Weeks Old Calculator: Age in Weeks

This calculator works out exactly how many weeks old a person, pet, or object is, based on a start date and an end date. You enter the birth date (or start date) and the date you want to measure to, and you get the age in whole weeks and the remaining days. The result is the number of complete 7-day periods between the two dates, which is the standard way to express age in weeks.

Formula

The core calculation is simple: subtract the start date from the end date to get the total number of days, then divide by 7. The number of complete weeks is the integer part of that division, and the remainder is the leftover days.

Age in weeks = floor( (End date − Start date) / 7 )

Where:

• Start date – the birth date or starting point (e.g., 2020-01-15).
• End date – the date you want to measure to (e.g., 2025-06-01).
• floor() – rounds down to the nearest whole number, so you only count complete 7-day periods.
• Remaining days – the days left over after the last full week (always between 0 and 6).

The calculator treats both dates as inclusive of the start day – meaning if you were born on January 1, you are 0 weeks old on January 1, 1 week old on January 8, and so on. This matches how age is typically counted in everyday life.

Worked Example

Using the default values: birth date 2020-01-15, measure date 2025-06-01.

1. Count the total days between the two dates. From 2020-01-15 to 2025-01-15 is exactly 5 years (2020, 2021, 2022, 2023, 2024 – one leap year, 2024, so total days = 365×4 + 366 = 1826 days).
2. From 2025-01-15 to 2025-06-01: January has 16 days left (16th to 31st), February 28, March 31, April 30, May 31, plus 1 day in June = 16+28+31+30+31+1 = 137 days.
3. Total days = 1826 + 137 = 1963 days.
4. Divide by 7: 1963 ÷ 7 = 280.428…, so whole weeks = 280.
5. Remaining days = 1963 − (280 × 7) = 1963 − 1960 = 3 days.

Result: 280 weeks and 3 days old. The calculator shows 280 as the headline answer, with 3 remaining days and 1963 total days.

Why Count Age in Weeks?

Counting age in weeks is common for newborns and infants, because development milestones happen quickly in the first year. Parents and pediatricians track growth spurts, feeding schedules, and sleep patterns in weeks rather than months. It is also used for puppies and kittens, where the first weeks are critical for weaning and socialization. For older children and adults, weeks are less common, but you might want to know your age in weeks for a fun fact, a milestone like a 1000-week birthday, or to calculate pregnancy duration (which is typically counted in weeks, starting from the first day of the last menstrual period).

This calculator is also useful for tracking project durations, rental periods, subscription lengths, or any interval where you need to know how many full weeks have passed.

How to Use the Calculator

Using the calculator is straightforward:

• Enter the birth date or start date in the first field (format YYYY-MM-DD).
• Enter the date you want to measure to in the second field. This can be today, a future date, or a past date.
• Click the calculate button (or the result updates automatically).

The output gives you three numbers:

• Age in weeks – the headline result, the number of complete 7-day periods.
• Remaining days – the days left over after the last full week (0 to 6).
• Total days – the exact number of days between the two dates, useful for verifying the math.

If you want to know your age in weeks as of today, simply set the end date to today’s date. If you want to know how many weeks until a future event, set the start date to today and the end date to the event date – the result will be the weeks until then.

Common Mistakes and Misconceptions

One common mistake is mixing up the order of dates. The calculator expects the start date to be earlier than the end date. If you enter them in reverse, the result will be negative, which is meaningless for age. Always put the earlier date first.

Another misconception is that “weeks old” includes the birth day as week 1. In this calculator, the birth day is week 0, and week 1 starts the next day. This is the standard convention: a baby born on Monday is 0 weeks old on Monday, 1 week old the following Monday. Some cultures or contexts might count differently, but this is the most common method.

Also, be careful with leap years. The calculator automatically handles them because it works with actual dates, not just multiplying years by 365. For example, someone born on February 29, 2020, will have their first birthday on February 28, 2021, in a non-leap year, and the calculator will correctly count the days.

