ISO Week Calculator
The week the ISO standard numbers — week number and week-year for any date, the Monday-through-Sunday span, and the first-Thursday rule that decides it all.
ISO Week Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Dates 1900 to 2100
- ISO 8601 only — the US Sunday-anchored count is named, not computed
In short: 24 September 2026 is a Thursday in week 39 of 2026 — a 53-week ISO year, the long shape a Thursday New Year makes. The edges bite: 1 January 2027 answers week 53 of 2026, and 30 December 2024 answers week 1 of 2025. The label follows the week's Thursday, not the calendar page.
Formula
week 1 holds the first Thursday of January (the week of 4 January); weeks run Monday through Sunday; the week-year is the Thursday's year
ISO 8601 numbers weeks so that week 1 always holds the first Thursday of January — equivalently, the week containing 4 January. Weeks run Monday through Sunday and every week has all seven days, so there are no partial weeks to argue about. The week-year is the year of the week's Thursday, which is why the year's first day or three can wear the previous year's number and the year's last day or three can wear the next one's. A year holds 53 weeks when its 1 January is a Thursday — or a Wednesday in a leap year.
Worked Example
- Enter the date.
- Read the ISO week number and week-year.
- Check the Monday-to-Sunday span and the year's week count.
Defaults: 24 September 2026 — Thursday, week 39 of 2026; the year holds 53 weeks. 1 January 2027 reads week 53 of 2026.
Strengths & Limits Of This Model
Where this engine is strong
- Week number AND week-year, the pair reports need
- The boundary quirk shown on real dates
- The 53-week shapes named
Where it stops
- No US-system week count — named as the other convention
Practical Use Cases
Weekly reporting
the enterprise week number
Year-boundary triage
why 1 January reads week 53
Scheduling
the Monday-to-Sunday span
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
ISO Week Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why isn't 1 January always week 1?
Because week 1 is defined by the first Thursday of January, not by the first day. When 1 January falls on Friday, Saturday or Sunday, those days belong to week 52 or 53 of the year before, and the new year's week 1 starts on the following Monday. 1 January 2027 is a Friday, so it answers week 53 of 2026.
When does a year have 53 weeks?
When 1 January is a Thursday in a common year, or a Wednesday in a leap year — either shape leaves room for 53 Thursdays. 2026 opens on a Thursday, so it holds 53 weeks; 2025 opened on a Wednesday and held 52.
How is this different from the US week numbers?
The US convention starts weeks on Sunday and calls whatever week contains 1 January week 1, partial weeks and all. The two systems can disagree by a full week around New Year and put the same date in different week-years.
What is the week-year for?
Reporting. A week that straddles midnight on 31 December must file under one year, and ISO files it under the year of its Thursday — otherwise weekly totals split or double at the boundary.
Which software uses ISO weeks?
Most enterprise systems treat ISO 8601 as the default — the major databases and ERP suites number weeks this way out of the box, which is why a week number from a report almost always means the ISO count and why the New-Year boundary trips so many spreadsheets.
Why does the week-year differ from the calendar year?
Because the label follows the week's Thursday. A week is filed under the year its Thursday sits in, so a Monday-to-Sunday week that straddles midnight on 31 December files whole under one year — weekly totals neither split nor double at the boundary.
Which day does an ISO week start on?
Monday, always — Monday is weekday 1 and Sunday is weekday 7 in the standard's own numbering. The span card prints the Monday-to-Sunday stretch so the week's edges are dates, not assumptions.
Can a week number repeat within a year?
No — the numbers run strictly 1 through 52 or 53. Week 53 appears only in the long shapes: 1 January on a Thursday in a common year, or on a Wednesday in a leap year. Any 'week 53' outside those shapes is a different numbering system talking.