# Week number calculator

> A week number tells you which week of the year a date falls in. Two systems exist — ISO 8601 (Monday-first, used internationally) and the US system (Sunday-first) — and they can.

Find the ISO and US week number for any date, plus day of year, day of week, quarter, and weeks remaining in the year.

**Interactive version:** https://onlinecalculator.me/date-time/week-number/
**Category:** Date & Time

## How to use

1. Pick a date with the date picker — it defaults to today.
2. ISO week, US week, day of week, day of year, quarter, and weeks remaining all appear instantly.

## ISO 8601 week numbering

Week 1 = the week containing the first Thursday of the year (equivalently, the week with January 4 in it).

- Weeks run **Monday → Sunday**
- The first and last few days of a year may belong to the adjacent ISO year
- Used internationally — ISO, most of Europe, most business software

## US week numbering

Week 1 = the week containing January 1.

- Weeks run **Sunday → Saturday**
- There is no ambiguity about which year a date belongs to
- Used in U.S. retail calendars, school schedules, and some payroll systems

## Worked examples

### January 1, 2026 (a Thursday)

- **ISO week**: Week 1, 2026 (ISO year starts on Monday Dec 29 2025; Jan 1 is Thursday of that week)
- **US week**: Week 1, 2026
- **Day of year**: 1
- **Quarter**: Q1

### December 31, 2023 (a Sunday)

- **ISO week**: Week 52, 2023 *(still in the previous ISO year — the new ISO year starts Monday Jan 1)*
- **US week**: Week 53, 2023
- **Day of year**: 365
- **Quarter**: Q4

### January 1, 2024 (a Monday)

- **ISO week**: Week 1, 2024 *(Monday is the first day of an ISO week — clean start)*
- **US week**: Week 1, 2024
- **Day of year**: 1

## Notes

- **Year-edge ambiguity** — if you're sharing a report dated "week 52" or "week 1" near New Year, always include the year to avoid off-by-one errors. ISO week 1 of 2024 can include dates from December 2023.
- **52 vs. 53 weeks** — most years have 52 ISO weeks. Years starting on Thursday (and leap years starting on Wednesday) have 53. 2020 had 53; 2026 will have 53.
- **Quarters** are calendar quarters, not fiscal quarters — Jan–Mar is Q1, Apr–Jun Q2, and so on.

## Frequently asked questions

### What is the difference between ISO and US week numbers?

ISO 8601 weeks start on Monday; week 1 contains the year's first Thursday. US weeks start on Sunday; week 1 contains January 1. The two systems can disagree by up to 1 week near the start of year.

### What is ISO week 1?

ISO week 1 is the week containing the first Thursday of the year. January 1 is not necessarily in week 1.

### Can the ISO year differ from the calendar year?

Yes. Dates in late December that fall in ISO week 1 belong to the next ISO year. Dates in early January that belong to week 52 or 53 of the prior year show the previous ISO year.

### How many weeks are in a year?

Most years have 52 ISO weeks. Years that start on Thursday, or leap years starting on Wednesday, have 53 weeks.

### How do I share my calculation?

Click "Share with my numbers" to copy a URL with your date.

## Sources

- [ISO week date](https://en.wikipedia.org/wiki/ISO_week_date) — Wikipedia
- [ISO 8601 date and time format](https://en.wikipedia.org/wiki/ISO_8601) — Wikipedia

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- [Business days calculator](https://onlinecalculator.me/date-time/business-days/) — Working days between two dates, excluding weekends.
- [Countdown calculator](https://onlinecalculator.me/date-time/countdown/) — Days, hours, and minutes until any future date.

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