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How to use this calculator#
- Enter the date of birth exactlyUse the date picker rather than typing, so there is no ambiguity between 03/04 as 3 April and 4 March. The calculator works on real calendar dates, so leap days inside the span are handled automatically.
- Set the second date if you want age on a past or future dayLeave it as today for a current age, or set it to a specific day to answer ‘how old was she when the photo was taken’ or ‘how old will he be at the wedding’.
- Read the years-months-days breakdown, not just the yearsThe breakdown is what forms, visa applications and paediatric charts ask for. A child of 4 years 11 months is treated very differently from one of 5 years 1 month.
- Use the total-days figure for anything cumulativeDays lived is the right unit for medication dosing intervals, milestone counts and any calculation where months of unequal length would introduce error.
The formula#
Calendar age with borrowing
Years = Y₂ − Y₁, Months = M₂ − M₁, Days = D₂ − D₁; if Days < 0 then Days += (length of the month before the target) and Months −= 1; if Months < 0 then Months += 12 and Years −= 1
- Y₁ M₁ D₁
- Year, month and day of birth
- Y₂ M₂ D₂
- Year, month and day of the date you are measuring to
- Days
- The leftover days after whole years and months, borrowed from the previous month if negative
- Months
- Whole calendar months after the last birthday, 0 to 11
- Years
- Completed years — your age has not increased until the birthday itself arrives
- Total days
- The plain elapsed-day count, independent of the years-months-days breakdown
Age counting is exclusive of the birth date: you are 0 on the day you are born and 1 on your first birthday, so the count is of completed intervals. Borrowing always takes the length of the month immediately before the target date, which is why the leftover days depend on when in the year you measure. Leap days need no special handling — 29 February simply exists in the elapsed-day count when the range contains one.
How exact age is calculated#
Your age is the elapsed time from your date of birth to a target date. Because months have different lengths, we count whole years and months first, then the remaining days, for a precise breakdown.
Worked examples#
An age with the borrow step shown
Born 14 March 1990, measured on 4 August 2026.
- Years: 2026 − 1990 = 36. Months: 8 − 3 = 5. Days: 4 − 14 = −10
- Days is negative, so borrow from the month before August — July, which has 31 days: −10 + 31 = 21
- Borrowing costs a month: 5 − 1 = 4
- Result: 36 years, 4 months, 21 days
- Elapsed days = 13,292, which is 1,898 weeks and 6 days
36 years, 4 months and 21 days — 13,292 days lived.
A leap-day birthday in a common year
Born 29 February 2000. What is the age on 28 February 2026, and on 1 March 2026?
- To 28 Feb 2026: Years 26, Months 0, Days 28 − 29 = −1
- Borrow the 31 days of January: Days = 30, Months = −1, so Years drops to 25 and Months becomes 11
- That gives 25 years, 11 months, 30 days — not yet 26
- To 1 Mar 2026: Days 1 − 29 = −28, borrow February's 28 days → Days = 0, Months = 1 − 1 = 0
- That gives exactly 26 years, 0 months, 0 days
By this convention a leap-day birthday rolls over on 1 March in a common year. In 2028 there is a real 29 February, so the age changes on the day itself.
Reference tables#
| Age | Days lived | Weeks | Leap days included |
|---|---|---|---|
| 10 | 3,653 | 521 weeks 6 days | 3 |
| 18 | 6,575 | 939 weeks 2 days | 5 |
| 21 | 7,671 | 1,095 weeks 6 days | 6 |
| 30 | 10,958 | 1,565 weeks 3 days | 8 |
| 40 | 14,610 | 2,087 weeks 1 day | 10 |
| 50 | 18,263 | 2,609 weeks | 13 |
| 65 | 23,742 | 3,391 weeks 5 days | 17 |
A calendar year averages 365.2425 days, so multiplying an age by 365 undercounts by roughly a day for every four years — ten days by the age of 40, seventeen by 65.
