How to calculate age in years, months and days

1. Check whether the birthday has passed

Start with a complete birth date, including the year, and choose one reference date. To find age on a particular day, compare against that day, not against today.

1. Subtract the birth year. Someone born October 20, 1998 reaches 2026 − 1998 = 28 years during 2026.
2. Compare the months. Before October, they are still 27. After October, they are 28.
3. If the month matches, compare the days. Before October 20, subtract one. On October 20 or later, keep 28.

Do not round up. Someone who is 27 years and 11 months old is still 27 in completed years. For a February 29 birthday, use an explicit anniversary rule, explained below.

2. Count complete months, then leftover days

For a more detailed age, count forward on the calendar. Find the last birthday, count complete additional months, then count the days left over. A calendar month is not a fixed 30 days.

The month count starts from the original day of birth, the 17th in this example. September 17 is the latest monthly anniversary on or before September 22.

What if the reference day is earlier than the birth day?

Keep the same birth date but move the reference date to September 10, 2024. September 17 has not arrived, so stop at August 17 instead:

• May 17 to August 17 adds 3 complete months after the 26th birthday.
• August 17 to September 10 adds 24 days: 14 days to August 31, then 10 more to September 10.

The result is 26 years, 3 months and 24 days. Counting forward avoids a common subtraction error: borrowing 30 days when the month actually has 31.

For age in complete months, multiply the years by 12 and add the remaining months. The first example is 26 × 12 + 4 = 316 complete months, plus 5 days. Do not multiply by 30 to turn that result into days.

3. Handle leap days and month ends consistently

Some dates do not have a matching day in every month or year. The examples here follow the rules shown on the ToolPixels Age Calculator.

Born on February 29

ToolPixels uses February 28 as the yearly anniversary in a non-leap year. A person born February 29, 2000 therefore reaches 25 years on February 28, 2025 under this convention.

In a leap year, February 29 exists and stays the anniversary. On February 28, 2024, that same person is still 23 in completed years; on February 29, they turn 24.

This is a calendar convention, not a rule for official eligibility. If an organization needs your age on a cutoff date, check its required reference date and treatment of leap-day birthdays.

Born on the 29th, 30th or 31st

If a monthly anniversary would land on a missing day, ToolPixels uses the last day of that month. Later anniversaries still use the original birth day, not the shortened month's day.

For a start date of January 31, 2025:

• February 28: 1 month, 0 days. February has no 31st.
• March 30: 1 month, 30 days. The second monthly anniversary is March 31, so it has not arrived.
• March 31: 2 months, 0 days. The anniversary returns to the 31st.

Do not keep adding one month to the adjusted February date: that would incorrectly move the next anniversary to March 28 for this method. Calculate each anniversary from the original start date.

Age in days is a different measurement

Calendar age counts birthdays and monthly anniversaries. Total days counts the elapsed days between the two dates, including any leap days along the way.

For example, January 1, 2024 to January 1, 2025 is exactly 1 calendar year but 366 days. Dividing those days by 365 or 365.25 does not give exactly 1. Use the birthday check, not division by an average year length, for completed years.

The starting date is not counted as an extra day: June 1 to June 2 is 1 day; the same date entered twice gives 0 days. Total weeks are complete seven-day periods plus the days left over.

Check your result with ToolPixels

The free ToolPixels Age Calculator applies these calendar rules to your dates. To check an example or calculate your own age:

1. Enter the Birth date. Check the month and day: numeric date formats can differ between browsers.
2. Set Age on date. It defaults to today, but you can choose any date on or after the birth date.
3. Select Calculate age. Read the years, months and days, then check the date range displayed below the totals.
4. Use Copy results if needed. Save the reference date separately so the copied age still makes sense later.

You also get complete months, weeks with leftover days, total days, and the weekday of birth. The inputs are calendar dates, not timestamps: the tool does not calculate exact hours lived from a birth time.

Calculate age from your dates

Common questions

Can I calculate age with only a birth year?

Not exactly. Someone born in 1998 is either 27 or 28 during 2026, depending on whether their birthday has passed. You need the month and day to choose between them.

Why do two age calculators disagree?

First check that both use the same birth date and reference date. Then compare their February 29 and month-end rules, and whether they add the starting day to their day count. A calendar age and an estimate based on elapsed days are also different measurements.

Can I use this for the time between two events?

Yes. Enter the earlier event as the birth date and the later event as the age-on date for a calendar duration. For elapsed-day counts with weekday and weekend totals, use the Date Difference Calculator.