Finally, don’t confuse “weeks old” with “weeks pregnant.” Pregnancy is counted from the first day of the last menstrual period, which is about two weeks before conception. So a pregnancy is typically 40 weeks, but the baby is only about 38 weeks from conception. This calculator measures actual time between dates, not the medical convention for pregnancy.

Why the Result Is a Whole Number

Age in weeks is almost always expressed as a whole number plus leftover days, not a decimal like 280.43 weeks. That’s because a week is a discrete unit – you don’t say “I am 280.43 weeks old” in everyday conversation. Instead, you say “280 weeks and 3 days.” The calculator gives you the whole weeks and the remaining days separately, which is the most useful format.

If you need a decimal value for some calculation (e.g., 280.43 weeks), you can divide the total days by 7: 1963 / 7 = 280.428…, which rounds to 280.43. But for most practical purposes, the whole-week-plus-days format is what you want.

Limitations and Edge Cases

The calculator works for any dates between January 1, 1900, and December 31, 2100. This covers most real-world uses. If you need a date outside this range, you can still calculate manually using a date difference calculator or a spreadsheet.

One edge case: if the start date is after the end date, the result will be negative. The calculator does not block this, but the output will show a negative number of weeks, which is not a valid age. Always check that your dates are in the correct order.

Another edge case: if both dates are the same, the result is 0 weeks and 0 days, which is correct – you are 0 weeks old on your birth date.

For very old dates (before 1900), you may need to use a different tool, but the math is the same: count the days and divide by 7.

Frequently Asked Questions

How many weeks old is a 1-year-old?
A 1-year-old is exactly 52 weeks and 1 day old (or 52 weeks and 2 days in a leap year). This is because a non-leap year has 365 days, which is 52 weeks (364 days) plus 1 extra day. So if someone was born on January 1, 2024, on January 1, 2025, they are 52 weeks and 1 day old.

How do I calculate my age in weeks manually?
Count the total number of days between your birth date and the target date, then divide by 7. The integer part is your age in weeks, and the remainder is the extra days. For example, if you have lived 10,000 days, you are 10,000 ÷ 7 = 1428 weeks and 4 days old.

What is the difference between weeks old and months old?
Months are not a fixed number of days – they vary from 28 to 31. Weeks are always exactly 7 days. So “3 months old” is not exactly 13 weeks; it depends on which months. This is why weeks are more precise for tracking early development.

Can I use this calculator for a pregnancy due date?
This calculator measures elapsed time between two dates, which is not the same as pregnancy dating. Pregnancy is counted from the first day of your last menstrual period (LMP), and a full-term pregnancy is about 40 weeks from LMP. If you know your LMP, you can enter it as the start date and today’s date as the end date to see how many weeks pregnant you are, but this is only an approximation – always confirm with a healthcare provider.

Why does the calculator show 0 weeks on the birth date?
Because age is counted as complete periods of 7 days. On the day you are born, you have not completed any full week, so you are 0 weeks old. The next day you are still 0 weeks old, but 1 day old. After 7 days, you become 1 week old.

What if I want to know weeks between two dates that are not a birth date?
That’s fine – the calculator works for any two dates. For example, you can enter the start of a project and today’s date to see how many weeks it has been running. Just make sure the first date is earlier than the second.

Related

• How to Calculate Days Old
• 8 Hour Shift End Calculator: Finish Time
• 4 213 Round to Nearest Thousand Calculator

in start | Birth date (YYYY-MM-DD) | - | 2020-01-15 | 1900-01-01 | 2100-12-31
in end | Date to measure (YYYY-MM-DD) | - | 2025-06-01 | 1900-01-01 | 2100-12-31
out weeks | Age in weeks | weeks | floor((end - start) / 7) | 0
out days | Remaining days | days | (end - start) - floor((end - start) / 7) * 7 | 0
out total_days | Total days | days | end - start | 0