| Milestone age | Born in (turns it in 2026) | Born in (turns it in 2027) |
|---|---|---|
| 16 | 2010 | 2011 |
| 18 | 2008 | 2009 |
| 21 | 2005 | 2006 |
| 30 | 1996 | 1997 |
| 40 | 1986 | 1987 |
| 50 | 1976 | 1977 |
| 62 | 1964 | 1965 |
| 65 | 1961 | 1962 |
| 67 | 1959 | 1960 |
| 73 | 1953 | 1954 |
Anyone whose birthday falls later in the year is still the previous age for part of it, so treat these as the year of the birthday rather than the whole calendar year.
| Age | What changes |
|---|---|
| 16 | Full driving licence in most states, subject to graduated licensing rules |
| 18 | Legal adulthood, voting, contracts, Selective Service registration for men |
| 21 | Legal purchase of alcohol nationwide |
| 26 | Coverage under a parent's health plan ends |
| 50 | Catch-up contributions to 401(k) and IRA accounts become available |
| 59½ | Withdrawals from most retirement accounts stop attracting the 10% early-distribution penalty |
| 62 | Earliest age to claim Social Security retirement benefits, at a permanently reduced rate |
| 65 | Medicare eligibility begins, with a seven-month enrolment window around the birthday |
| 67 | Full Social Security retirement age for everyone born in 1960 or later |
| 73 | Required minimum distributions from most retirement accounts begin |
Ages 59½ and 73 are set in tax law and have changed several times; check the current IRS guidance before acting on either.
Common mistakes#
- Subtracting the birth year from the current year2026 − 1990 = 36, but someone born on 14 December 1990 is still 35 in August 2026. The shortcut is wrong for everyone whose birthday has not yet arrived — which on any given day is roughly half the population.
- Dividing total days by 36513,292 ÷ 365 = 36.42 years, which looks right until you need the exact date. The average calendar year is 365.2425 days, and over 40 years the difference accumulates to ten whole days — enough to move a milestone across a month boundary.
- Mishandling a 29 February birthdayLeap-day birthdays have no anniversary in three years out of four. Most legal systems and this calculator treat the age as rolling over on 1 March, but some jurisdictions and insurers use 28 February. If a threshold like turning 18 falls on a leap-day birthday, check which rule applies.
- Letting a time zone shift the dateParsing a date of birth as a UTC timestamp and displaying it in a western time zone can render it as the previous day, quietly changing the age by a year around a birthday. Dates of birth should be handled as plain calendar dates with no time component at all.
Frequently asked questions#
Can I calculate age between two specific dates?
Yes. Set the ‘age at date’ field to any date to find how old someone was — or will be — on that day.
Does it account for leap years?
Yes, the day-level calculation uses real calendar dates, so leap years are handled automatically.
Key terms#
- Chronological age
- Completed years, months and days since birth. Distinct from developmental or bone age, which are clinical assessments.
- Anniversary date
- The same day and month in a later year. Age increases on the anniversary, not on the first day of the birth month.
- Leap year
- A year divisible by 4, except centuries, which must also be divisible by 400. 2024 and 2028 are leap years; 2026 and 2027 are not; 2100 will not be.
- Leap day
- 29 February, inserted to keep the calendar aligned with the solar year of about 365.2422 days.
- Elapsed days
- The plain count of midnights between two dates. The most reliable unit for age arithmetic because it is immune to month lengths.
- ISO 8601
- The international standard date format, YYYY-MM-DD. Unambiguous worldwide, unlike 03/04/2026 which reads as two different dates on two continents.
- East Asian age reckoning
- Traditional systems in which a person is 1 at birth and gains a year at New Year. South Korea moved to international age counting in law in June 2023.
Sources#
- Full retirement age by year of birth — US Social Security Administration
- Get started with Medicare — when coverage begins — US Centers for Medicare & Medicaid Services
- ISO 8601 date and time format — International Organization for Standardization
- Retirement plan and IRA required minimum distributions FAQs — Internal Revenue Service
Figures last checked .
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