How is the ISO 8601 week number defined?

Under the international ISO 8601 standard, weeks begin on Monday. Week 1 of any year is defined as the week that contains the first Thursday of the new year (or the week containing January 4th).

How many weeks are in a calendar year?

A standard Gregorian year has 52 weeks plus 1 day (or 2 days in a leap year). In the ISO week-numbering system, most years have 52 weeks, but years that start on Thursday (or leap years starting on Wednesday) contain 53 weeks.

How do you calculate the number of weeks between two dates?

Calculate the total calendar days between the two dates and divide by 7. The integer portion represents complete weeks, and the remainder represents additional days.

Why do US and ISO week numbers sometimes differ?

In the United States and Canada, the week traditionally begins on Sunday and Week 1 starts on January 1st regardless of weekday. In the international ISO standard, weeks begin on Monday.

ISO 8601 Calendar Intelligence

Week Calculator – Week Number, Range & Date Shifter

Find the current ISO week number, calculate total weeks between any two dates, and shift dates forward/backward by exact week counts.

Calculated Week -

Current Week Number

Select Date -

Start Date

End Date

ISO Week Number-

Remaining Weeks in Year

Year Progress

Financial Quarter

Day of Year

Total Days

Understanding ISO Week Numbers & Duration Math

The ISO 8601 week-numbering system is the internationally recognized benchmark used across manufacturing, enterprise resource planning (ERP), corporate finance, and sprint management. It standardizes calendar weeks by designating Monday as the starting day and assigning Week 1 to the week containing the year's first Thursday.

Adding and Subtracting Weeks: Planning project roadmaps or pregnancy trimesters requires shifting forward or backward by an exact multiple of 7 days. Our tool computes exact target calendar dates, accounting for month-end rollovers and leap years automatically.

Key Metrics Provided: Current ISO week number, percentage of the year elapsed, remaining weeks until year-end, financial quarter (Q1–Q4), and exact days between milestones.

What is an ISO week date?

An ISO week date identifies dates by year, week number (W01–W53), and weekday (1 for Monday to 7 for Sunday). For example, 2026-W36-1 represents Monday of Week 36 in 2026.

Can a date in January belong to the previous year's week?

Yes. If January 1 falls on a Friday, Saturday, or Sunday, those days belong to Week 52 or 53 of the preceding year because the week contains fewer than four days of the new year.

How many weeks are in 90 days?

90 days is equal to 12 weeks and 6 days (or approximately 12.86 decimal weeks).

How do project managers use week numbers?

Software teams and global supply chains use week numbers (e.g., "Feature release scheduled for Week 42") to coordinate timelines without ambiguity across differing international holiday schedules.