WEEKNUM vs ISOWEEKNUM: Which Week Number Should You Use?

The short answer to WEEKNUM vs ISOWEEKNUM: both turn a date into a week number, but they disagree on where week 1 starts. WEEKNUM starts it on 1 January, while ISOWEEKNUM follows the international ISO 8601 standard, where every week runs Monday to Sunday and week 1 holds the year's first Thursday. This guide shows both on the same dates, explains the New Year traps, and tells you which to use for weekly reports.

WEEKNUM vs ISOWEEKNUM at a glance#

For most dates the two give the same number. The differences show up at the start and end of the year.

How WEEKNUM works#

WEEKNUM counts weeks from 1 January. Whatever day 1 January falls on, that partial week is week 1, and week 2 begins on the next week-start day.

=WEEKNUM(A2)
=WEEKNUM(A2,2)

The first counts weeks that start on Sunday, and the second counts weeks that start on Monday. The optional return type sets the first day of the week:

Because week 1 always starts on 1 January, it can be very short. In 2027, 1 January is a Friday, so with =WEEKNUM(A2,2) week 1 is only Friday 1 to Sunday 3 January. December can end with a short week too. In a leap year that starts on Saturday, such as 2028, Sunday 31 December is alone in week 54 with the default return type.

How ISOWEEKNUM works#

ISOWEEKNUM follows ISO 8601, the international standard for dates. It has only one argument and two rules:

1. Weeks always run Monday to Sunday.
2. Week 1 is the week that contains the first Thursday of the year. (That's the same as the week containing 4 January.)

=ISOWEEKNUM(A2)

Every ISO week has exactly seven days, and a year has 52 or 53 of them. The catch is that the first and last few days of a calendar year can belong to the other year's week. If 1 January falls on a Friday, Saturday or Sunday, those days are still in the last week of the previous year. If it falls on a Tuesday, Wednesday or Thursday, ISO week 1 starts a few days before, in December.

The same dates with both functions#

Here is what each function returns for dates around New Year 2026 and 2027:

Three things stand out:

• Sunday vs Monday. On Sundays such as 4 January 2026, the default WEEKNUM has already moved to the next week, while Monday-based systems haven't.
• Late December. Monday 29 December 2025 is week 53 for WEEKNUM but week 1 of 2026 for ISO.
• Early January. Friday 1 January 2027 is week 1 for WEEKNUM but week 53 of 2026 for ISO.

The ISO year trap#

Because an ISO week can belong to another year, =YEAR(A2)&"-W"&ISOWEEKNUM(A2) labels 1 January 2027 as 2027-W53, a week that doesn't exist. Use the ISO year instead, which is the year of that week's Thursday:

=YEAR(A2-WEEKDAY(A2,2)+4)

WEEKDAY(A2,2) numbers Monday as 1, so A2-WEEKDAY(A2,2)+4 is always the Thursday of A2's week. For 1 January 2027 that Thursday is 31 December 2026, so the ISO year is 2026. A safe label is:

=YEAR(A2-WEEKDAY(A2,2)+4)&"-W"&TEXT(ISOWEEKNUM(A2),"00")

This returns 2026-W53. The Thursday trick also explains ISOWEEKNUM itself: the ISO week number is simply which week of its year that Thursday falls in, which is the formula in the exercise on this page. The WEEKDAY guide explains the return types it relies on.

When to use WEEKNUM#

WEEKNUM is the right choice when your organisation's weeks are tied to the calendar year:

• Your calendar says week 1 starts on 1 January. Many US calendars and planners work this way.
• You need Sunday-start weeks. ISOWEEKNUM can't start weeks on Sunday; =WEEKNUM(A2) can.
• Every week must stay inside one calendar year. WEEKNUM never assigns a date to another year's week, so =YEAR(A2) is always the right year label.
• Your company uses a custom week start, such as Saturday for retail. Return types 11 to 17 cover every day.

Accept that the first and last weeks can be short, and say so in your report so a 3-day week 1 isn't compared with full weeks.

When to use ISOWEEKNUM#

ISOWEEKNUM is the better choice when weeks must be comparable:

• Weekly reports and trends. Every ISO week has seven days, so week 1 vs week 2 is a fair comparison.
• International teams and suppliers. ISO week numbers are widely used in Europe and in planning, logistics and manufacturing, so "week 14" means the same thing to everyone.
• Year-over-year comparisons. ISO week 10 always starts on a Monday and always has seven days, in every year.

When you group data this way, also group by the ISO year, not the calendar year. Otherwise the last days of December and the first days of January end up in the wrong totals. The guide to summing by week shows how to total a column by week, and suggests week-start dates as an alternative to numbers.

Common mistakes with week numbers#

Most week number errors come from mixing systems:

• Mixing WEEKNUM and ISOWEEKNUM in one report. Team A says week 1, team B says week 53, for the same day. Agree on one system.
• Forgetting the return type. =WEEKNUM(A2) uses Sunday weeks. If your weeks start on Monday, write =WEEKNUM(A2,2).
• Labelling ISO weeks with YEAR(A2). Use the ISO year, =YEAR(A2-WEEKDAY(A2,2)+4), for dates near New Year.
• Text dates. Both functions return #VALUE! for dates stored as text. Convert them with DATEVALUE first.
• Week numbers across several years. Week 10 of 2025 and week 10 of 2026 both show 10. Include the year in the label or group by week-start date.
• Using a week number when a date is clearer. A week-start date like =A2-WEEKDAY(A2,2)+1 (Monday of the week) never repeats and sorts correctly. For month-based periods, EDATE vs EOMONTH covers the equivalent tools.

If your weekly numbers depend on working days rather than calendar weeks, see NETWORKDAYS vs WORKDAY.

Our recommendation#

Use ISOWEEKNUM for weekly reporting unless you have a reason not to. Its weeks are always seven days and always start on Monday, so weekly numbers are comparable within and across years, and it matches the standard many international teams use. Pair it with the ISO year formula for labels.

Use WEEKNUM when your organisation defines week 1 as the week of 1 January, when you need weeks that start on Sunday or another day, or when every date must stay in its own calendar year. In that case, set the return type explicitly so nobody has to guess. Whichever you pick, write the rule in a note on the sheet.

Practise it#

Try the exercise on this page: rebuild an ISO week number from the Thursday of the week and see why 1 January 2027 is in week 53. Then practise WEEKDAY, EDATE and EOMONTH in a live spreadsheet, and if date maths still feels odd, learn how Excel stores dates as numbers. Each formula is checked instantly as you type.