Week of Year Calculator
It is ISO week 2 of 2026 (a 53-week year), covering Jan 5 to Jan 11. We compute the week number by the ISO 8601 first-Thursday rule and swing a needle to it on a 53-spoke wheel.
ISO week
#2
Weeks in year
53
Through weeks
3.8%
Weeks left
51
Quick Conversion
Formula: % = (week ÷ weeks-in-year) × 100
The ISO Week-Number Wheel
ISO week date
2026-W02-1
Week range
Jan 5 – Jan 11
Weeks done
2 / 53
Date → ISO Week Converter
Choose a date to see its ISO 8601 week number.
ISO Week Quarters
Approximate ISO week numbers at the boundaries of each quarter in a 52-week year.
ISO Week → Position in the Year
| ISO week | % of 52-week year | % of 53-week year | Approx. month |
|---|---|---|---|
| W01 | 1.9% | 1.9% | early January |
| W05 | 9.6% | 9.4% | late January / early February |
| W09 | 17.3% | 17.0% | late February / early March |
| W13 | 25.0% | 24.5% | late March (end Q1) |
| W18 | 34.6% | 34.0% | early May |
| W22 | 42.3% | 41.5% | early June |
| W26 | 50.0% | 49.1% | late June (mid-year) |
| W31 | 59.6% | 58.5% | early August |
| W35 | 67.3% | 66.0% | early September |
| W39 | 75.0% | 73.6% | late September (end Q3) |
| W44 | 84.6% | 83.0% | early November |
| W48 | 92.3% | 90.6% | early December |
| W52 | 100.0% | 98.1% | late December |
See how far through the current week you are with Week Progress, or number today out of 365/366 with the Day of Year counter.
The ISO Week-Number Formula
shift date to its Thursday, then week = ceil(((Thursday − Jan 1) ÷ 86,400,000 + 1) ÷ 7)week 1 = the week containing the year's first Thursday (the week with Jan 4)Worked example: for a date in early June 2026 whose week-Thursday is June 4, the days since January 1 number 154; (154 ÷ 1 + 1) ÷ 7 rounded up gives week 23. Because 2026 begins on a Thursday it is a 53-week ISO year, so the wheel draws 53 spokes and the final week of December is W53.
52- vs 53-Week Years
| Year | Jan 1 weekday | ISO weeks |
|---|---|---|
| 2023 | Sunday | 52 |
| 2024 | Monday | 52 |
| 2025 | Wednesday | 52 |
| 2026 | Thursday | 53 |
| 2027 | Friday | 52 |
| 2028 | Saturday | 52 |
| 2032 | Thursday | 53 |
Your Saved Lookups
No lookups saved yet. Tap "Save to history" to record the current ISO week and revisit it later.
How to Read the Week Wheel
- Open the page — the wheel draws one spoke per ISO week (52 or 53 for the current year) with week 1 at the top, counting clockwise.
- Read the needle: it points to the current week, whose number is printed large in the central hub, with the green spoke marking today's week.
- Check the ISO week date (YYYY-Www-D) and the Monday-to-Sunday range beneath the wheel to confirm the exact dates.
- Use the date-to-week converter to look up the ISO week number for any calendar date, applying the first-Thursday rule automatically.
- Tap Save to history to store the week in localStorage; the wheel advances one spoke each Monday at midnight.
A Brief History of the ISO Week
In 2026, a supply-chain planner whose ERP schedules production in calendar weeks needs one answer before standup: which ISO week are we in? Week of Year shows it as a radial wheel of 52 or 53 spokes, with a needle swinging to the current week and the number printed in the hub. The figure is the ISO 8601 week number, the international standard that virtually every European business, spreadsheet, and software date library uses to slice the year into weeks.
ISO 8601, first published by the International Organization for Standardization in 1988 and refined since, defines the week date unambiguously. Weeks run Monday (day 1) to Sunday (day 7), and week 1 of any year is the week containing that year's first Thursday — equivalently, the week containing January 4, or the first week with the majority of its days in the new year. This 'first Thursday' rule is the single idea that makes ISO week numbers consistent worldwide, and it is the rule this wheel applies.
The rule has a striking consequence at year boundaries: the first days of January can belong to the last week of the previous year, and the last days of December can belong to week 1 of the next year. For instance, if January 1 falls on a Saturday, then January 1 and 2 belong to week 52 or 53 of the prior ISO year, and ISO week 1 does not start until Monday January 3. The wheel and the converter both honour this, so the number always matches a standards-compliant calendar app.
Most years contain 52 ISO weeks, but some contain 53. A year has 53 weeks if January 1 falls on a Thursday, or if it is a leap year and January 1 falls on a Wednesday. Across a 400-year Gregorian cycle, 71 of the years are 53-week years — about once every 5.6 years. The wheel automatically draws 52 or 53 spokes for the current ISO year, so a 53-week year like 2026 shows one extra tick.
Week numbering drives a surprising slice of industry. German and Scandinavian manufacturing plans by Kalenderwoche (KW); retailers run 'week 47' promotions; broadcast and fiscal calendars such as the 4-4-5 accounting calendar group weeks into months and quarters; and project tools from Jira to SAP expose ISO week as a first-class field. Saying 'ship it in week 38' is unambiguous across borders precisely because ISO 8601 fixed the definition.
Because the ISO week is anchored to Thursday, it pairs naturally with the day-of-year ordinal count and the week-progress battery. The Day of Year counter numbers today out of 365 or 366; the Week Progress battery shows how far through the current Monday-to-Sunday week you are; and the Year Progress thermometer expresses the same position as an annual percentage. Together they place any moment precisely within the calendar year.
Everything is computed client-side from a single new Date() call on a one-second interval, so the wheel, the week number, and the date range reflect your own time zone and update as the week rolls over at Monday midnight. A date-to-week converter lets you look up the ISO week for any date, and a small localStorage history panel keeps your saved lookups.
Trusted by planners, schedulers, and project managers
“Our whole ERP runs on KW numbers, so a wheel that points to the right ISO week and correctly shows 2026 as a 53-week year is exactly what I needed. I check it every Monday morning.”
“The first-Thursday rule trips up so many homemade week calculators. This one nails the year-boundary cases — January 1 landing in week 52 of the prior year is handled perfectly.”
“We plan 'semaine 47' campaigns months ahead, and the date-to-week converter lets me confirm any date's ISO week instantly. The 52/53 spoke wheel is a genuinely lovely way to see where we are in the year.”
“Jira shows ISO week, our German partners say KW, and I needed one reference everyone could trust. The wheel plus the YYYY-Www explanation ended every 'which week is it?' debate on the team.”
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