How to calculate an age
- Choose the date of birth in the first field.
- Leave the second field blank for the age today, or set a date to find the age then.
- Read the exact age in years, months and days.
- Check the totals below for months, weeks, days and hours.
- See which day of the week the birth fell on and when the next birthday is.
What this calculator gives you
- Exact age in years, months and days rather than a rounded year figure.
- Totals in months, weeks, days and hours for forms that ask for them.
- The day of the week of the birth date.
- A countdown to the next birthday, with the weekday it falls on.
- An age-at-date mode for working out age on a past or future date.
- Calculation entirely in your browser, so no date of birth is ever transmitted.
Why exact age is harder than it looks
Subtracting years gives the wrong answer most of the time. Someone born in December 1990 is not 35 in January 2026. They are 35 only after their December birthday. Getting this right means comparing the month and day as well, and handling the case where the birthday has not yet arrived this year.
The month arithmetic is fiddlier still. Months have different lengths, so "one month after 31 January" has no obvious answer. This calculator borrows days from the preceding month when the day-of-month goes negative, which is the convention that matches how people actually count.
Where an exact age is required
- Medical forms, where dosage and screening thresholds depend on age in months for young children.
- School admissions, which usually cut off at an exact date rather than a birth year.
- Legal documents, insurance applications and pension calculations.
- Sports eligibility, where age groups are defined on a specific day.
- Visa and immigration forms that ask for age at the date of application.
The 29 February question
Someone born on a leap day has a birthday that exists roughly once every four years. Legally, most jurisdictions treat their age as advancing on 1 March in common years, and a minority use 28 February. This calculator follows the more common convention, so a leap-day birthday in a non-leap year is counted from 1 March.
It matters for anything with a hard age threshold. A driving licence, a legal majority, an insurance band. If it applies to you, it is worth checking which rule the specific institution uses rather than assuming.
Reading the totals
The total-days figure is the one most forms want when they ask for age in days, and it counts actual elapsed days including every leap day. The hours figure is derived from it rather than counted, so it treats every day as 24 hours, which is very slightly wrong across daylight saving changes and irrelevant at this scale.
Total months counts complete months only. Someone aged 3 years and 11 months shows 47 months, not 48, which is what medical and developmental forms expect.
Nothing is transmitted
A date of birth is a piece of personal data that belongs in identity theft checklists, and it is exactly the sort of thing that should not be typed into an unknown server. Everything here is calculated in your browser and nothing is sent anywhere. For arithmetic between two arbitrary dates rather than an age, the date calculator is the right tool, and the timestamp converter handles Unix time.
Age in months for infants
Paediatric guidance is written in months rather than years for the first two or three years of life, because development moves fast enough that "eighteen months" and "two years" describe genuinely different children. Vaccination schedules, growth charts and developmental milestones all use the month figure.
The complication is that a month is not a fixed length, so "six months old" means the same date number six months later rather than 180 days. This calculator counts calendar months, which is what clinical guidance means. If a form asks specifically for age in weeks, common for newborns. The total-weeks figure is the one to use, and it counts actual elapsed weeks rather than dividing months by four.