Age Calculator

How to calculate age exactly

Saying someone is “34” hides most of the answer. An exact age is a calendar quantity — a number of whole years, then whole months, then days — and working it out by hand is fiddly because months and years are not all the same length. This calculator does the counting the way a calendar does, so the result matches what you would get by counting on a wall calendar, one birthday and one month at a time. Enter a date of birth, choose today or any other date to measure against, and it returns the age broken down into years, months and days, the same age re-expressed in months, weeks, days and hours, and the date and weekday of the next birthday.

Step 1: count whole months

Take the number of calendar months between the birth date and the calculation date. If stepping forward that many months from the birth date overshoots the calculation date, use one fewer. For someone born on 20 May 1990, on 10 March 2025 the count is 417 months: 20 May 1990 plus 417 months is 20 February 2025, and 418 would reach 20 March, which is too far.

Step 2: count the days left over

From 20 February 2025 to 10 March 2025 is 18 days. 417 months is 34 years and 9 months, so the exact age is 34 years, 9 months and 18 days. This two-step method is why the result never goes wrong at a month boundary the way a simple day-count divided by 30 would.

Step 3: express it in other units

The same birth date and calculation date also give a total day count: 12,713 days from 20 May 1990 to 10 March 2025. Total months is 417, the figure from step 1. Weeks are the total days divided by seven, with the remainder shown beside them — 1,816 weeks and 1 day for this example. Hours are the total days multiplied by 24, counted from midnight on the birth date, so 305,112 hours here; no birth time is asked for, so this figure can be off by up to a day’s worth of hours either way. Decimal years are the complete years plus the fraction of the current birthday-to-birthday year that has passed, measured with that particular year’s real length — 34.8055 years for this example, not 34.75, because a 366-day year and a 365-day year contribute a slightly different fraction per day.

Birthdays at the end of a month

Stepping forward a month from 31 January lands on 28 or 29 February, because 31 February does not exist. The calculator clamps to the last day of the shorter month, which is how people count in practice: a baby born on 31 January is one month old at the end of February. The “How was this calculated?” panel shows the exact anchor date used, so every day in the result can be checked.

Leap years and 29 February

A year is a leap year when it divides by 4, except century years, which must also divide by 400 — so 2000 was a leap year and 1900 was not. Leap days are included in the total day count automatically, and the result panel also counts how many real 29 Februaries fell between the birth date and the calculation date on their own line.

For people born on 29 February, the calculator treats 28 February as the birthday in every year that is not a leap year, and states this on the result. Someone born on 29 February 2000, checked on 1 March 2025, gets 25 years, 0 months and 1 day: their most recent anniversary fell on 28 February 2025 (2025 is not a leap year), one day before the calculation date. Their next birthday is dated 28 February 2026, for the same reason. Some jurisdictions instead treat 1 March as a 29 February birthday’s legal anniversary — check the local rule if the exact day decides something important, such as when a contract or licence takes effect.

The totals and the next-birthday panel

The “same age in other units” panel exists because different forms ask for age in different ways — total months for a paediatric growth chart, total weeks for a pregnancy-adjacent calculation, decimal years for a statistical dataset. All five figures are read off the same underlying day count, so they always agree with each other and with the years/months/days headline.

The next-birthday panel shows the number of days remaining, the weekday the birthday falls on, and a progress bar for how far through the current birthday-to-birthday year the calculation date sits. The bar’s length for that year is measured, not assumed, so it is 366 days wide whenever a 29 February falls inside it and 365 days wide otherwise — which is also why the days-remaining figure jumps by one on either side of a leap day.

Common mistakes

• Dividing total days by 365. That drifts by about one day every four years, so by age 40 it can overstate the age by roughly ten days — enough to show someone a year older than they are in the run-up to a birthday.
• Assuming every “1 month” is 30 days. The calculator counts calendar months, which run from 28 to 31 days each, so the day component in the result is whatever is left over after those real months, not a fixed 30.
• Entering the wrong date of birth for a cut-off form. If a form asks for age “as at” a specific date rather than today, use “A specific date” — the age today and the age on a past or future cut-off date are usually different numbers.
• Treating the hours figure as exact to the minute. It is a day count multiplied by 24, with no birth time in the calculation, so it is a close estimate rather than a precise reading.

When an exact age matters

Application forms often ask for an age “as at” a cut-off date — school admissions, exams with age limits, sports categories, insurance and pension quotes. Use “A specific date” for those, since the age on the cut-off date is what counts, not today’s. The date of birth cannot be entered later than the calculation date; the form rejects that rather than showing a negative age.

To compare two people instead of measuring one, use the Age Difference Calculator. For a length of employment rather than a length of life, the Work Experience Calculator uses the same calendar arithmetic. To find a specific round-number birthday, such as a 10,000th day alive, see the 10,000 Days Calculator and the Birthday Milestone Calculator.

Frequently asked questions