Week Calculator
This week calculator works in three modes: find how many weeks are between two dates, add a number of weeks to a start date, or subtract weeks from a date to find an earlier date. Every result shows full weeks, leftover days, total days, and the equivalent in months and decimal weeks. Switch to "between dates" mode to also see ISO week numbers, the day of the week for each date, and a breakdown of workweeks versus weekend days.
How to count weeks between two dates
The formula is straightforward: subtract the earlier date from the later date to get the total number of days, then divide by 7. The whole part of that result is the number of full weeks, and the remainder (days mod 7) tells you how many extra days are left over. For example, from January 1 to March 1 in a non-leap year is 59 days: 59 divided by 7 is 8 full weeks with 3 days remaining, or 8.43 weeks in decimal form. Leap years add an extra day to any span that crosses February 29, which this calculator handles automatically.
Add or subtract weeks from a date
To find a future date that is a set number of weeks away, multiply the weeks by 7 to get days and add them to the starting date. To find a past date, subtract the same number of days. A common use case is project planning: if a milestone is due in 6 weeks, adding 42 days to today gives the exact deadline. Another common use is payroll and billing, where invoices or payments recur every 2 or 4 weeks. This calculator shows the resulting date including the day of the week, so you can quickly confirm whether the deadline falls on a weekend.
Workweeks vs calendar weeks
A calendar week is any 7-day period, but a workweek contains only Monday through Friday - 5 business days. When planning work projects, workweeks matter more than raw weeks. This calculator counts the actual weekdays in the span (not simply multiplying by 5/7), which means results are accurate even when the start or end date falls mid-week. For example, a 14-day span starting on a Wednesday contains only 10 weekdays, not 10 exactly equal to 14 x 5/7. Weekend days and public holidays are counted separately so project managers can see both the calendar view and the working-day view side by side.
ISO week numbers explained
ISO 8601 is the international standard for week numbering. Under this system, weeks always start on Monday, and week 1 is defined as the week containing the first Thursday of the year (equivalently, the week containing January 4). A year under ISO rules can have 52 or 53 weeks. This matters for business reporting, payroll cycles that run on ISO weeks, and any cross-border schedule where different countries follow the standard. This calculator shows the ISO week number for both the start and end date of any "between dates" query so you can align results with spreadsheets and project tools that use the same numbering.
Common week spans as days and months
| Weeks | Days | Approx. months | Workweeks (5-day) |
|---|---|---|---|
| 1 | 7 | 0.23 | 1 |
| 2 | 14 | 0.46 | 2 |
| 4 | 28 | 0.92 | 4 |
| 8 | 56 | 1.84 | 8 |
| 13 | 91 | 2.99 | 13 |
| 26 | 182 | 5.98 | 26 |
| 52 | 364 | 11.96 | 52 |
| 104 | 728 | 23.92 | 104 |
Quick reference for typical planning horizons.
Frequently asked questions
How many weeks are in a year?
A common year (365 days) contains exactly 52 weeks and 1 day. A leap year (366 days) contains 52 weeks and 2 days. Under ISO 8601 week numbering, most years have 52 ISO weeks, but years that start on Thursday (or leap years that start on Wednesday) have 53 ISO weeks.
Does this calculator include or exclude the start and end dates?
The "weeks between" calculation counts the gap from the start of the first date to the start of the last date - so the start date is included in the count and the end date is excluded, just like counting days on a number line. This matches how most calendar and scheduling apps count date differences. For example, Monday to the next Monday is exactly 7 days (1 week), not 8.
How many weeks is 90 days?
90 days divided by 7 is 12 weeks and 6 days, or 12.857 weeks in decimal form. It is approximately 2.96 months.
How many weeks is 6 months?
Six calendar months is approximately 26 weeks. Because months have different lengths, the exact count depends on which six months are involved. January through June (181 days in a common year, 182 in a leap year) is 25 weeks and 6 days or 26 weeks respectively.
How do I calculate workweeks in a date range?
Count the number of weekdays (Monday-Friday) in the range, then divide by 5. This calculator does that automatically. Note that it counts calendar weekdays only and does not remove public holidays, since those vary by country and employer. If you need to exclude holidays, subtract them from the weekday total after the calculation.
What is the difference between a week and a workweek?
A week is any 7-day period, while a workweek is the 5-day Monday-to-Friday portion of a standard week. For date-span calculations, this matters because a 3-week vacation (21 days) contains only 15 workdays if it falls on standard weeks, fewer if it starts mid-week.
How are leap years handled?
The calculator works from the actual calendar dates, so any span that includes February 29 in a leap year automatically accounts for the extra day. There is no special setting to enable.