What Week Number Is It? ISO Week Rules Explained (Free)
See today's week number and understand ISO 8601 week rules: weeks start Monday, and week 1 holds the year's first Thursday. Free week number calculator.
Published ยท 5 min read
The current week number is shown at the top of the free week number calculator, worked out from your device's date. Most of the world uses ISO 8601 week numbers: weeks start on Monday, and week 1 is the week that contains the year's first Thursday. That rule is why 1 January sometimes belongs to week 52 or 53 of the previous year, and why some years have 53 weeks.
Week numbers turn up in project plans, school timetables, shipping schedules, payroll and retail reporting. They're simple until New Year, when two people can give the same date different numbers. Here's how they work.
The ISO 8601 rules
ISO 8601 is the international standard for dates, and its week numbering has three rules:
- Weeks start on Monday and end on Sunday.
- Week 1 is the week with the year's first Thursday. An equivalent way to say it: week 1 is the week that contains 4 January.
- Every week belongs to exactly one year, the year its Thursday falls in.
Rule 3 is the clever part. Because a week is never split between two years, every week has seven days and every year has either 52 or 53 whole weeks.
The ISO way to write a week is 2026-W44, and a single day can be written as 2026-W44-3 (the third day, Wednesday).
A worked example: New Year 2026-2027
1 January 2026 was a Thursday, so the week from Monday 29 December 2025 to Sunday 4 January 2026 is week 1 of 2026. The last three days of December 2025 belong to 2026's first week.
At the other end, 1 January 2027 is a Friday. The week from Monday 28 December 2026 to Sunday 3 January 2027 has its Thursday on 31 December, so it's week 53 of 2026. The first three days of 2027 belong to the previous year's last week, and week 1 of 2027 starts on Monday 4 January.
| Date | ISO week |
|---|---|
| Mon 29 Dec 2025 | Week 1 of 2026 |
| Thu 1 Jan 2026 | Week 1 of 2026 |
| Wed 28 Oct 2026 | Week 44 of 2026 |
| Thu 31 Dec 2026 | Week 53 of 2026 |
| Fri 1 Jan 2027 | Week 53 of 2026 |
| Mon 4 Jan 2027 | Week 1 of 2027 |
This trips up spreadsheets and reports all the time. If you group data by "year" and "week number", the first days of January 2027 land in 2026's buckets under ISO rules.
When does a year have 53 weeks?
A year has 53 ISO weeks when it starts on a Thursday, or when it's a leap year that starts on a Wednesday. Otherwise it has 52. 2026 starts on a Thursday, so it has 53 weeks; the next year with 53 ISO weeks is 2032.
This matters for anything planned "per week": a weekly salary paid every Friday, a 52-week savings challenge, or comparing this year's week 53 with a last year that didn't have one.
The US-style system
Many calendars in the US, Canada and some other countries count weeks differently, and the calculator offers this as US (Sunday start):
- Weeks start on Sunday.
- Week 1 is the week that contains 1 January, however few days of January it has.
The two systems often agree for most of the year. 28 October 2026 is week 44 in both. They disagree around New Year: under the calculator's US numbering, Thursday 31 December 2026 is already in week 1 of 2027, because that Sunday-to-Saturday week contains 1 January 2027. Under ISO, the same day is week 53 of 2026.
There's more than one US-style convention in software. Excel's WEEKNUM function, for example, starts week 1 on 1 January and can label the last days of December as week 53 of the old year. Excel and Google Sheets both have ISOWEEKNUM for ISO numbers. When you share week numbers with someone, agree which system you both mean.
How to find the week number of any date
Open the free week number calculator:
- Read This week is at the top, with the Monday-to-Sunday (or Sunday-to-Saturday) dates under it.
- Under Week numbering, choose ISO 8601 (Monday start) or US (Sunday start).
- To check another date, pick it in Week number for a date. You see its week number and the first and last day of that week. If the week belongs to a different year, it says so, as in "Week 53 of 2026". Press Today to jump back.
- Press Show all weeks of the year to list every week with its start and end dates. The week you picked is highlighted.
The full list is handy for planning: copy the week ranges into a project plan, a school term calendar or a sprint schedule.
Who uses week numbers
- Business and logistics: delivery dates are often given as "week 44" in Europe.
- Retail and finance: weekly sales reports compare the same week number year on year.
- Software teams: sprints and releases are often named by week.
- Schools: many timetables run on numbered weeks.
- Payroll: weekly and fortnightly pay runs follow week numbers.
If you need the number of days between two dates rather than a week number, use the free date calculator, or read the guide on counting the days between two dates.
Check your week number
Pick a date, choose ISO or US numbering, and see its week and dates straight away. The free week number calculator runs on your device and needs no sign-up.
Frequently asked questions
- What week number is it today?
- It's shown at the top of the free week number calculator, using your device's date and ISO 8601 numbering unless you switch to US numbering.
- When does ISO week 1 start?
- On the Monday of the week that contains the year's first Thursday. It can start as early as 29 December or as late as 4 January.
- Why is 1 January in week 53?
- Under ISO rules, a week belongs to the year its Thursday falls in. If 1 January is a Friday, Saturday or Sunday, its week has its Thursday in December, so it's the last week of the old year.
- How many weeks are in a year?
- 52 or 53 ISO weeks. A year has 53 when it starts on a Thursday, or is a leap year starting on a Wednesday. 2026 and 2032 both start on a Thursday.
- Does the US use ISO week numbers?
- Many US calendars and tools start weeks on Sunday and count the week with 1 January as week 1. Switch the calculator to US numbering to see those numbers.
Calculators
Try the Week Number Calculator
Free, no sign-up, works in your browser. See today's week number at a glance, find the week number of any date, and list every week of the year with its dates.