Week Number Calculator
Find the ISO-8601 week number for any date — the standard used across Europe and in business calendars.
Reviewed by the CalcCafe editorial team · Last updated 1 July 2026 · How we test our tools
ISO week number
Day of year
Weekday
How it works
The tool implements the ISO-8601 rule: shift to the nearest Thursday and count weeks from the first Thursday of that year.
Everything is computed locally in your browser — nothing is uploaded.
Frequently asked questions
How is the ISO week defined?
Under ISO-8601, weeks start on Monday and week 1 is the week containing the year's first Thursday. This can place early-January dates in the previous year's last week.
People also ask
What week number is it now?
Enter today's date in the calculator and it returns the current ISO-8601 week, computed locally from your device's clock. Because ISO weeks start on Monday and week 1 is the week containing the first Thursday of January, the answer can differ by one from a Sunday-start calendar, and a year has either 52 or 53 weeks. The number rolls over every Monday.
How to calculate week number?
Find the Thursday of the date's Monday-to-Sunday week, then take that Thursday's day of the year, subtract 1, divide by 7, round down, and add 1. For example, 2026-01-01 is a Thursday, so its week starts on Monday 2025-12-29, and (1 minus 1) divided by 7 rounded down plus 1 gives week 1; that also means 29 to 31 December 2025 belong to week 1 of 2026, not week 53 of 2025. The calculator applies exactly this rule for any date.
How do I find the week number of a date?
Type the date into the calculator and read the ISO week and its year. If you are working in software, most languages have it built in: Excel has ISOWEEKNUM(A1), Python has date.isocalendar().week, and PostgreSQL has EXTRACT(WEEK FROM some_date). Avoid Excel's plain WEEKNUM(), which defaults to Sunday-start weeks and can give a different number from the ISO standard.
What is the week from now?
One week from now is exactly 7 days later, so the ISO week number is simply the current week plus 1, unless that crosses the year boundary, in which case it wraps to week 1 of the next ISO year. Enter today's date plus 7 days in the calculator to see the exact week and year. Note that the ISO year can differ from the calendar year for dates in late December and early January.
Related tools
Sources & references
These tools follow our methodology and provide educational estimates only — verify important figures with a qualified professional.