Week Number Calculator
Updated November 2026 · Free · No sign-up
Find the week number for any date — ISO 8601 standard, plus the US convention, and why your payroll’s week 1 may differ from your calendar app’s.
Quick answer: Under ISO 8601, week 1 is the week containing the first Thursday of January (weeks start Monday). Years have 52 or 53 ISO weeks. US convention instead starts week 1 on January 1 with Sunday weeks — the two disagree for several days every year.
How Week Numbers Work
ISO 8601 (Europe, logistics, most payroll software): weeks run Monday-Sunday; week 1 contains the year’s first Thursday — so Jan 1-3 can belong to the PREVIOUS year’s week 52/53, and Dec 29-31 can be next year’s week 1. US convention: Sunday weeks, week 1 starts Jan 1 — simpler, never off-by-one at boundaries, but incompatible with ISO systems.
Worked Examples
Example 1 — the January trap. January 1, 2027 is a Friday: ISO says it belongs to week 53 of 2026; ISO week 1 of 2027 starts Monday January 4. The US system calls it week 1 immediately — a 1-week disagreement for four days.
Example 2 — 53-week years. ISO years have 53 weeks when Jan 1 falls on a Thursday (or Wednesday in leap years) — relevant to weekly payrolls facing a 53-paycheck year and to week-numbered manufacturing plans.
Example 3 — payroll weeks. UK payroll’s "week 1" starts April 6 (the tax year), not January — three different week-1 definitions can coexist in one company.
Who Uses Which
| System | Users |
|---|---|
| ISO 8601 | EU business, logistics, SAP/ERP, manufacturing planning |
| US Sunday weeks | US calendars, broadcast, retail 4-5-4 variants |
| Tax-year weeks | UK payroll (from Apr 6) — the week-53 payroll quirk |
The 53rd Week Problems
Weekly-paid staff in a 53-week year get an extra paycheck that annual-salary division never budgeted — UK payroll handles it with specific week-53 tax instructions; US biweekly systems meet the cousin problem as 27-paycheck years. Planning systems comparing "week 32 this year vs last" also silently shift a day each year (two after leap years) — year-over-yearweek comparisons need date anchoring, not week numbers.
Frequently Asked Questions
What week number is it?
Enter any date above — ISO 8601 computes week 1 as the week holding January’s first Thursday (Monday starts); the US convention numbers from January 1.
Why do week numbers differ between systems?
ISO’s first-Thursday rule assigns year-boundary days to the neighboring year’s weeks; the US Jan-1 rule never does — they disagree a few days every year.
How many weeks are in 2027?
ISO 2027 has 52 weeks (Jan 1 is a Friday, so ISO week 1 starts Jan 4). Calendar-wise, 2027 spans parts of 53 ISO weeks.
What is a 53-week year?
An ISO year gains week 53 when it starts on Thursday (or Wed in leap years) — weekly payrolls then pay 53 checks; budget accordingly.
Which week numbering does payroll use?
ERP/payroll usually ISO; UK payroll uses tax-year weeks from April 6 — confirm the system before quoting "week 1".
✓ Formula verified • Last updated: November 3, 2026