Week Number Calculator: ISO and US Week of the Year
The ISO 8601 week number and the US (Sunday-start) week number for any date, with the week's dates.
Inputs
- ISO week-year
- 2026
- ISO week runs
- Monday Aug 24, 2026 – Sunday Aug 30, 2026
- US week number (Sunday start, Excel WEEKNUM)
- Week 36
- Date
- Sunday, August 30, 2026
- ISO 8601: weeks start on Monday and week 1 is the one with the year's first Thursday. The US convention starts on Sunday and counts the week with 1 January as week 1, so the two can differ by one.
Use this calculator on your own site
Free embed
- Two lines of HTML, unlimited views
- Shows a small CalculateEveryday bar, with a credit link under the calculator
What each Week Number input means
- Date
- Any date. The page returns its ISO week, the ISO week-year it belongs to, the Monday-to-Sunday dates of that week, and the US Sunday-start week number.
Week Number formula
ISO: Monday-start weeks, week 1 = the week with the first Thursday · US: Sunday-start weeks, week 1 = the week with 1 January
Two week-numbering systems are in common use and they disagree for part of every year. ISO 8601, used across Europe and in most planning tools, starts weeks on Monday and calls week 1 the week that contains the year's first Thursday — so 1 January can belong to week 52 or 53 of the previous year, and that year is the 'ISO week-year'. The US convention, which is Excel's default WEEKNUM, starts weeks on Sunday and calls the week containing 1 January week 1, even if it holds only one day. The page gives both, with the Monday-to-Sunday dates of the ISO week.
Week Number worked example
Sunday 30 August 2026 is in ISO week 35 — the week of Monday 24 to Sunday 30 August. In the US system it is week 36, because 1 January 2026 was a Thursday and US week 1 began on Sunday 28 December 2025.
More Week Number examples
Friday 1 January 2027 is in Week 53 — of ISO year 2026, the week of Monday 28 December to Sunday 3 January. In the US system it is week 1 of 2027.
Friday 2 October 2026 is in ISO Week 40, Monday 28 September to Sunday 4 October, and US week 40 as well.
Thursday 31 December 2026 is in ISO Week 53 — 2026 is one of the years with 53 ISO weeks — and in US week 53 too, the week that began on Sunday 27 December.
Where the Week Number calculator can give the wrong answer
- Early January can be in week 52 or 53 of the previous ISO year; late December can be in week 1 of the next. Read the ISO week-year line.
- Some ISO years have 53 weeks — those that start on a Thursday, or leap years that start on a Wednesday. 2026 is one.
- Excel has other WEEKNUM modes (Monday-start, and 21 for ISO); this page gives the default and ISO.
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Week Number reference table
| ISO year | Week 1 begins | Weeks in the year |
|---|---|---|
| 2025 | Monday 30 December 2024 | 52 |
| 2026 | Monday 29 December 2025 | 53 |
| 2027 | Monday 4 January 2027 | 52 |
| 2028 | Monday 3 January 2028 | 52 |
| 2029 | Monday 1 January 2029 | 52 |
| 2030 | Monday 31 December 2029 | 52 |
Week Number FAQ
- Enter today's date. The page shows the ISO number used in Europe and the US Sunday-start number, which can differ by one.
- ISO weeks start Monday and week 1 has the first Thursday; US weeks start Sunday and week 1 has 1 January. For 30 August 2026 that gives ISO week 35 and US week 36.
- 52 or 53. In ISO numbering 2026 has 53 weeks, because it starts on a Thursday; 1 January 2027 is in ISO week 53 of 2026.
- WEEKNUM's default is the US system (Sunday start, 1 January in week 1). ISOWEEKNUM, or WEEKNUM with return type 21, gives the ISO week.
- ISO week 1 of 2027 starts on Monday 4 January 2027, because 1–3 January belong to week 53 of 2026. In the US system, week 1 is the week of Sunday 27 December 2026 to Saturday 2 January 2027.
- An ISO week n begins on the Monday of week 1 plus 7 × (n − 1) days. Week 1 of 2026 began on 29 December 2025, so week 40 begins 273 days later, on Monday 28 September 2026, and ends on Sunday 4 October.
- Friday 25 December 2026 is in ISO week 52 and US week 52.
- Not always. ISO week 1 starts on the Monday of the week that holds the year's first Thursday, which can be as early as 29 December or as late as 4 January.
- The Sunday-to-Saturday week that contains 1 January, even when it holds only one day of the new year — as in 2028, when 1 January is a Saturday.