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Week Number Calculator
The ISO-8601 week number for any date — Monday-start, week 1 is the week containing the year’s first Thursday (Jan 4) — plus the ISO year, the week’s Monday–Sunday date range and weeks-in-year (52 or 53). Matches Excel/Sheets ISOWEEKNUM, and handles the year-boundary edge where the ISO year differs from the calendar year. Computed entirely in your browser. Accurate, instant and free — for United States.
2026-01-01 is ISO week 1 of 2026 (Mon 2025-12-29 – Sun 2026-01-04). 2026 has 53 ISO weeks.
ISO week
Week 1
ISO year
2026
Weeks in year
53
Week range (Mon – Sun)
2025-12-29 – 2026-01-04
2026-01-01 falls in ISO week 1 of 2026.
ISO-8601 vs your spreadsheet (ISOWEEKNUM vs WEEKNUM)
ISOWEEKNUM(). The older WEEKNUM(date, return_type) can give a different number: its default (return_type1) starts weeks on Sunday and calls the week containing January 1 “week 1”, so the same date can read a different week near year boundaries. Use return_type 21 (or ISOWEEKNUM) to reproduce this ISO answer.Year-boundary weeks — why the ISO year can differ
How we calculate
How we calculate this
Reviewed by Reckonist Editorial · Last reviewed July 2026. Figures follow the methods and sources set out in our editorial standards.
ISO-8601 week date: Monday-start, week 1 contains the year’s first Thursday; weeks-in-year is 53 when Jan 1 is a Thursday (or a leap year with Jan 1 on Wednesday), else 52.
ISO-8601 · ISO week date
Take it further: count the gap between two dates with the Date Difference Calculator, or shift a date forward or back with the Date Add / Subtract Calculator.
The ISO-8601 week rule
Under ISO-8601, weeks start on Monday and week 1 is the week that contains the year’s first Thursday — equivalently, the week that contains January 4. This is the definition where naïve calculators quietly go wrong, and getting it right is the whole point of this tool.
Week number
Thursday rule
take the Thursday of the date’s week · week = whole weeks since the ISO year’s first Thursday + 1
Weeks are Monday-start; the Thursday anchor is what pins the week to the correct ISO year.
Weeks in year
52 or 53
53 if Jan 1 is Thursday, or (leap year and Jan 1 is Wednesday); else 52
Don’t assume 52 — long ISO years have a week 53.
- 1Week 1 rule: 1 Jan 2026 is a Thursday, so its week contains the year’s first Thursday → ISO week 1.
- 2Weeks in year: Jan 1 is a Thursday → 2026 has 53 ISO weeks.
- 3Boundary check: 29 Dec 2025 (Monday) also reads ISO week 1 of 2026 — the ISO year runs ahead of the calendar year.
The start and end dates of a week
Every ISO week runs Monday to Sunday. The calculator turns your date into a concrete range — the Monday (start) and Sunday(end) of the week it falls in — so a reference like “week 27” becomes real calendar dates you can plan around.
Monday–Sunday, always
How many weeks are in a year
A year has 53 ISO weeks when 1 January is a Thursday, or when it is a leap year and 1 January is a Wednesday; otherwise it has 52. Assuming 52 is a common bug — these “long” years are exactly where week-numbering breaks.
53-week (long ISO) years
ISOWEEKNUM vs WEEKNUM — why Excel disagrees
Most people who get a “wrong” week number arrive from a spreadsheet. ISOWEEKNUM(date) follows the strict ISO rule (Monday-start, first-Thursday week) and matches this tool. The older WEEKNUM(date, return_type) uses a different definition: its default (return_type 1) starts weeks on Sunday and treats the week containing 1 January as week 1, so the same date can read a different number — especially near year boundaries.
Reproduce the ISO answer in a spreadsheet
=ISOWEEKNUM(A1), or the equivalent =WEEKNUM(A1, 21) (the ISO return_type). If your sheet shows a different number, it is almost certainly using WEEKNUM with a Sunday-start return_type.Year-boundary weeks (ISO year ≠ calendar year)
Late-December dates can belong to week 1 of the next ISO year, and early-January dates to week 52 or 53 of the previous one. For example 29 December 2025 (a Monday) is ISO week 1 of 2026. This is the ISO-year-boundary subtlety, and getting it wrong is the classic week-number bug — so the tool always shows the ISO year alongside the week.
Always read the ISO year with the week
Frequently asked questions
The ISO-8601 week number counts the weeks of the year with weeks starting on Monday, where week 1 is the week that contains the year’s first Thursday — equivalently, the week that contains January 4. Under this rule 1 January 2026 (a Thursday) is ISO week 1 of 2026. It is the standard used across Europe and in business, manufacturing and logistics.
Enter today’s date and read off the ISO week number. The tool shows the ISO week (1–53), the ISO year, and the week’s Monday-to-Sunday date range. Because the “current” week depends on your local date, the answer follows the date you enter rather than a fixed timezone.
Most years have 52 ISO weeks, but a year has 53 when 1 January is a Thursday, or when it is a leap year and 1 January is a Wednesday. By that rule 2026 has 53 ISO weeks (1 January 2026 is a Thursday); the next 53-week years are 2032 and 2037. Every other year from 2026 to 2048 has 52.
Spreadsheets have two functions. ISOWEEKNUM(date) follows the strict ISO-8601 rule (Monday-start, week 1 contains the first Thursday) and matches this calculator. The older WEEKNUM(date, return_type) uses a different definition: its default (return_type 1) starts weeks on Sunday and treats the week containing January 1 as week 1, so it can report a different number, especially near year boundaries. Use WEEKNUM(date, 21) or ISOWEEKNUM to reproduce the ISO answer.
Yes. Late-December dates can fall in ISO week 1 of the next ISO year, and early-January dates can fall in week 52 or 53 of the previous ISO year. For example 29 December 2025 (a Monday) is ISO week 1 of ISO year 2026. That is why the tool shows the ISO year, which can differ from the calendar year at the boundary.
Every ISO week runs from Monday to Sunday. The calculator shows the Monday (start) and Sunday (end) date of the week your entered date falls in, so you can turn a “week 27” reference into concrete calendar dates.
Often, yes. A common US/spreadsheet convention starts weeks on Sunday and calls the week containing 1 January “week 1”, which can differ from the ISO number by one near year boundaries. This tool defaults to the international ISO-8601 standard; if your spreadsheet disagrees, it is almost always using the WEEKNUM (non-ISO) definition.
Method & references
Methodology: the ISO-8601 week number is found by taking the Thursday of the entered date’s (Monday-start) week, then counting whole weeks from that ISO year’s first Thursday and adding one; weeks-in-year is 53 when 1 January is a Thursday (or a leap year with 1 January on a Wednesday), else 52; the week range is the Monday and Sunday of the entered date’s week. All calculations run on UTC midnights, client-side, and nothing is stored.
Cross-links
How we calculate this
Reviewed by Reckonist Editorial · Last reviewed July 2026. Figures follow the methods and sources set out in our editorial standards.
ISO-8601 week date: Monday-start, week 1 contains the year’s first Thursday; weeks-in-year is 53 when Jan 1 is a Thursday (or a leap year with Jan 1 on Wednesday), else 52. Reference: ISO-8601.
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