Week Number Calculator

The Week Number Calculator tells you what week of the year it is right now, or the week number for any date you choose. It shows the ISO 8601 week number (the international standard, with weeks starting on Monday) alongside the North American week number (weeks starting on Sunday), the exact Monday-to-Sunday date range of the week, and where that week sits in the year. You can also work in reverse — enter a week number and a year, and the tool returns the calendar dates that week covers.

What Is a Week Number?

A week number is a count of the weeks of the year, from 1 up to 52 or 53. Businesses, manufacturers, project planners, and payroll teams use week numbers as a compact way to refer to a period — "we ship in week 32" is shorter and less ambiguous than naming a date range. The catch is that there is more than one rule for assigning week numbers, so the same date can have different week numbers depending on which convention you follow.

ISO 8601 Week Numbers (the International Standard)

The ISO 8601 standard is used across Europe and in most of the world's business and software systems. Its rules are precise:

• Weeks start on Monday and end on Sunday.
• Week 1 is the week containing the year's first Thursday — equivalently, the week that contains January 4.
• A week always belongs entirely to one ISO year. This means the first days of January can fall in the last week (52 or 53) of the previous year, and the last days of December can fall in week 1 of the next year.

North American (US) Week Numbers

The common North American convention is simpler but less standardised:

• Weeks start on Sunday and end on Saturday.
• Week 1 is the week that contains January 1.
• Weeks are counted within each calendar year, so this method can run up to week 53 or 54.

Because the start-of-week and the week-1 rule both differ from ISO, the two systems frequently disagree — most noticeably in early January and late December. This calculator always shows both so you are never caught out by the difference.

How Many Weeks Are in a Year?

Most years contain 52 ISO weeks, but roughly every five to six years there is a 53-week year. A year has 53 ISO weeks when:

• January 1 falls on a Thursday, or
• it is a leap year and January 1 falls on a Wednesday.

Recent and upcoming 53-week years include 2015, 2020, and 2026. These extra weeks matter for payroll, retail sales reporting, and any system that buckets data by ISO week.

Week of the Year vs Week of the Month

Don't confuse the two. The week of the year counts from the start of the year and runs 1–53. The week of the month counts only within a single calendar month and runs 1–6. This tool reports both, plus the day of the year (1–366) and the calendar quarter.

How to Use This Calculator

1. Choose a mode: "By Date" finds the week number for a date; "By Week Number" converts a week number back into calendar dates.
2. Enter your date: Pick any date with the date picker, or leave it on today to see the current week number instantly.
3. Click Calculate: The tool computes the ISO and US week numbers right away.
4. Review the results: See the week number, the Monday-to-Sunday date range, the visual week strip, and where the week falls in the year.

Common Uses for Week Numbers

Frequently Asked Questions

What week of the year is it right now?

This calculator opens on today's date and shows the current ISO 8601 week number immediately. ISO weeks run Monday to Sunday, so the current week number tells you how many full weeks of the year have completed plus the one in progress.

How is the ISO 8601 week number calculated?

ISO 8601 weeks start on Monday. Week 1 is the week that contains the year's first Thursday, which is the same as the week containing January 4. Counting Monday-to-Sunday weeks from there gives the week number. Because of this rule, a date in late December can belong to week 1 of the next year, and an early-January date can belong to week 52 or 53 of the previous year.

Why do ISO and US week numbers differ?

They use different rules. ISO 8601 weeks start on Monday and week 1 must contain the first Thursday. The North American convention starts weeks on Sunday and simply makes week 1 the week that contains January 1. As a result the two systems often give different week numbers for the same date, especially in early January and late December.

How many weeks are in a year?

Most years have 52 ISO weeks, but some have 53. A year has 53 ISO weeks when January 1 falls on a Thursday, or when it is a leap year whose January 1 falls on a Wednesday. For example, 2020 and 2026 are 53-week years.

What dates does a given week number cover?

Switch to the "By Week Number" mode, enter the year and the week number, and choose ISO or US. The calculator returns the exact start and end dates of that week. For ISO weeks this is always a Monday-to-Sunday range.

What is the difference between week of the year and week of the month?

The week of the year counts weeks from the start of the year (1 to 52 or 53). The week of the month counts weeks within a single calendar month (1 to 5 or 6). This tool shows both.

Additional Resources

• ISO 8601 - Wikipedia
• ISO Week Date - Wikipedia