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ISO 8601 weeks start on Monday, and week 1 is the week containing the year's first Thursday, so a year has 52 or 53 numbered weeks and the days around New Year can belong to the other year's numbering: this is the system UK businesses, payrolls and diaries use.
What The Week Number Calculator Works Out
- ISO week number: the week of the year under ISO 8601, the system on UK diaries, payrolls and project plans. Weeks run Monday to Sunday and week 1 is the week containing the year's first Thursday (equivalently the week of 4 January), so 29 to 31 December can belong to week 1 of the next year and 1 to 3 January to week 52 or 53 of the last. Quote the ISO year with the week ("week 1 of 2027") to avoid that ambiguity.
- Day: the weekday and the day-of-year count, which is useful for interest calculations, countdowns and date stamps on products.
- Year progress: the percentage of the calendar year elapsed and the days remaining, with leap years handled (2028 is next).
- UK tax week (advanced): payroll runs on its own numbering from 6 April, with tax week 1 covering 6 to 12 April and tax month 1 running 6 April to 5 May. A tax year has 52 weeks plus a day or two, so weekly payrolls hit a "week 53" in some years. This is the number on payslips and in PAYE submissions.
- US-style week number (advanced): the convention where weeks start on Sunday and week 1 is whichever week contains 1 January. It is what Excel's WEEKNUM returns by default, as opposed to ISOWEEKNUM, and the mismatch is a classic spreadsheet gotcha when comparing "week 37 this year versus last".
- Weeks in this ISO year (advanced): 52 or 53. Years starting on a Thursday, or leap years starting on a Wednesday, get 53 weeks, roughly every five to six years, which quietly breaks annual comparisons in dashboards built on week numbers.
- Looked-up date (advanced): enter an ISO year, a week number and a weekday to get the calendar date, the reverse conversion delivery schedules, brewing and logistics week codes and retail planning rely on.
- Every week of the year (advanced): a full table of the Monday and Sunday of each ISO week in the year, with the current week highlighted, for printing into a planner.