Week Calculator - Weeks, Days, and ISO Week Numbers

A week calculator is a planning tool that turns any pair of calendar dates into a precise count of complete weeks and remaining days, while also reporting the ISO 8601 week number and weekday for each input date.

• • Project and Sprint Planning: Track how many weeks remain until a software release, marketing launch, or renovation milestone without doing date arithmetic by hand.
• • School and Term Calendars: Measure the number of weeks between the first day of classes and finals week, or between the start of summer break and the first day of the next term.
• • Lease, Contract, and Notice Periods: Confirm how many complete weeks fit inside a 12-week notice period, a 26-week probation window, or a 52-week lease term.
• • Personal Milestones and Countdowns: Count down the weeks until a wedding, baby due date, retirement, or vacation, and label each milestone with its ISO week number for shared reference.

Unlike a simple day counter, this tool groups the elapsed days into a weeks-and-days pair that matches how people actually schedule work, school, and life events.

The ISO 8601 week number layer is included because most international project plans, payroll cycles, and school terms are referenced by week number rather than calendar date.

For a broader set of date arithmetic that adds or subtracts any number of days, use the Date Calculator alongside this week counter.

The calculator finds the absolute number of days between the two dates, divides that count by 7, and reports the integer quotient as complete weeks with the integer remainder as leftover days. It then derives the ISO 8601 week number and the weekday name for each input date.

• startDate: The first calendar date in the range, counted as day zero.
• targetDate: The final calendar date in the range. Must be on or after startDate.
• Total Days: Integer day difference between the two dates, measured in UTC midnights.
• Complete Weeks: Whole number of 7-day groups that fit into Total Days.
• Remaining Days: Days left over after subtracting Complete Weeks times 7.

The week number is computed by anchoring each date to the week that contains the year's first Thursday, which is the ISO 8601 rule used by most international calendar systems.

Weekday names are looked up in UTC to keep the result stable for users who travel or compare notes across time zones.

According to International Organization for Standardization (ISO 8601), the week containing the year's first Thursday is week 1, weeks run Monday through Sunday, and week numbers can reach 53 in years that contain 53 Thursdays.

If you want the raw total-day count without the week grouping, the Days Between Dates Calculator returns the same interval in days only.

A few calendar concepts explain why this tool gives different results from a simple day counter and why the ISO week number sometimes changes the year of a date.

These four concepts are enough to read every output of the calculator with confidence and to spot when an unusual result, like a week 53, comes from the calendar itself and not from a math error.

When the range stretches across multiple years, the Years Between Dates Calculator breaks the same interval into completed years, months, and days.

The tool runs in your browser, recalculates on every change, and shows results in a black result card and a details list.

1. 1 Pick the Starting Day: Enter the first date in your range using the date picker. This is the day the count begins, so a two-week count starting Monday means the period runs from Monday to the second Monday.
2. 2 Pick the Last Day: Enter the final date of the range. The calculator rejects ranges where the end date is before the start date and shows a clear error message.
3. 3 Read the Weeks and Days: Look at the black result card to read the complete weeks and remaining days. The details list underneath also shows the total day count and the ISO week number plus weekday for both dates.
4. 4 Reset for the Next Range: Click Reset to restore the default start and end dates if you want to count a different range or share the calculator with someone else.

For a 90-day probation period that starts on Monday, March 2, 2026, the calculator returns 12 complete weeks and 6 remaining days, with the starting Monday in ISO week 10 and the final Sunday in ISO week 22.

To count down the weeks remaining until a single future date such as a wedding or a deadline, the Date Countdown Calculator provides a live-updating view.

Counting weeks and days in one place saves time, removes calendar arithmetic errors, and gives you a label (the ISO week number) you can share with a team or a class.

• • Project Schedule Planning: Estimate sprint lengths, marketing campaigns, and product release windows in weeks without having to count days on a wall calendar.
• • School Term and Course Calendars: Measure semesters, summer break lengths, and exam-prep windows in complete weeks for syllabus planning.
• • Lease, Contract, and Notice Periods: Confirm that 12 weeks, 26 weeks, or 52 weeks fit inside a contractual window, including the extra days that are often left over.
• • Health and Pregnancy Tracking: Match prenatal and postnatal timelines to ISO week numbers that medical and workplace forms often request.
• • Leap-Year Safe Arithmetic: The calculator handles February 29 correctly, so 12-month ranges that span a leap year return 52 weeks and 2 days rather than the standard 52 weeks and 1 day.
• • Client-Side Results: All math runs in your browser, so you can iterate on different ranges without a page reload or any data being sent off your device.

Pairing the weeks-and-days total with the ISO week number for each input date also lets you reference a date by week, which is how most project plans and academic calendars are written.

For sub-day precision inside a multi-week range, the Time Between Dates Calculator adds hours, minutes, and seconds to the week-level result.

The arithmetic is constant, but a few calendar quirks can change the result by a single day or add an unexpected week 53.

• • The calculator counts whole days only; it does not show hours, minutes, or seconds inside the range. For sub-day precision, use the time-between-dates-calculator.
• • Week numbers are computed with the ISO 8601 rule. Some U.S. payroll systems and a few religious calendars use other week-numbering schemes, which can give a different number for the same date.

These caveats explain every edge case the tool surfaces, including the occasional week 53 and the single extra day a leap year can introduce.

According to National Institute of Standards and Technology (NIST), a standard calendar year is exactly 365 days (or 366 in a leap year), which is 52 weeks plus one or two extra days.