Use this free week calculator to quickly find week numbers, count weeks between two dates, and add or subtract weeks from any date.
Find week number for a date
Calculate weeks between two dates
Add or subtract weeks from a date
This calculator uses ISO week numbering (weeks start on Monday, and Week 1 is the week with the first Thursday of the year).
What is a week calculator?
A week calculator is a practical date tool that helps you answer common scheduling questions in seconds. Instead of manually counting calendar boxes, you can instantly determine the week number of any date, find how many weeks are between two dates, and calculate a future or past date by adding or subtracting weeks.
This is useful for project planning, payroll periods, school terms, fitness programs, pregnancy timelines, and quarterly reporting. If your work or life runs on recurring weekly cycles, a week calculator removes guesswork and reduces planning errors.
How to use this week calculator
1) Find the week number
Pick any date in the first section and click Calculate Week Number. You will get:
- ISO week number and ISO year
- The day of the year
- Start and end dates of that week (Monday to Sunday)
2) Count weeks between dates
Enter a start date and end date in the second section, then click Calculate Difference. The result includes total days, exact weeks, full weeks plus remaining days, and an inclusive day count (helpful for events or booking windows).
3) Add or subtract weeks
In the third section, enter your base date and the number of weeks to shift. Use positive values to move forward in time and negative values to move backward. This is ideal for deadlines, installment schedules, or milestone planning.
Understanding ISO week numbers
Different systems define weeks differently. This page uses the international ISO-8601 standard, which is common in business and software systems. Here are the key rules:
- Weeks start on Monday.
- Week 1 is the week containing the year’s first Thursday.
- Some years have 53 weeks instead of 52.
- Dates near New Year may belong to the previous or next ISO year.
These details matter for reporting systems, manufacturing cycles, and cross-border collaboration where consistency is required.
Common use cases
Project management
Managers often estimate timelines in weeks. With a week calculator, you can convert “8 weeks from kickoff” into an exact delivery date and verify sprint windows quickly.
Payroll and HR
For biweekly or weekly payroll, you can count full week spans between dates and prevent mistakes in pay period boundaries.
Education planning
Schools and universities frequently map lessons by week number. Instructors can align syllabi and assessments without manually scanning calendars.
Health and personal goals
From training plans to habit streaks, week-based planning gives structure. You can measure progress in week blocks and set clearer checkpoints.
Tips for accurate week calculations
- Use the same week standard across all tools and teams.
- Confirm whether your process needs inclusive or exclusive date counting.
- When calculating deadlines, account for weekends and holidays separately.
- For international teams, clarify local timezone assumptions.
Final thoughts
A reliable week calculator is a small tool with high impact. It improves planning speed, reduces date errors, and keeps recurring schedules on track. Bookmark this page and use it whenever you need clean, quick week-based date math.