Week Number Calculator MX
Find the ISO-8601 week number for any date, plus the day of the year and days remaining.
How it works
Pick a date and the calculator returns its ISO-8601 week number and week-year, the weekday, the day of the year and how many days are left in the year. It defaults to today.
Frequently asked questions
What is the current week number?
Enter today’s date and the calculator returns its ISO week number, counting from week 1 at the start of the year.
How are week numbers calculated?
By the ISO 8601 standard: weeks start on Monday and week 1 is the week containing the year’s first Thursday (the week with 4 January).
When does week 1 start?
Week 1 is the week containing the first Thursday of the year, so it may begin in late December or early January.
How many weeks are in a year?
Most years have 52 weeks; some have 53 when the extra days complete another full ISO week.
Why might an early-January date be in week 52 or 53?
Because ISO week 1 must contain the first Thursday, the first days of January can still belong to the final week of the previous year.
Week Number Calculator
This calculator tells you the week number of any date — useful for planning, payroll periods and projects that track by week.
How week numbers work (ISO 8601)
Under the international ISO 8601 standard, weeks start on Monday, and week 1 is the week containing the year’s first Thursday (equivalently, the week that contains 4 January). Most calendars and business systems use this rule.
How many weeks in a year?
A year has 52 weeks, or 53 in years where the extra days push a full week into the count. The week number therefore runs up to 52 or 53.
Worked example
Because week 1 always contains the first Thursday, a date in very early January can fall in the last week (52 or 53) of the previous ISO year — the calculator applies the rule for you.
Results are estimates for general guidance in Mexico and may not reflect the latest local rates, fees or rules. Check official sources before making decisions.