Week Calculator
Week Number Calculator works out Week number and Week-numbering year from What to work out, Week numbering and Date. The default figures on this page give week number of 38; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
2 steps
- 2026-09-14 is a Monday, so its Monday-start week runs 2026-09-14 to 2026-09-20.
- That week's Thursday is 2026-09-17, which lies in 2026, so the week is 2026-W38.
Its 4 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
3 sources
What the Week Number Calculator calculates
The week of the year a date falls in, under ISO 8601 numbering (Monday-start weeks, week 1 holds the first Thursday, so the week-numbering year can differ from the calendar year) or US/Excel WEEKNUM numbering (Sunday-start weeks, week 1 holds 1 January). It also runs backwards, giving the start and end dates of a given week number, and reports how many weeks the year has.
How to use this Week Number Calculator
Enter What to work out, Week numbering, Date and Week number. Week number updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Week number, Week-numbering year and First day of the week.
The method follows ISO 8601-1:2019, Date and time - Representations for information interchange, clause 4.2.3 (week dates), Microsoft Excel WEEKNUM function specification and Howard Hinnant, chrono-Compatible Low-Level Date Algorithms.
1 January 2021 is in ISO week 53 of 2020 example
With What to work out: week-of-date, Date: 2021-01-01, Week numbering: iso, the calculator returns Week number: 53, Week-numbering year: 2020, First day of the week: 2020-12-28. This example is checked against an independent source in the test suite.
What you can compare
Change one input at a time to see how it affects Week number, Week-numbering year and First day of the week. The inputs you can vary are What to work out, Week numbering, Date, Week number and Year of that week.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How to calculate weeks from a date?
The week of the year a date falls in, under ISO 8601 numbering (Monday-start weeks, week 1 holds the first Thursday. Open “How this is worked out” beside the result to see each step for your own figures.
How can we calculate weeks?
It needs What to work out, Week numbering, Date, Week number and Year of that week. Nothing else is required, and the figures stay in your browser.
How to calculate the number of weeks per month?
The result gives Week number, Week-numbering year, First day of the week, Last day of the week, Weeks start on and ISO week date, each labelled with its unit so the figure can be read on its own.
How to calculate weeks between two dates online?
The page opens with worked default figures: week number of 38. Replace them with your own and every figure updates as you type.
What information do I need for the Week Number Calculator?
You need What to work out, Week numbering, Date, Week number and Year of that week. The page opens with default figures that give week number of 38, so you can see the scale before entering your own.
What does this Week Number Calculator show?
It shows Week number, Week-numbering year, First day of the week, Last day of the week, Weeks start on, ISO week date, ISO weeks in that year and Whole weeks left in the year, with the working and any warnings for your entries.