PlanMaker ISOWEEKNUM Function: ISO Week Numbers and Years
Use ISOWEEKNUM in PlanMaker with documented syntax, reproducible worksheets, independently checked examples and practical compatibility limits.
Verification status: See the evidence and test scope described in this guide. How formula examples are checked
Quick answer: =ISOWEEKNUM(DATE(2017,1,1)) returns week 52, matching SoftMaker’s documented example. January 1, 2017 belongs to ISO week 52 of 2016. ISOWEEKNUM returns the week number alone, so an ISO reporting key also needs the correct ISO year.
Availability and version history
- Introduced: Documented in the PlanMaker 2016 manual; the original introduction release and date remain unknown.
- Availability: Documented in the 2026 English reference; no final supported-version endpoint is established.
- Scope: Versioned documentation does not establish identical behavior across desktop, mobile or localized builds.
- Checked: October 14, 2026, against the official ISOWEEKNUM reference, with independent calculations. No native PlanMaker test.
Syntax and ISO rules
=ISOWEEKNUM(Date)
Date is the one required argument. There is no optional week-start mode or default date. Use a genuine date cell or DATE to construct the value, rather than relying on an ambiguous string such as 01/02/2025.
The official ISOWEEKNUM reference establishes two rules:
- Weeks start on Monday.
- Week 1 is the week containing January 4.
Equivalently, an ISO week belongs to the year containing its Thursday. As a result, early January may belong to the previous ISO year, while late December may belong to the next one. The week number normally ranges from 1 to 53 and repeats each year.
Build a boundary-date worksheet
Create headers in A1:D1: Date, ISO week, Calendar year, ISO year. Enter the date formulas below in A2:A7 and apply a Date display format:
| Row | A: Enter | Expected B: ISO week | Expected C: Calendar year | Expected D: ISO year |
|---|---|---|---|---|
| 2 | =DATE(2017,1,1) |
52 | 2017 | 2016 |
| 3 | =DATE(2019,12,30) |
1 | 2019 | 2020 |
| 4 | =DATE(2020,1,5) |
1 | 2020 | 2020 |
| 5 | =DATE(2021,1,1) |
53 | 2021 | 2020 |
| 6 | =DATE(2021,1,4) |
1 | 2021 | 2021 |
| 7 | =DATE(2024,12,30) |
1 | 2024 | 2025 |
In B2 enter =ISOWEEKNUM(A2) and fill to B7. In C2 enter =YEAR(A2) and fill to C7. Leave D2:D7 for the ISO-year formula in the next section.
January 1, 2017 was a Sunday in the ISO week beginning December 26, 2016. January 4, 2021 was the Monday starting that year’s first ISO week. These are calendar rules, not errors caused by the spreadsheet changing the input date.
Calculate a usable ISO year-week key
In D2 enter:
=YEAR(A2-WEEKDAY(A2,2)+4)
Fill to D7. WEEKDAY mode 2 numbers Monday as 1 through Sunday as 7. Subtracting that number and adding 4 moves to the Thursday in the same week; YEAR then returns its calendar year. This gives the ISO-year values shown in column D.
Put ISO key in E1. In E2 enter =D2&"-W"&TEXT(B2,"00") and fill to E7. The expected labels are 2016-W52, 2020-W01, 2020-W01, 2020-W53, 2021-W01, 2025-W01. TEXT’s formatting argument supplies a two-digit week display. Keep A:B as numeric data; E is a text label for display or grouping.
Using YEAR(A2) instead of the ISO year would incorrectly label the first row as 2017-W52 and the last as 2024-W01. An ISO week number alone is not a unique reporting period.
Compare with WEEKNUM
Keep the main table unchanged. In G2 enter =DATE(2020,1,5). Enter the following formulas in H2:H5:
| Cell | Formula | Expected result |
|---|---|---|
| H2 | =WEEKNUM(G2) |
2 |
| H3 | =WEEKNUM(G2,2) |
1 |
| H4 | =WEEKNUM(G2,21) |
1 |
| H5 | =ISOWEEKNUM(G2) |
1 |
The WEEKNUM reference defaults to a Sunday start and a week containing January 1. Mode 2 starts Monday but still uses January 1. Mode 21 explicitly uses the ISO January-4 rule.
A Monday start by itself is therefore insufficient to identify ISO weeks. On January 1, 2017, WEEKNUM(...,2) is 1, while ISOWEEKNUM is 52. Choose one convention consistently and label it clearly.
PlanMaker’s ISOWEEK alternative and export warning
ISOWEEK(Date,1) can directly return the ISO year, so =ISOWEEK(A2,1) would give 2016 for the first worksheet row. Its default instead returns the ISO week number.
However, SoftMaker explicitly warns that ISOWEEK is not an Excel function: formulas using it are replaced by their current results as fixed values when saving in Excel format. The Thursday/YEAR/WEEKDAY composition above avoids that PlanMaker-only function, though an imported workbook still needs recalculation checks. SoftMaker documents ISOWEEKNUM support in Excel 2013 and later.
Troubleshooting and limits
- January gives week 52 or 53: inspect the ISO year, not just the date’s calendar year.
- December gives week 1: that date may belong to the following ISO year.
- A second argument was added: ISOWEEKNUM takes one argument; use WEEKNUM to select another week convention.
- Date-like text is misread: use DATE or a verified date cell and apply a readable display format. Locale affects input and formula separators.
- Reports mix week systems: check headers and all related formulas, especially a default WEEKNUM call hiding among ISO calculations.
- Invalid, empty or out-of-range input: the short function page does not specify every exact error or coercion rule; no such results are invented here.
These formulas group dates into ISO weeks. They do not count workdays, exclude holidays, or define a company’s fiscal calendar.