Week Number Calculator

Overview

How to use this calculator

1. Enter a date by specifying the calendar year, month, and day.
2. The calculator interprets that date using the Gregorian calendar and applies ISO 8601 week rules.
3. Review the ISO week number, ISO year, and a formatted label such as 2025-W02 or 2026-W01.
4. Use the label consistently across sprint boards, roadmaps, and reports so everyone is working off the same week reference.

Inputs explained

Year

The calendar year of the date you’re checking (for example, 2024 or 2025). Note that the ISO year returned may differ for dates near New Year’s.

Month

Calendar month number from 1 to 12 (1 = January, 12 = December). You can think in terms of standard calendar dates; the ISO logic handles the underlying week mapping.

Day

Day of the month (1–31, depending on month and leap year). The calculator validates the day-month-year combination against the Gregorian calendar.

Outputs explained

ISO week

The ISO 8601 week number for the given date, from W01 up to W52 or W53. This week index resets to 1 at the start of each ISO year, which is defined by the week containing January 4.

ISO year

The ISO week-year associated with the date. For dates near the start or end of the calendar year, the ISO year may be the previous or next calendar year (for example, December 31 may belong to ISO year 2026 rather than 2025).

Week label

A combined label like 2025-W02 or 2026-W01 that joins the ISO year and ISO week in a standard format you can reuse in issue trackers, spreadsheets, and filenames.

When to use it

• Coordinating software releases, agile sprints, or OKR check-ins that are scheduled by ISO week rather than by calendar month.
• Aligning transportation, manufacturing, or maintenance operations with partners and suppliers who communicate in week numbers.
• Building content, editorial, or analytics reporting calendars using ISO week labels so recurring items are easy to group and compare year over year.
• Normalizing logs and exports from systems that tag activity by ISO year and week, allowing you to map those labels back to calendar dates when needed.
• Quickly checking whether two dates fall into the same ISO week or cross a week-year boundary before shifting deadlines or grouping work.

Tips & cautions

• Remember that ISO weeks start on Monday. If your local workweek starts on Sunday, expect some weeks to appear shifted compared with a Sunday-start calendar.
• Because Week 1 is defined by the first Thursday, early January dates can belong to the prior ISO year, and late December dates may belong to Week 1 of the following ISO year.
• Use the ISO week label (for example, 2025-W10) in both internal tools and client communication to avoid ambiguity about which week you mean.
• Pair this tool with the day-of-week calculator to get a complete view: ISO week, ISO year, and specific weekday for any key date.
• If your organization uses a custom retail or 4-4-5 calendar, treat this as an ISO baseline and maintain a separate mapping between ISO weeks and your internal week numbers.
• Uses ISO 8601 rules only; retail calendars, 4-4-5 patterns, and custom fiscal week systems may define week numbers differently.
• Assumes the proleptic Gregorian calendar for all dates. Historical transitions from other calendars are not modeled.
• Does not provide direct mapping to quarter-based or fiscal week systems; those require separate business-specific rules.
• Output follows Monday-start ISO conventions and does not reflect locales that define Week 1 or the start of the week differently.

Worked examples

Deep dive

FAQs

What calendar rules does this follow?

The calculator uses ISO 8601 week-date rules: Monday is the first day of the week, and ISO Week 1 is the week that contains the first Thursday of the calendar year (or equivalently January 4). All dates are interpreted under the proleptic Gregorian calendar.

Why is December 31 sometimes in Week 1 of the next ISO year?

ISO weeks are tied to week structure, not strictly to calendar year boundaries. If the last few days of December form part of the ISO week that contains January 4 of the next year, those days belong to the next ISO year’s Week 1.

Can I use this for systems that start weeks on Sunday?

You can, but you should be aware that the output follows ISO Monday-start rules. If your internal reports use Sunday-start weeks or a different Week 1 definition, the week numbers may not match one-for-one.

Does time of day or time zone affect the week result?

No. The calculator works purely on the calendar date, not the clock time. Time zones and daylight saving time do not change which ISO week a date belongs to in this model.

Is this suitable for retail or 4-4-5 calendars?

Retail, 4-4-5, and other fiscal calendars use their own week numbering schemes. This calculator gives ISO week numbers only; you would need a separate mapping to your specific retail or fiscal calendar.

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