# What is my birth year from my age?

Works out the year someone was born from their age and today’s date (or any date), and whether their birthday has come yet that year; if you are not sure, it gives both possible years.

- Page: https://www.acalculator.org/days/birth-year-calculator
- JSON spec: https://www.acalculator.org/days/birth-year-calculator.json
- Version: c333bd1c6f9e

## Default answer

Example with the default inputs (Age 16, On date October 19, 2026, Birthday this year? Already had it) on the example date Monday, October 19, 2026: Someone aged 16 on October 19, 2026 was born in 2010.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| age | Age | The age in whole years, as you would say it. |
| on | On date | The date the age is for. Today, unless you change it. |
| had | Birthday this year? | Whether the birthday has already come in that year, on or before the date. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| year | Birth year | The year of birth, or the two possible years. |
| earliest | Earliest possible year | The earlier year it can be. |
| latest | Latest possible year | The later year it can be. |

## Method

With Y the year of the date and A the age: born in Y − A if the birthday has come by the date, Y − A − 1 if not; not sure gives both.

## Assumptions

- Age is counted in complete years: it goes up by 1 on each birthday and is never rounded up.
- You say whether the birthday has come; the page does not need the birthday itself.

## Worked examples

1. age = 16, on = 2026-10-18, had = yes gives year = 2010, earliest = 2010, latest = 2010. Source: U.S. Census Bureau, American Community Survey 2023 Subject Definitions, Age: age in complete years as of the date, not rounded up before a birthday. https://www2.census.gov/programs-surveys/acs/tech_docs/subject_definitions/2023_ACSSubjectDefinitions.pdf.
2. age = 16, on = 2026-10-18, had = no gives year = 2009, earliest = 2009, latest = 2009. Source: U.S. Census Bureau, American Community Survey 2023 Subject Definitions, Age: age in complete years as of the date, not rounded up before a birthday. https://www2.census.gov/programs-surveys/acs/tech_docs/subject_definitions/2023_ACSSubjectDefinitions.pdf.
3. age = 30, on = 2026-03-01, had = unsure gives year = 1995 or 1996, earliest = 1995, latest = 1996. Source: U.S. Census Bureau, American Community Survey 2023 Subject Definitions, Age: age in complete years as of the date, not rounded up before a birthday. https://www2.census.gov/programs-surveys/acs/tech_docs/subject_definitions/2023_ACSSubjectDefinitions.pdf.
4. age = 0, on = 2026-01-01, had = no gives year = 2025. Source: U.S. Census Bureau, American Community Survey 2023 Subject Definitions, Age: age in complete years as of the date, not rounded up before a birthday. https://www2.census.gov/programs-surveys/acs/tech_docs/subject_definitions/2023_ACSSubjectDefinitions.pdf.

## FAQ

### What year was I born if I am 16?

On a date in 2026: 2026 − 16 = 2010 if you have had your birthday this year, or 2009 if it is still to come.

### How do I work out a birth year from an age?

Subtract the age from the current year. If the birthday has not come yet this year, subtract 1 more. On March 1, 2026, a 30-year-old was born in 1996 if their birthday has passed, else in 1995.

### Why does the birthday matter?

Age goes up by 1 on each birthday. Before the birthday, a person is one year younger than the year difference, so they were born a year earlier than year − age.

### What if I do not know if their birthday has passed?

Pick Not sure. The page gives both possible years, for example 1995 or 1996. Only the birthday itself tells them apart.

### What year was a baby under 1 born?

An age of 0 means less than a year old. If the birthday (the day of the month they were born) has not come yet this year, they were born last year; otherwise this year.

## Sources

- U.S. Census Bureau, American Community Survey 2023 Subject Definitions, Age: age is the age in complete years as of the date, not rounded up before a birthday. https://www2.census.gov/programs-surveys/acs/tech_docs/subject_definitions/2023_ACSSubjectDefinitions.pdf (cited from the copy already in this repo, src/families/how-old-am-i)