Free Age Calculator
Exact age in years, months, and days between two dates.
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Inputs
Result
'Age as of' must be on or after the birth date.
Estimates only. Verify with a professional for consequential decisions.
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Short answer
Age equals the elapsed time between a birth date and a reference date, decomposed into calendar years, months, and remaining days.
What is the Age Calculator?
Age is the duration between someone's birth and a chosen reference moment. Calendar arithmetic is variable-length because months and years differ in day count.
How does the Age Calculator work?
Subtract calendar fields (year, month, day) individually. When the day or month subtraction is negative, borrow from the next higher unit using the previous month's day count.
Formula
Variables
yFull calendar years elapsedmAdditional months beyond the last full yeardAdditional days beyond the last full month
Explanation
Borrow days from the previous month and months from the previous year when arithmetic goes negative, mirroring how humans count age.
Examples
Example 1: Born 1990-06-15, today
34 years, 5 months, 5 days elapsed.
Applications
- Legal age verification
- Insurance underwriting
- Pediatric dosing charts
- Retirement planning
Advantages
- Handles leap years and variable month lengths
- Also reports total days for use in scientific contexts
Limitations
- Does not adjust for time zones or hours within a day
Common mistakes
- Ignoring the borrow when days-difference is negative
- Assuming every month has 30 days
Tips
- Use ISO 8601 (YYYY-MM-DD) to avoid ambiguity
Related concepts
The Age Calculator sits inside the Date & Time Calculators hub, in the date differences & durations cluster. Measuring the span between two dates. Understanding the terms below makes the output easier to interpret and easier to compare against neighbouring measures.
Practical use cases and industry applications
Understanding Age
Age is the duration between someone's birth and a chosen reference moment. Calendar arithmetic is variable-length because months and years differ in day count. Within date & time calculators, age belongs to the date differences & durations cluster, where it shares terminology and assumptions with closely related tools.
Learning how age is calculated
Borrow days from the previous month and months from the previous year when arithmetic goes negative, mirroring how humans count age. Working through the variables one at a time — y, m, d — makes the result reproducible by hand and easier to sanity-check.
Using age to make a decision
Legal age verification Insurance underwriting Pediatric dosing charts Because outputs depend on the assumptions you enter, run more than one scenario before committing to a figure.
How age compares with related measures
duration, span, inclusive counting, deadline all describe adjacent aspects of date differences & durations. Comparing this calculator's output against those measures — using the related tools listed on this page — prevents a single metric from being read in isolation.
Frequently asked questions
How is 'age in months' defined?
Full calendar months elapsed since the birth date. Any remaining days beyond the last full month are not counted as an additional month.
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