Week dates converts an ISO week number and calendar year into the exact start and end civil dates for that week. ISO weeks begin on Monday; week 1 is the week containing the first Thursday of January. Use this page when a spreadsheet column says week 14 and you need the Monday-through-Sunday span for scheduling, reporting, or API filters.

ISO week rules in practice

Early January days may belong to ISO week 52 or 53 of the previous ISO year. Late December days may belong to week 1 of the next ISO year. The ISO year label on this page can differ from the calendar year you typed — that is correct, not a bug.

US week conventions that start Sunday from January 1 disagree with ISO near year boundaries. When stakeholders cite week numbers without naming the standard, confirm ISO versus US before booking travel or filing reports.

How to use week dates

Enter calendar year and ISO week number, then calculate. Read Monday start date, Sunday end date, and the ISO year label when it differs from the typed year. Copy the range into project plans, SQL BETWEEN clauses, or calendar invites.

Browse full week lists on 2026 week numbers or jump to a single day snapshot with date info.

Planning and analytics

Sprint retrospectives keyed to ISO week need exact boundaries so metrics do not bleed into adjacent weeks. Warehouse labor models that staff by ISO week align overtime rules with the Monday start shown here.

Marketing cohorts labeled week 20 should filter events between the computed start and end inclusive, not calendar month June 1–30 unless the campaign explicitly uses months.

Cross-year weeks

Week 1 sometimes starts in late December of the prior calendar year. Week 52 or 53 may end in early January of the next calendar year. Dashboards that group by calendar month must split ISO weeks at month boundaries or accept partial-week attribution — document the choice.

Compare cross-year edge cases with week number hub navigation and mark spans on the 2026 yearly calendar.

Worked examples

A project charter says deliverables due ISO week 26 2026. Convert week 26 to dates, then subtract review days with business days calculator if only workdays count toward internal freeze.

An analyst receives a CSV keyed by iso_week and iso_year columns. They join to daily facts by expanding each week to its date range from this tool rather than assuming seven-day blocks aligned to calendar months.

Spreadsheet alignment

Excel ISO.WEEKNUM and WEEKNUM return types differ. Before trusting a workbook, pick one known week from this page and verify the sheet's week label and start date match. Mismatch at January week 1 is the most common failure mode.

Power BI and Looker models should store iso_year alongside iso_week in dimension tables — filtering on calendar year alone drops edge days belonging to adjacent ISO years.

API and database filters

REST query parameters like week=14&year=2026 should resolve to UTC-safe date ranges only after confirming whether your product uses local midnight or UTC midnight at boundaries. This page returns civil dates; timezone conversion stays your application's job.

Indexed date columns filter faster than computing ISO week in SQL on every query — materialize week start/end in ETL using authoritative conversion once per row.

Limits

This tool maps ISO weeks only — not US Sunday-based week numbers or custom fiscal weeks. For company fiscal periods starting in July or October, use the fiscal year calculator.

It does not count weekdays inside the span; use weekday counter when you need how many Fridays fall in that ISO week.

Documentation for teams

Write ISO 8601 week date format (YYYY-Www-D) in internal specs when APIs expose week-based resources. Link this page in runbooks so on-call engineers decode week parameters without opening spreadsheet macros.

Sprint naming conventions like 2026-W14 should cite the Monday start date in parentheses on Jira tickets so remote teams in multiple time zones share one anchor.

Retail and fiscal week conflicts

Retail 454 calendars and custom fiscal weeks are not ISO — do not feed fiscal week numbers into this tool without conversion. When finance reports ISO week 53 in a 53-week ISO year, confirm whether your ERP generated a valid week or rolled an extra week label — week 53 exists only in some years.

Inventory snapshots keyed by iso_week need iso_year in the primary key — duplicate week numbers across ISO years collapse rows in naive GROUP BY queries and inflate KPI denominators silently.

Teaching ISO weeks

Computer science curricula introduce ISO weeks when teaching strftime and date libraries. Assign students to convert three given weeks to date ranges here, then manually verify one day inside the range with date info. Misunderstanding Monday start causes off-by-one week errors in student projects every semester.

Print week range results beside wall calendars during onboarding so non-technical stakeholders stop assuming week 1 always equals January 1.

Capacity planning by week

Support teams staffing chat queues by ISO week convert forecast tickets per week to headcount after reading exact week boundaries — partial weeks at month end need explicit policy for attributing tickets.

Manufacturing MRP systems exploding weekly buckets should store week start date from this tool as foreign key rather than week integer alone.

Sports leagues and broadcast

Sports leagues publishing weekly highlight reels keyed to ISO week should convert week numbers to date ranges for broadcast rights windows — licensors compare air dates against week boundaries contractually.

Fantasy sports weekly scoring periods aligned to ISO week need Monday lock times documented in rules using week start from this tool.

Compliance reporting windows

SOC audit samples keyed to ISO week need week boundaries attached to evidence folders — auditors reproduce selections when start and end civil dates appear on the cover sheet generated from this tool.

Related tools

See week number, days between dates, and the date tools hub.