Week Number Finder

What week number is it, and how do I find the ISO 8601 week of a date?

AnyTool Week Number Finder gives the week number for any date, in your browser. Pick a date and it shows the ISO 8601 week number and the ISO week-year, the US (Sunday-start) week number, the Monday–Sunday range that week covers, the day of the year, the days left in the year, and whether the year has 52 or 53 ISO weeks. ISO 8601 — the international standard used in Europe, business and software — starts weeks on Monday and defines week 1 as the week containing the year’s first Thursday (the week containing January 4), so its week-year can differ from the calendar year around January 1 and December 31. You can also work backwards: enter a year and an ISO week number and it returns that week’s Monday and Sunday dates. Nothing is uploaded.

ISO 8601 week number and ISO week-year for any date
US (Sunday-start) week number shown alongside
Monday–Sunday week range, day of year and days remaining
Reverse lookup: year + week number → Monday and Sunday dates
100% in your browser — no upload, no signup, works offline

What is

ISO 8601 week number

A week number identifies which week of the year a date falls in. Under ISO 8601 — the international standard — weeks run Monday to Sunday and week 1 is the week containing the year’s first Thursday, equivalently the week containing January 4, so week 1 always has at least four of its days in the new year. A consequence is the ISO week-year, which can differ from the calendar year: a late-December date can belong to week 1 of the next ISO year, and an early-January date can belong to week 52 or 53 of the previous one. An ISO year has 52 weeks, or 53 when its first day is a Thursday or it is a leap year starting on a Wednesday. The US convention differs: weeks start on Sunday and week 1 is simply the week containing January 1, with no Thursday rule, so the two systems can disagree by a full week near the year boundary.

Calculators

Related terms

ISO 8601Week-yearWeek 53US week number

Frequently Asked Questions

The ISO 8601 week number is found by taking the Thursday of the week the date falls in and counting how many weeks have passed since that year’s first Thursday. AnyTool Week Number Finder does this automatically: choose a date and it shows the ISO week number, the ISO week-year, and the canonical token such as 2025-W08, along with the Monday-to-Sunday range that week covers. The date defaults to today, and everything updates live as you change it.

Pick the date and AnyTool shows its ISO 8601 week number and the ISO week-year, computed in your browser.

ISO 8601 keeps each week whole and assigns it to the year that holds most of it, so dates near January 1 and December 31 can spill across the boundary. For example, 31 December 2024 belongs to ISO week 2025-W01, and 1 January 2027 belongs to 2026-W53. AnyTool reports the ISO week-year alongside the week number and flags when it differs from the calendar year, so you never mislabel a year-end week.

Because ISO week 1 is the week with the year’s first Thursday, a few days at each year boundary belong to a neighbouring year’s weeks.

ISO 8601 is the international standard used in Europe, business and software: weeks run Monday to Sunday and week 1 is the week containing the first Thursday, so it always has at least four days in the new year. The US convention starts weeks on Sunday and makes week 1 the week containing January 1, with no Thursday rule, so its first week can be short and it always stays inside the calendar year. The two can differ by a full week in late December and early January. AnyTool shows both numbers so you can use whichever your spreadsheet, payroll or planner expects.

ISO weeks start Monday with week 1 holding the first Thursday; US weeks start Sunday with week 1 simply containing January 1.

AnyTool works in both directions. In reverse mode you enter an ISO week-year and a week number and it returns the Monday (start) and Sunday (end) of that week. If you ask for a week the year does not have — for example week 53 in a 52-week year — it clamps to the last valid week and tells you. All arithmetic runs in UTC in your browser, so the dates are stable and nothing is uploaded.

Switch to “Week number → dates”, enter the year and ISO week, and it returns that week’s Monday and Sunday dates.

Find the ISO 8601 week number and ISO week-year for any date, the US (Sunday-start) week number, the Monday–Sunday week range, day of year and days remaining, plus a reverse lookup from a year and week number back to its Monday and Sunday dates — all in your browser.

Features

ISO 8601 week number and ISO week-year (52 or 53 weeks)
US (Sunday-start) week number alongside the ISO one
Monday–Sunday week range, day of year and days remaining
Reverse: year + ISO week number → Monday and Sunday dates
Clock-free engine — 100% client-side, nothing uploaded

PriceFree

CategoryCalculator

PlatformAll (browser-based)

PrivacyYour date never leaves your device

Input: A calendar date (defaults to today), Or a year + ISO week number

Output: ISO week number and week-year, US week number, Monday–Sunday week range, Day of year and days remaining

Detailed Explanation

Methodology

How the ISO 8601 Week Number Is Calculated

The ISO 8601 week number is found with the standard Thursday rule: shift the date to the Thursday of its own Monday-to-Sunday week, take the year of that Thursday as the ISO week-year, and count how many weeks have elapsed since that year’s first Thursday. Week 1 is therefore the week containing the year’s first Thursday — equivalently, the week containing January 4 — so it always has at least four days in the new year. AnyTool reports the week number, the ISO week-year and the canonical token (for example 2025-W08), and works out whether the year has 52 or 53 ISO weeks. Because the engine is clock-free, the page supplies the date and the math is fully reproducible.

