Why use week numbers?

Project planning uses weeks more naturally than days for medium-term work. Manufacturing/retail planning operates on weekly cycles. ISO standard week numbers enable internationally consistent reporting. Software dev sprints often use week numbers.

What's ISO 8601 week numbering?

International standard. Weeks start Monday. Week 1 is the first week containing a Thursday (or equivalently, contains January 4th). Some years have 53 weeks. Used in business and international standards. The calculator shows ISO week numbers for any date.

Why do US and ISO week numbers differ?

US weeks start Sunday and number from January 1. ISO weeks start Monday and use first Thursday rule. Difference can be 0-2 weeks depending on year. Verify which system your context uses to avoid scheduling errors across systems.

Can a year have 53 weeks?

Yes. Years where January 1 is a Thursday, or leap years where January 1 is a Wednesday, have 53 weeks in ISO. Recent examples: 2020, 2026. Most years have 52 weeks. The calculator handles both correctly.

What happens with weeks that span months?

Many weeks span two months (e.g., week of Jan 30 - Feb 5). The week number applies to the entire week regardless of month boundary. ISO weeks ignore months for numbering — purely time-based.

How does this help business planning?

Sales forecasting, inventory management, payroll cycles, and project deadlines often use week numbers. Compare 'this week vs last week' or 'week 12 forecast'. The calculator helps match calendar dates to your business week numbers.

Is the data sent anywhere?

No. Calculation happens entirely in your browser using standard date math. Your dates never leave your device.

Week Number Calculator

Find the ISO week number for any date and list all dates in a given week. Free week number lookup with calendar year overview.

Date & Time Calculators

Instant results

Select Date

Today is in

Week 20

of 2026

W20 of 53 weeks

Days in Week 20

Year ProgressWeek 20 of 53

W138% through the yearW53

Day of Week

Monday

Day of Year

131

Days Remaining

234

Quarter

Week 20 Range

Mon, May 11, 2026 — Sun, May 17, 2026

How to Use Week Number Calculator

Enter a date

Select any date or use the date picker. Use today's date if you want the current week number.

View week number

See ISO 8601 week number (international standard) and US week number (Sunday-Saturday).

Compare with year

Some years have 53 weeks; most have 52. The calculator shows total weeks in the year for context.

Use for planning

Apply to project sprints, business cycles, agricultural planning, or any context where weekly cadence matters.

When to Use Week Number Calculator

Project sprint planning

Software development teams use week numbers for sprint identification ('Sprint 12 = ISO week 12'). Aligns with international standards and lets distributed teams reference common time periods. Useful for retrospectives and planning.

Business and retail planning

Sales reporting often uses week numbers ('Week 47 had highest revenue'). Inventory management plans on weekly cycles. Holiday retailers reference specific weeks ('Black Friday week'). Calendar week numbers enable consistent reporting.

Agricultural and seasonal planning

Farmers use week numbers for planting/harvest cycles. 'Plant by week 18' is more useful than specific dates that vary by year. The calculator helps align activities with the correct week.

International coordination

Different countries use different week start days. ISO 8601 week numbers provide international standard. The calculator shows ISO numbers for any date, enabling unambiguous communication across timezones and cultures.

Week Number Calculator Examples

Today's week number

Input

Date: 2026-04-30

Output

ISO Week: 18 (Mon Apr 27 - Sun May 3)\nUS Week: 18 (Sun Apr 26 - Sat May 2)

Most weeks ISO and US numbering agree. Difference of 0-1 weeks possible early/late in year. Always specify which system when sharing internationally.

End of year boundary

Input

Date: 2026-12-31

Output

ISO Week: 53 (special - 53rd week)\nUS Week: 53

2026 has 53 weeks (years where Jan 1 is Thursday or leap year with Wed). Most years have 52 weeks. The calculator handles 53-week years correctly.

Different year, same date

Input

Compare Apr 30 across years

Output

2024: Week 18\n2025: Week 18\n2026: Week 18

Same date in different years sometimes lands on different week numbers depending on day-of-week of January 1. The calculator gives accurate result for any specific year.

Tips & Best Practices for Week Number Calculator

1.When sharing week numbers internationally, specify ISO 8601 to avoid US/EU confusion.
2.Some years have 53 weeks (occurring every 5-7 years). Plan for this in long-term scheduling.
3.Weeks span months — week 9 might be entirely February or partly Feb/Mar. Don't assume one week = one month.
4.Software dev sprints commonly align with ISO weeks. Easy reference: 'sprint 18 = week 18'.
5.Excel WEEKNUM() function uses different conventions per region. Use the calculator for consistent ISO results.
6.For business reporting, define your week-numbering policy and stick with it across all reports.

Frequently Asked Questions

A week number identifies which week of the year a date falls in (1-53). Different countries use different systems: ISO 8601 (used internationally) numbers weeks Monday-Sunday with week 1 being the first week with a Thursday in it. US convention: weeks Sunday-Saturday with week 1 starting Jan 1.

Week Number Calculator

Days in Week 20

How to Use Week Number Calculator

Enter a date

View week number

Compare with year

Use for planning

When to Use Week Number Calculator

Project sprint planning

Business and retail planning

Agricultural and seasonal planning

International coordination

Week Number Calculator Examples

Today's week number

End of year boundary

Different year, same date

Tips & Best Practices for Week Number Calculator

Frequently Asked Questions

Related Tools

Day of Year Calculator

Time Duration Calculator

Moon Phase Calculator

Timestamp Converter

Date Calculator

Date Difference Calculator