How to Use the ISO Week Number Calculator

Pick a date using the date field.
Click Calculate. The tool runs the ISO 8601 week algorithm instantly in your browser.
Read the ISO week number and ISO year on the left, and Excel/Sheets' different default WEEKNUM() result on the right — with a note explaining whether they agree for that particular date.
Nothing you enter is uploaded anywhere — this is pure date arithmetic running as JavaScript in your browser tab.

What ISO 8601 Week Numbering Actually Is

ISO 8601 (the same international standard behind the

YYYY-MM-DD

date format) also defines a precise, unambiguous rule for numbering weeks:

Weeks start on Monday, always.
Week 1 of a year is the week containing that year's first Thursday — equivalently, the week containing January 4th.
Every ISO week has exactly 7 days. There is never a short "partial" week 1 or week 53.

This rule has a genuinely counter-intuitive consequence: because week 1 is defined by where the Thursday falls, a handful of days at the very start of January can belong to the last ISO week of the previous year, and a handful of days at the very end of December can belong to the first ISO week of the next year. This tool calls that "ISO year" — it can legitimately differ from the calendar year.

Worked Examples

December 31, 2018 is a Monday. Under ISO 8601 it is week 1 of ISO-year 2019 — a plain calendar-year date of "2018" belongs to ISO week 1 of the following year.
January 1, 2021 is a Friday. Under ISO 8601 it is week 53 of ISO-year 2020 — a date in early January belongs to a week of the previous year, and that year (2020) has 53 ISO weeks rather than the usual 52.
January 1, 2024 is a Monday, so it neatly starts week 1 of ISO-year 2024 with no mismatch — this is the simple case, but it is not the general rule.

Why Excel/Sheets' Default WEEKNUM() Gives a Different Answer

This is the part most people trip over: Excel and Google Sheets' plain

WEEKNUM()

function is NOT ISO 8601 by default, and the two can disagree by an entire week near every year boundary.

WEEKNUM()

's default behavior (return_type 1) uses a completely different rule:

Weeks start on Sunday.
Week 1 is simply "the week containing January 1st" — even if that week only has 1-6 actual days inside the new year (no full-week requirement like ISO has).

For January 1, 2021 (a Friday), plain

WEEKNUM()

calls it week 1 of 2021 (because it contains Jan 1), while true ISO 8601 calls the exact same date week 53 of 2020. Neither is "wrong" — they're answering different, explicitly-defined questions — but silently assuming they're interchangeable is a real, recurring source of off-by-one-week bugs in financial reporting, payroll, and scheduling spreadsheets that need genuine ISO week numbers.

Getting True ISO Weeks Inside Excel or Sheets

Both Excel and Google Sheets can produce correct ISO 8601 week numbers — just not through plain

WEEKNUM()

. Use:

ISOWEEKNUM(date)
— a dedicated function, or
WEEKNUM(date, 21)
— the same plain
WEEKNUM()
function, but with return_type
21
, which switches it to the ISO 8601 rule.

Either one will match the ISO week number this tool calculates.

Frequently Asked Questions

Can a year have 53 ISO weeks? Yes — any year whose January 1st falls on a Thursday, or any leap year whose January 1st falls on a Wednesday, has 53 ISO weeks instead of the usual 52.

Does this tool handle leap years correctly? Yes — all date arithmetic accounts for real Gregorian leap years (including the 400-year rule), independent of Excel's separate 1900 serial-date leap-year bug covered by this site's Excel serial date converter tool.

Is my input uploaded or stored anywhere? No. The calculation runs entirely as JavaScript inside your browser tab — nothing is sent to a server, logged, or stored.

Is this tool free? Yes, completely free, no login, no limit.