Week Number Calculator (ISO 8601 & US)

This Week Number Calculator turns any date into its week number, and it shows two of them side by side, because "week 1" doesn't point at one fixed set of dates. Two common systems count the weeks of the year, and they disagree most in the first days of January, right when an American team and a European partner both say "week 1" and mean dates a week apart. So both numbers come up together, the ISO 8601 one that the rest of the world runs on with its Monday start, and the US one that starts on Sunday, along with the weekday and the exact dates that week covers. It runs entirely in your browser, nothing you type is uploaded, and the answer updates the instant you pick a date.

1. Pick a date. Use the date picker, or tap Use today to see what week it is right now.
2. Read the two week numbers. The ISO 8601 week sits up top (shown as 2026-W30 too), the US Sunday-start week right below it, so a mismatch is impossible to miss.
3. Check the date range. The Monday to Sunday span of that week is spelled out in full, which is how you turn "week 30" back into real calendar dates.
4. Watch the flags. If your date's ISO week belongs to a different year, or the year happens to run 53 weeks, the tool tells you plainly.
5. Go the other way. Switch to Week number to dates, type an ISO year and a week number, and it hands you back the days that week covers.

You also get the weekday, day of the year, quarter, and how many ISO weeks remain, updated the instant you change the date.

ISO 8601 is the version most of the world runs on. European business, database engines, the ISOWEEKNUM function in spreadsheets, basically anything built to a formal spec. Three rules define it, and they're worth understanding rather than memorizing. Weeks start on Monday. Week 1 is the week that holds the year's first Thursday, which is the same as the week containing January 4. And every week belongs whole to a single year, assigned to whichever year owns its Thursday.

That third rule is where people get caught. Because an entire week goes to one year, the first days of January can land in the last week of the old year. January 1, 2023 was a Sunday, and that week's Thursday was still back in December, so under ISO that date reads as week 52 of 2022, not week 1 of 2023. Feed it to the tool and the ISO year comes back as 2022. That is the standard working correctly, and the tool spells out the mismatch instead of leaving you to assume the calendar year.

The US convention is looser. Weeks start on Sunday, week 1 is simply whatever week contains January 1, and the count stays inside the calendar year so January 1 is always week 1. Two differences stacked together, the start day and the week-1 rule, push the systems out of sync. Deep in the year they often agree or sit one apart. Near New Year they can diverge by a full week, and ISO can even hand the date to a different year entirely.

That gap is what quietly costs teams real time. An American team on Sunday-start calendars says "week 1" and means the week of January 1. Their European partner, counting by ISO, means the first-Thursday week, which can be a full seven days later. Both are right, both mean different dates, and nobody notices until a deadline slips. Most week tools show you a single number and never say which system produced it. This one puts both in front of you so the disagreement is visible before it bites.

One honest caveat. "The US convention" isn't a single ratified standard the way ISO is. Some American systems start weeks on Sunday, some make week 1 the first full week, and retail runs its own 4-4-5 fiscal calendars that ignore both. The Sunday-start, contains-January-1 rule shown here is the common default, but if you're feeding a specific payroll or ERP system, confirm which rule it uses first.

Most years hold 52 ISO weeks. Every five or six years one gets a 53rd, and anyone who assumes 52 is fixed trips over it. The arithmetic is plain enough: 52 weeks is 364 days, a day or two short of a full year, and those leftover days pile up until roughly every fifth or sixth year they add up to a whole extra week. A year runs 53 ISO weeks when January 1 lands on a Thursday, or when it's a leap year and January 1 is a Wednesday. The one-glance version of the same test is that either January 1 or December 31 falls on a Thursday. Checking only December 31 is the trap that catches people, because a leap year starting on a Thursday ends on a Friday and still runs 53 weeks, which is exactly what 2032 does. 2015, 2020, 2026 and 2032 are all 53-week years, and 2037 comes after that.

Where it does damage is payroll and finance. If your books bucket sales by ISO week and the model assumes 52 a year, a 53-week year silently slips in an extra reporting period, year-over-year comparisons break because you're lining 53 weeks against 52, and weekly-paid staff can see an extra pay run. None of that is a crisis if you saw it coming. Get blindsided and you're untangling it after the numbers already went out wrong. So the tool flags the year the moment you land on a date inside one.

Half the time you don't have a date, you have a week number someone handed you, and it means nothing until you know what days it covers. "The release is week 32" is useless until week 32 becomes early August. Switch to Week number to dates, enter the ISO year and the week, and the tool returns the Monday to Sunday span. Because it's ISO, week 1 can start in late December of the previous calendar year, so 2026 week 1 begins Monday, December 29, 2025. Correct, not broken, just the first-Thursday rule running in reverse. Type a week the year doesn't have, week 53 in a 52-week year, and you get told the valid range rather than a made-up week.

It's also how you decode a manufacturing date code. A lot of products print a year-and-week code on the part, so 2630 usually means made in week 30 of 2026. Warranty claims, recalls, and batch tracing all come down to turning that back into a real date range.