Weeks run Monday to Sunday under ISO 8601
Week 1 contains the year’s first Thursday (and January 4)
ISO week-year can differ from the calendar year
An ISO year has 52 or 53 weeks
Engine is clock-free and unit-testable — the page supplies the date

How It Works

ISO 8601 Versus the US Week Number

Two week-numbering systems are in common use and they can disagree by a full week near the year boundary. ISO 8601 — used across Europe, business and software — starts weeks on Monday and ties week 1 to the first Thursday. The US convention starts weeks on Sunday and makes week 1 simply the week containing January 1, with no Thursday rule, so its first week can have fewer than four days and the US week always stays inside the calendar year. AnyTool shows both numbers and both week ranges at once, so users can read whichever their spreadsheet, payroll system or planner expects rather than guessing.

ISO: Monday-start, week 1 holds the first Thursday
US: Sunday-start, week 1 holds January 1
The two systems can differ by a week at year-end
US week numbers never leave the calendar year
Both numbers and ranges are shown side by side

Use Cases

Reverse Lookup: Week Number Back to Dates

Beyond date-to-week, the tool runs in reverse. Enter an ISO week-year and a week number and it returns the Monday (start) and Sunday (end) calendar dates of that week, anchored on the Monday of week 1. If the requested week is out of range — for example week 53 in a year that only has 52 — it clamps to the last valid week and flags that it did so, rather than returning a nonsensical date. This makes it easy to turn a label like 2025-W08 into the actual dates a sprint, payroll period or rota covers.

Enter year + ISO week → Monday and Sunday dates
Anchored on the Monday of ISO week 1
Out-of-range weeks are clamped and flagged
Turns tokens like 2025-W08 into real dates
Useful for sprints, payroll periods and rotas

Privacy & Security

Privacy, Time Zones and Offline Use

Every calculation runs in the browser, so the date you enter is never sent to a server, there is no account or tracking, and the page works offline after first load. All arithmetic is done in UTC to avoid daylight-saving jumps, but because the tool reads your device’s calendar, your time zone can shift what counts as a boundary day near midnight. The tool is built on a shared, clock-free, unit-tested engine, so its behaviour is consistent and verifiable.

Your date never leaves your device
No signup, no tracking; works offline after first load
UTC arithmetic avoids daylight-saving errors
Time zone can shift a boundary day near midnight
Built on a pure, clock-free, unit-tested engine

Week numbers: in-browser (AnyTool) vs typical online week-number tools
Capability	AnyTool	Typical online tools
Processing	Runs in your browser	Often server-side
ISO 8601 week number	Yes, with ISO week-year	Usually supported
ISO week-year flagged	Yes — when it differs	Often hidden
US (Sunday-start) week	Shown alongside ISO	Sometimes missing
Week range (Mon–Sun)	Shown for both systems	Sometimes missing
Weeks in year (52/53)	Computed	Rarely shown
Reverse week → dates	Yes, with clamping	Sometimes missing
Works offline	Yes (PWA)	No
Cost / signup	Free, no signup	Often ad-heavy or gated

AnyTool computes the ISO and US week numbers and the reverse lookup locally and uploads nothing.

Cite This Page

Title: Week Number Finder (ISO 8601 week number + week-year, US Sunday-start week, Mon–Sun range, reverse week → dates, no server)
Author: AnyTool
Published: January 1, 2026
Modified: February 8, 2026
Section: Calculators
URL: https://www.anytool.tech/week-number-finder

Formatted Citation

AnyTool. "Week Number Finder (ISO 8601 week number + week-year, US Sunday-start week, Mon–Sun range, reverse week → dates, no server)." AnyTool, January 1, 2026. https://www.anytool.tech/week-number-finder

A privacy-first, 100% client-side week-number finder built on a pure, clock-free, unit-tested engine. For any date it computes the ISO 8601 week number using the standard Thursday rule — shift to the Thursday of the date’s Monday-to-Sunday week, take that Thursday’s year as the ISO week-year, and count weeks since the year’s first Thursday — so week 1 is the week containing the first Thursday (the week containing January 4) and always has at least four days in the new year. It reports the canonical token such as 2025-W08, the ISO week-year (which can differ from the calendar year, so 31 December 2024 is 2025-W01 and 1 January 2027 is 2026-W53), whether the year holds 52 or 53 weeks, and the Monday-to-Sunday range the week covers. Alongside the ISO figure it shows the US convention, where weeks start on Sunday and week 1 is simply the week containing January 1 with no Thursday rule, so the two systems can disagree by a full week at the year boundary; the tool presents both so users can pick the one their context expects. It also runs in reverse: given an ISO week-year and week number it returns the Monday and Sunday calendar dates, clamping out-of-range weeks and saying so. It additionally reports the day of the year, the days remaining in the year and the calendar quarter. All arithmetic is done in UTC to avoid daylight-saving jumps, the page is the only place that decides what today is, nothing is uploaded, and the page works offline after first load.