this date, Second date of birth (optional), Age in years and Extra months of age.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How to find your age on a calculator?
Age in completed years, months and days on any date, with the total days lived, the weekday of birth, the next birthday and the gap between two birth dates. Open “How this is worked out” beside the result to see each step for your own figures.
How do I find out my age?
It needs What to work out, Date of birth, Age at this date, Second date of birth (optional), Age in years and Extra months of age. Nothing else is required, and the figures stay in your browser.
How do you calculate age in mm, dd, yyyy?
The result gives Age in years, Extra months, Extra days, Age, Chronological age (years;months) and Age in decimal years, each labelled with its unit so the figure can be read on its own.
What information do I need for the Age Calculator?
You need What to work out, Date of birth, Age at this date, Second date of birth (optional), Age in years, Extra months of age, Extra days of age and Gestational age at birth (weeks). The page opens with default figures that give age in years of 20 years, so you can see the scale before entering your own.
What does this Age Calculator show?
It shows Age in years, Extra months, Extra days, Age, Chronological age (years;months), Age in decimal years, Age in months and Age in weeks, with the working and any warnings for your entries.
Can I check how the Age Calculator result was calculated?
Yes. The “How this is worked out” section shows the calculation steps, and the sources section identifies the references used for the method.