Week Calculator

Add or subtract weeks from a date to find the resulting date, its weekday, its week number of the year, and the exact number of days shifted.

Quick Facts

Core formula
Resulting date = Start date ± (weeks × 7 days)
A calendar week is always exactly 7 days, so the math is pure date arithmetic.
ISO 8601 rule
Week 1 is the week containing the year's first Thursday
ISO weeks run Monday to Sunday; a year can have 52 or 53 of them.

Your Results

Calculated
Resulting date
-
Start date plus or minus the weeks entered
Day of week
-
Weekday of the resulting date
Week number
-
Week of the year, selected convention
Total days shifted
-
Weeks × 7, exact day count

Ready

Set a start date and number of weeks, then press Calculate.

About the Week Calculator

A week is a fixed unit of exactly 7 days, which makes week arithmetic simpler than month or year arithmetic — there is no equivalent of "how many days are in this week" varying by week the way it does for months. This calculator adds or subtracts a chosen number of weeks from a start date, then reports the resulting calendar date, its day of the week, and which numbered week of the year it falls in.

The core formula

The resulting date is calculated as:

  • Resulting date = Start date ± (Number of weeks × 7 days)

Adding is simple day arithmetic once weeks are converted to days: 4 weeks is exactly 28 days, 10 weeks is exactly 70 days, and so on. Fractional weeks (like 2.5) convert to a whole number of days (17.5 rounds to the nearest day, since calendar dates only exist at day granularity). Subtracting works the same way but moves backward in time — useful for finding a date "N weeks ago."

Week numbers: ISO 8601 vs. US convention

There are two common ways to number the weeks of a year, and they can disagree near January 1st:

  • ISO 8601 (international standard): weeks run Monday through Sunday. Week 1 is defined as the week containing the year's first Thursday — equivalently, the week containing January 4th. A year has 52 or 53 ISO weeks, and the first few days of January sometimes belong to week 52 or 53 of the previous year.
  • US convention: weeks run Sunday through Saturday, and week 1 is simply the week containing January 1st (even if that week only has one or two days in the new year). This matches how most US calendars and spreadsheet WEEKNUM defaults display week numbers.

Common uses

  • Finding a due date or deadline a set number of weeks from today (e.g., a 6-week project sprint or a 2-week notice period)
  • Working out a past date, such as "8 weeks before my due date" for prenatal appointment scheduling
  • Looking up which week number a date falls in for payroll, academic terms, or ISO-based scheduling systems
  • Converting a duration expressed in weeks into a concrete calendar date for planning

Precision note

This calculator uses pure calendar-day arithmetic (7 days per week) and does not account for time zones, daylight saving transitions, or business-day exclusions. If your use case is legal or contractual, confirm whether it means calendar weeks or business weeks — the two can differ significantly when weekends are excluded.

Frequently Asked Questions

How does the calculator find the resulting date?
It multiplies the number of weeks you enter by 7 to get a day count, then adds or subtracts that many days from your start date using standard calendar-date arithmetic. Because a week is always exactly 7 days, this calculation needs no special handling for month length or leap years — the underlying date object handles month and year rollovers automatically.
What is the difference between ISO and US week numbers?
ISO 8601 weeks start on Monday, and week 1 is the week that contains the year's first Thursday (equivalently, the week containing January 4th). US-convention weeks start on Sunday, and week 1 is simply the week containing January 1st. Near the start or end of a year, the same date can have different week numbers under each system — for example, December 31st might be "week 1 of next year" under ISO if it falls on a Monday, but stay in the final week of the current year under the US convention.
Can a year have 53 weeks?
Yes. Under ISO 8601, a year has 53 weeks whenever January 1st falls on a Thursday (in a common year) or on a Wednesday or Thursday (in a leap year). Otherwise it has 52. This happens because 365 or 366 days doesn't divide evenly by 7, so the extra day(s) occasionally push a 53rd week into the count.
Does this calculator account for business days or holidays?
No — it works in calendar weeks, where every week is exactly 7 days including weekends. If you need to exclude weekends or specific holidays (for example, to calculate a business deadline), you need a business-day calculator instead, since those exclusions change the day count.

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