Most of us know our age in years and guess the rest. But forms, applications and curiosity often demand more precision: how old are you in months, weeks or total days? How many days until your next birthday? This calculator takes your date of birth and any target date and gives your exact age, broken down properly — not a rough division by 365.

How it works

Enter your date of birth and the date you want to calculate to — today by default, or any past or future date. The calculator returns your age in years, months and days, plus totals: your age in months alone, in weeks, and in days. It also shows the countdown to your next birthday.

This is useful in more places than you might expect: school and nursery admission cut-offs, pension and benefit eligibility dates, age verification on official forms, insurance applications, working out a baby’s age in weeks (as health visitors do), or checking the exact age of a historical figure on a given date.

How the calculation works

Exact age is calendar arithmetic, not simple subtraction. The calculator works the way you would count birthdays: it counts the full years from your birth date to the same date in the target year; then the full calendar months from that anniversary to the same day-of-month in the target month; then the remaining days.

The tricky part is that months have different lengths — 28 to 31 days — and leap years add a 29th of February roughly every four years (years divisible by 4, except century years, unless they are divisible by 400 — so 2000 was a leap year but 1900 and 2100 are not). A naive “days divided by 365.25” estimate can easily be a day or two out, which matters when a form asks for your exact age. Proper calendar arithmetic, handling each month’s real length and every leap day in between, is the only way to get the years-months-days breakdown right.

Worked example

Someone born on 14 March 1990, calculated on 21 October 2026:

1. Full years: from 14 March 1990 to 14 March 2026 = 36 years (their 36th birthday has passed).
2. Full months: from 14 March 2026 to 14 October 2026 = 7 months.
3. Remaining days: from 14 October to 21 October 2026 = 7 days.

Exact age: 36 years, 7 months and 7 days.

In total days, that span includes nine leap days (1992, 1996, 2000, 2004, 2008, 2012, 2016, 2020 and 2024), giving 13,356 days — and their next birthday is 144 days away, on 14 March 2027.

Tips

• Babies’ ages are counted in weeks, then months. Health professionals track a newborn’s age in weeks for the first couple of months, then in months up to age two. Use the total-weeks and total-months figures rather than rounding to years.
• Watch admission and eligibility cut-offs. School places, junior sports leagues, child tickets and pension eligibility often hinge on your age on one specific date — 1 September for English school years, for example. Set the target date to the cut-off, not today.
• People born on 29 February officially age on 1 March in non-leap years for most UK legal purposes, though many celebrate on 28 February. If your birthday is the 29th, check which date an organisation uses before relying on a result around that time.
• For family history, calculate both ways. Comparing a grandparent’s age at death or at a child’s birth is easier with the total-days figure, which removes all the calendar unevenness.
• Double-check the birth date on official forms. A surprising number of “wrong age” results trace back to a transposed day and month — particularly when dates have been copied from US-format documents (MM/DD/YYYY) into UK ones (DD/MM/YYYY).

FAQ

How many days old am I?
Enter your birth date and the calculator’s total-days figure is the answer — it counts every day including all the leap days you have lived through. Most people are surprised by the number: by age 30 you have already passed 10,950 days.

How does the calculator handle leap years?
It uses real calendar arithmetic: February has 29 days in leap years, and a year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. Every 29 February between your two dates is counted.

Can I work out the age difference between two people?
Yes. Calculate one person’s exact age in total days on the day the other was born — or simply run the calculator with the older person’s birth date and the younger person’s birth date as the target date. The result is the exact gap in years, months and days.

When exactly do I turn a year older?
Legally, in the UK you are considered to reach a new age at the start of your birthday — the anniversary of your birth date. For most practical purposes, your age changes at midnight at the beginning of that date.

Why does my age in months not match years × 12?
Because age in months counts only whole months completed. Someone who is 36 years and 7 months old is 439 months old — 36 × 12 = 432, plus the 7 full months since their last birthday. Partial months are counted in the days figure, not rounded up.