What Is a Week Number?
A week number tells you which week of the year a date falls in, counted from 1 to 52 or 53. This week number calculator finds the week for any date instantly, and works in reverse to show the exact date range for a given week and year.
It reports two standards side by side: the ISO 8601 week (weeks start on Monday, and Week 1 is the week containing the first Thursday of the year) used across Europe and in most software, and the US week (weeks start on Sunday, and Week 1 contains January 1). These can differ by a week or two near the start and end of the year, so seeing both removes any guesswork.
Week numbers are the shorthand of scheduling: instead of "the week of February 2–8," you can simply say "Week 6." That makes them handy for sprint cycles and project milestones, weekly business reports and KPIs, production and delivery timelines, and coordinating across regions and time zones.
How to Use the Week Number Calculator
Find the week for a date
On the Select Date tab, today's date is chosen by default with the result shown right away. Open the date picker to pick any other date, or tap Today to jump back to the current date.
Find the dates for a week
Switch to the Select Week tab, then choose a week number (1–53) and a year. The start and end dates for that week appear immediately.
Browse the year calendar
Scroll the week calendar below the result to see every week with its start and end dates. The current week is highlighted, clicking any row selects that week, and the year arrows move between years.
Copy your result
Tap Copy Result to put the week number, date range, and both the ISO 8601 and US values on your clipboard, ready to paste anywhere.
Features
Dual Week Numbering Standards
Every result shows the ISO 8601 week (used internationally and in software) alongside the US standard week, so there is no confusion across regions or systems.
Two Input Modes
Pick a date to find its week number, or enter a week number and year to find the matching date range — whichever direction you need.
Date Details at a Glance
See the day of year, days remaining in the year, the quarter (Q1–Q4), whether it is a leap year, and the total ISO weeks (52 or 53) for that year.
Interactive Year Calendar
A scrollable table lists every week with its start and end dates. The selected week and today's week are highlighted, and you can click a row or use the year arrows to navigate.
Instant Results
Every calculation happens in real time as you change inputs — no calculate button to press — and your last selection is restored when you return.
One-Tap Copy
Copy the week number, date range, and both the ISO and US values to your clipboard at once, ready to share or paste into a report.
Frequently Asked Questions
What week number is it today?
Open the tool and the current date is selected automatically, so today's week number is shown the moment the page loads — in both the ISO 8601 and US standards. Today's week is also highlighted in the year calendar.
What is the difference between ISO and US week numbers?
ISO 8601 weeks start on Monday and define Week 1 as the week containing the first Thursday of the year. US weeks start on Sunday and define Week 1 as the week containing January 1. Near the start and end of the year these systems can differ by one or even two weeks, which is why this tool shows both.
When does Week 1 of the year start?
Under ISO 8601, Week 1 is the week containing the first Thursday of the year, so it always includes January 4 and may begin in late December. Under the US standard, Week 1 simply contains January 1. Select a week of 1 on the Select Week tab to see the exact start date for any year.
Why do some years have 53 weeks?
A standard year has 365 days — 52 weeks plus 1 extra day — and a leap year has 366 days (52 weeks plus 2 days). Under ISO 8601 rules, when January 1 falls on a Thursday, or December 31 falls on a Thursday, the year has 53 ISO weeks instead of 52. The tool shows the total weeks for the selected year in the date details.
Can a December date belong to Week 1 of the next year?
Yes. Under ISO 8601, the last few days of December can fall in Week 1 of the following year — for example, December 29, 2025 is part of ISO Week 1 of 2026. This happens because an ISO week is assigned to whichever year contains its Thursday. The reverse also occurs: January 1, 2 or 3 can sometimes land in Week 52 or 53 of the previous year.
Does the week start on Monday or Sunday?
It depends on the standard. ISO 8601 weeks run Monday through Sunday, while US weeks run Sunday through Saturday. This calculator reports both at once, so you can use whichever your team or software follows.
Which week numbering system should I use?
Use ISO 8601 if you work with international teams, European partners, or software that follows the ISO standard — most programming languages and databases use ISO weeks. Use the US standard if your organization follows a Sunday-through-Saturday calendar.
Is my data private?
Yes. All calculations are performed entirely in your browser, and no dates or results are sent to any server. Your last selection is saved locally on your own device so the tool picks up where you left off.
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