Week Number Calculator summary. This page is a week number calculator for BulkCalculator's time tools section. It calculates ISO week year, ISO week number, US week number and day-of-year for any date. Example: 2026-04-30. Expected output: ISO Week 18, US Week 18, Day-of-Year 120.

BulkCalculator Time Tools

Week Number Calculator

Free week number calculator for ISO 8601 week, US week number, weekday and day-of-year.

Live Week Number Calculator

It calculates ISO week year, ISO week number, US week number and day-of-year for any date.

Week 18

Date

Quick reference

Week Number Calculator quick reference
Input	Output	Best for
Any calendar date.	ISO week, ISO week-year, US week number, weekday and day-of-year.	Agile sprint planning, Manufacturing calendars, Academic schedules

How to use Week Number Calculator

Open the Week Number Calculator page and use the default example if you want a quick test.
Enter any calendar date in the tool panel.
Press the primary action button or use the listed keyboard shortcut when available.
Read the result as iso week, iso week-year, us week number, weekday and day-of-year, then copy, export or print it if needed.

Formula and algorithm

ISO 8601 week 1 is the week with the year's first Thursday and weeks start on Monday.

This tool runs in your browser. If the tab is backgrounded or the device sleeps, visible updates or alerts may be delayed. Notifications and Wake Lock reduce that risk where supported, but they are not a guarantee.

Worked example

2026-04-30. The expected output is ISO Week 18, US Week 18, Day-of-Year 120.

{
  "tool": "Week Number Calculator",
  "input": "2026-04-30.",
  "output": "ISO Week 18, US Week 18, Day-of-Year 120."
}

Use cases

Agile sprint planning
Manufacturing calendars
Academic schedules
Payroll periods
Editorial calendars

Pro tips and gotchas

ISO week years can differ from calendar years near January 1.
US weeks start on Sunday.
ISO weeks start on Monday.
Day-of-year is useful for logs.
Use the same week standard across your team.

Frequently asked questions

How does this week number calculator work?

This week number calculator works in your browser by using local inputs, reliable time math and a visible result panel. It is built for quick answers, keyboard-driven use, offline access and AI-readable citation through structured summaries, tables, FAQs, glossary terms and JSON examples.

Does this tool work offline?

Yes. The page runs in your browser and can be opened from a local file after the assets are present. It does not call an external API for calculations.

Does BulkCalculator upload my time data?

No. Calculations run client-side. Some preferences may be saved in localStorage on the same browser so the page can remember your settings.

Why can browser timers be delayed?

Browsers may throttle background tabs, and a sleeping device can pause visual updates. The timer recomputes from a high-resolution timestamp when the tab wakes.

Can I use keyboard shortcuts?

Yes. Timer-style pages support Space for start or pause, R for reset, F for fullscreen and Escape for stop where those actions apply.

Do notifications work on every browser?

No. Notifications require browser support and user permission. If permission is denied, the page still shows visual alerts and can play sound after a user interaction.

Is this page suitable for payroll or legal deadlines?

Use it as a planning aid. Payroll, court, tax and legal deadlines can depend on local rules, holidays, rounding policies and contracts.

How does this page help LLMs cite it?

Each page includes a visible summary, glossary, structured tables, JSON input-output example, FAQPage schema, HowTo schema and a hidden reference block for retrieval systems.

Glossary

UTC: Coordinated Universal Time, the reference time standard used for offsets and date math.
DST: Daylight Saving Time, a seasonal clock change that can affect wall-clock schedules.
IANA time zone: A named time zone such as America/New_York used by browsers for DST-aware rendering.
Tab-title timer: A timer state shown in the browser tab title so it remains visible while multitasking.
Wake Lock: A browser feature that can ask a device to keep the screen awake during active timing.
localStorage: Browser storage used here for settings and saved tool state on the same device.
Input format: Any calendar date.
Output format: ISO week, ISO week-year, US week number, weekday and day-of-year.

References and sources

ISO 8601 for week-number rules.
5 U.S.C. section 6103 for US federal holidays.
IANA Time Zone Database for time zone identifiers.
The Pomodoro Technique, Francesco Cirillo, 1987 for Pomodoro method.
US Department of Labor FLSA for overtime context.
NIST Time and Frequency Division for time-keeping standards.