Week Number Calculator

What Is the Week Number Calculator?

This tool finds the ISO-8601 week number for any date -- the standard used by payroll systems, project schedules, and manufacturing calendars ("delivery in week 12"). ISO weeks start on Monday and week 1 is defined as the week containing the year's first Thursday.

How to Use This Calculator

Enter any date. The result shows its ISO week number and ISO year (which can differ from the calendar year for dates in very late December or very early January).

How the Week Number Is Calculated

Worked Example

Practical Uses

• Matching a date to a payroll or timesheet week number
• Reading manufacturing date codes that reference a week number
• Planning against a project schedule expressed in week numbers

For the date's position within its year as a day count instead, see the Day of Year Calculator. To find which fiscal quarter a date falls in, use the Quarter Calculator.

Step-by-Step: Reading a Week Number Correctly

Enter the date and note both numbers in the result: the ISO week and the ISO year. Always keep the two together when you write them down or pass them on, for example week 1 of 2027 rather than just week 1. Systems that store the pair (year plus week) sort and compare correctly across the new year, while a bare week number does not tell you which year it belongs to.

A Worked Example Around the New Year

Take the hypothetical date January 1, 2027, a Friday. The Thursday of that week is December 31, 2026, so the date belongs to the last ISO week of 2026, not to week 1 of 2027. The tool will show a 2026 ISO year even though you entered a 2027 date. Now take January 4, 2027, a Monday. Its Thursday is January 7, 2027, so it sits in week 1 of 2027. Entering both dates makes the boundary rule easy to see: the week number follows the Thursday, not the first of January.

Where This Mismatch Bites

• Spreadsheets. Some spreadsheet functions default to weeks that start on Sunday and count the week containing January 1 as week 1. That can disagree with the ISO result by one, particularly in the first and last days of the year. If your report must match an ISO-based supplier or payroll system, use ISO weeks consistently on both sides.
• Shipping and production. A delivery promised for week 52 or 53 may physically land in early January. Enter the actual date to see which ISO year the week belongs to before you plan around it.
• Year-end payroll. Pay periods and reports keyed to a week number can straddle two tax years. Check with your payroll provider or the relevant tax authority about how a boundary week is treated, because that is a legal and accounting question rather than a calendar one.

Limits of the Calculation

The tool only covers the ISO 8601 definition: Monday as day one and the week containing the first Thursday as week 1. It does not produce US-style week numbers, fiscal week numbers that start on a company-chosen date, or academic term weeks. It also works with a calendar date only, so there is no time-of-day or time zone involved. If your organization has its own week calendar, treat this result as a reference point and apply your internal offset.

Useful Ways to Apply It

• Translate a project plan written in weeks into real dates by entering the Monday of a week you are told about.
• Confirm a date code printed on a product or invoice that uses a year plus week format.
• Line up meetings across teams that plan by week number, avoiding the one-off errors that appear around January.

Frequently Asked Questions