Calculate week numbers, find dates by week, convert between ISO weeks and dates, and analyze weekly patterns.
The Week Calculator converts between dates and ISO week numbers, finds the current week of the year, and provides detailed weekly analysis. Enter any date to find its week number, or enter a week number and year to find the date range. Essential for project management, payroll, and manufacturing scheduling.
ISO week numbers (ISO 8601) are widely used in business: Week 1 is the week containing the first Thursday of January. Weeks run Monday to Sunday. A year has 52 or 53 weeks. Many industries use week numbers for production schedules, delivery timelines, and reporting periods. This calculator handles all week-related calculations.
The tool also shows how many weeks remain in the year, which quarter the week falls in, and provides context about the day of year and percentage of year elapsed. It supports both ISO standard weeks (common worldwide) and US convention (weeks starting Sunday). Check the example with realistic values before reporting.
Week numbers are essential for business planning, payroll periods, manufacturing schedules, and project management. This tool converts instantly between dates and week numbers. Keep these notes focused on your operational context. Tie the context to the calculator’s intended domain. Use this clarification to avoid ambiguous interpretation. Align this note with review checkpoints.
ISO Week Number: The week containing January 4th is Week 1. Weeks start Monday. Day of Year = (date - Jan 1) / 86400000 + 1. % of Year = Day of Year / (365 or 366) × 100. Weeks Remaining = Total Weeks - Current Week.
Result: Week 24 of 2025
June 9, 2025 is in ISO Week 24. This week runs from June 9 (Monday) to June 15 (Sunday). Day 160 of 365, which is 43.8% through the year.
The ISO 8601 standard defines a clear, unambiguous week numbering system. Week 1 is defined as the week containing the year's first Thursday, or equivalently, the week that contains January 4. Weeks always start on Monday. This means January 1, 2 or 3 can fall in the last week of the previous year. The system ensures every day belongs to exactly one week of exactly one year.
Manufacturing uses week numbers extensively for production planning, quality tracking, and supply chain management. "Ship by W26" means ship by the end of Week 26. Automotive, electronics, and FMCG industries all rely on week-based scheduling. Date stamps on products often use the format "YYWW" (e.g., "2524" means Week 24 of 2025).
Most years have 52 ISO weeks, but some have 53. A year has 53 weeks when January 1 falls on Thursday, or when it's a leap year and January 1 falls on Wednesday or Thursday. On average, 53-week years occur every 5.6 years. Notable 53-week years: 2004, 2009, 2015, 2020, 2026.
ISO 8601 defines weeks starting Monday. Week 1 is the week containing January 4th (or the first Thursday in January). Most of the world uses this standard.
Yes — January 1 might be in Week 52 or 53 of the previous year. For example, January 1, 2021 was in Week 53 of 2020.
Most have 52, but some years have 53 ISO weeks. This happens roughly every 5-6 years when the year starts on Thursday (or Thursday/Friday for leap years).
In the US, weeks often start Sunday and Week 1 is the week containing January 1. This can differ from ISO by up to a week.
Week numbers are unambiguous ("Week 24" vs. "the second week of June") and align with standard reporting periods. Manufacturing, logistics, and payroll extensively use them.
ISO 8601 is the international standard and is recommended for cross-border business. Check your organization's convention.