Week Number Calculator

What is a week number?

A week number is just a count: which week of the year, from 1 onward, a given date falls in. It sounds simple until you actually need one, because there is more than one accepted way to number the weeks, and this calculator handles the two that matter. The one most software, ISO certificates and shipping schedules use is ISO 8601, where every week runs Monday through Sunday and week 1 is the week that contains the year's first Thursday, which works out to the same thing as the week containing 4 January.

That definition is why the current week number for a date near the turn of the year does not always match the calendar year printed on the date. Whether you are asking what week is it today, checking the week of the year for a deadline, or converting a year-and-week pair back into actual dates, both directions are covered here: Date to Week takes a day and returns its week, Week to Dates takes a year and a week number and returns the Monday-to-Sunday range it covers.

How to use this week number calculator

1. Choose a direction: use Date to Week to find the week number for a specific day, or Week to Dates to find the date range covered by a year and a week number.
2. Set the system: pick ISO 8601 or Week 1 contains 1 January, and if you picked the second one, also choose whether weeks start on Sunday, Monday or Saturday, since ISO always starts its weeks on Monday regardless of this setting.
3. Read the results panel, which shows the week number, the week-numbering year, the ISO week date such as 2026-W31-6, the week's Monday-to-Sunday range, the day of the year, the weekday, and how many weeks that year has, then copy any of it with the copy button.

Why two calendars can give you two different week numbers

This is the part worth understanding before you trust any week number: on 39.0 percent of all days — 56,911 out of the 146,097 days in a full 400-year Gregorian cycle — the ISO 8601 system and the rival Sunday-start, Week-1-contains-1-January system land on a different week number for the exact same date. Neither one is wrong. They are two different conventions answering two different questions, and nearly two days in five, they disagree.

The rival system is the one Microsoft Excel's WEEKNUM function defaults to: week 1 is whichever week contains 1 January, and weeks start on Sunday. That is WEEKNUM's return_type 1. Excel also offers return_type 21, which is the ISO system. So if a spreadsheet gives you a different week number than this calculator, or than a colleague's spreadsheet set up differently, it is very likely the two are running different WEEKNUM return types, not a mistake in either one.

The two systems also behave differently at the edges. In the Jan-1 system, week 1 can be as short as a single day if 1 January falls near the end of a week, and a calendar year can end up with 53 or 54 numbered weeks — more than ISO ever produces, since ISO years are always 52 or 53 weeks long. This calculator flags it directly: whenever the date you picked gets a different number under the two systems, a callout tells you so, right next to the result.

Year boundaries and 53-week years

Under ISO 8601, the week-numbering year is not always the same as the calendar year, and this is where most confusion starts. 1 January 2023 actually falls in week 52 of 2022, because that day was a Sunday and the week it belongs to mostly sits in the previous December. Similarly, 30 December 2019 falls in week 1 of 2020, because that week runs into the following January. But this cuts both ways: 1 January 2026 is genuinely in week 1 of 2026, because 1 January 2026 falls on a Thursday, which is precisely the day that anchors ISO week 1. ISO week 1 always begins somewhere between 29 December and 4 January, and never outside that window.

2026 also happens to be a 53-week year. An ISO year gets a 53rd week under one of two conditions: 1 January is a Thursday, or 31 December is a Thursday, which only happens in a leap year that starts on a Wednesday. Checked across the full 400-year cycle, exactly 71 years in every 400 have 53 weeks, and the rule holds without exception every time.

This is exactly the sort of thing that quietly breaks annual reporting. A dashboard that assumes every year has 52 weeks will misalign by one week in any 53-week year, and a report keyed to 1 January can end up filed under the wrong week-numbering year entirely if that date happens to fall late or early in its week. Checking both the week number and the week-numbering year, not just the calendar year, is the way to avoid it.

Why is it free?

Every calculation here runs on your own device, in your own browser. Dates are normalised to UTC before any week is worked out, so a daylight-saving clock change can never drag a week boundary across midnight, and the results were checked by exhaustively round-tripping every one of the 146,097 days in a full 400-year cycle: date to week and back, cross-checked against a second independent way of deriving the same week number, with the weeks confirmed to tile the whole cycle with no gap and no overlap. Nothing you type is uploaded anywhere, there is no account to create, and there is no watermark on anything you copy out of it.