Week Calculator

May 8, 2026

Planning a project timeline, counting weeks until a vacation, or monitoring a pregnancy? A week calculator instantly tells you how many weeks lie between any two dates and can add or subtract weeks to find a future or past date. It turns messy calendar math into a quick, accurate result.

The calculator works with any dates from year 1 to 9999, automatically factoring in leap years and the exact length of each month. You can choose to include or exclude the start date in the count, and the result shows full weeks plus remaining days.

How does a week calculator work?

The tool uses your device’s built‑in calendar to count the exact number of days between the two dates you pick. It then divides that day count by 7 to get whole weeks and a remainder. For adding or subtracting weeks, the calculator first multiplies the number of weeks by 7 and shifts the start date forward or backward by that many days.

Step‑by‑step: count weeks between dates

Choose a start date and an end date.
The calculator computes the absolute difference in days – ignoring the time of day.
It takes the total days D and performs integer division by 7: W = D // 7.
The leftover D % 7 appears as extra days.
The result: “10 weeks and 3 days” means exactly 10 full weeks plus 3 days.

Adding weeks to a date

When you add weeks, the calculator multiplies the number of weeks by 7, adds those days to the start date, and lands on the correct day of the week. For example, adding one week always moves you to the same weekday (Monday → Monday), regardless of month boundaries and leap years.

Common uses for a week calculator

Project planning – break a multi‑week timeline into weekly milestones.
Pregnancy tracking – gestational age is typically measured in completed weeks.
School terms – find how many weeks remain until the end of the semester.
Contract and warranty terms – many legal agreements use weeks as a unit.
Fitness challenges – count down the weeks of a 12‑week training program.
Age in weeks – discover exactly how many weeks you’ve been alive.

How many weeks in a year?

A standard civil year has 365 days, which equals 52 weeks and 1 day. A leap year (366 days) gives 52 weeks and 2 days. Over a four‑year cycle that includes one leap year, the total days are 1,461, or exactly 208 weeks and 5 days. That yields an average of 52.178571 weeks per year. In many business contexts, a year is treated as 52 weeks, with the extra days absorbed into the calendar schedule.

Weeks vs. days: quick conversion

1 week = 7 days
2 weeks = 14 days
4 weeks = 28 days (a typical “month” in some financial calendars)
12 weeks = 84 days
52 weeks = 364 days (one day short of a common year)

To convert any number of weeks to days, multiply by 7. To go from days to weeks, divide by 7 and take the integer part for full weeks.

Date calculations are based on the Gregorian calendar as implemented in your browser. For official, legal, or medical decisions, always verify the result with an authoritative source.

Frequently Asked Questions

How many weeks are in a year?

A standard year has 52 weeks and 1 day (365 days), while a leap year has 52 weeks and 2 days (366 days). On average, a year has 52.1429 weeks, or exactly 52 weeks and about 1.25 days when accounting for leap years over a four‑year period.

How do I manually calculate weeks between two dates?

First, find the total number of days between the dates by counting or using a date-difference tool. Then divide the total days by 7. The whole‑number result is the number of full weeks, and the remainder is extra days. For example, 100 days ÷ 7 equals 14 weeks and 2 days.

What is the formula to add weeks to a date?

Multiply the number of weeks by 7 to get the equivalent days. Then add that number of days to the start date. For instance, to add 12 weeks to March 1, calculate 12 × 7 = 84 days; March 1 + 84 days = May 24 (in a non‑leap year).

Does the week calculator account for leap years?

Yes, the calculator uses your device’s calendar which automatically includes leap years. Any date range that crosses February 29 will correctly count the extra day, keeping week totals accurate.

How can I use the week calculator for pregnancy tracking?

Enter the first day of your last menstrual period as the start date and today’s date as the end date. The calculator shows how many weeks have passed, which corresponds to the gestational age. Always confirm with your healthcare provider.

Why is a week 7 days?

The seven‑day week originated from ancient Babylonian astronomy linked to the lunar cycle. It was later adopted by the Roman calendar and standardized in the Gregorian system, now used worldwide for civil timekeeping.