Select a date
Pick any date using the date picker. The calculator immediately returns the week number, week year, and calendar quarter for that date.
Free, privacy-first
Week Number Calculator
Find the week number for any date and compare ISO and US week systems without doing calendar math by hand.
Inputs
Results
Results update as you use the interface above.
How to use this calculator
Choose a date, switch between ISO and US week rules if needed, and read the resulting week number and week range.
Choose ISO or US week rules
ISO 8601 weeks start on Monday and assign Week 1 to the week containing the first Thursday of the year. US weeks start on Sunday, which can produce different week numbers and week years near year boundaries.
Read the week number and range
The result shows the week number, the full date range for that week (start to end), the week year (which may differ from the calendar year in ISO mode), and the calendar quarter.
Apply it to your planning context
Use the week number in payroll reports, project tracking, manufacturing schedules, or any system that references weeks instead of calendar dates. Note the system (ISO vs US) your context uses to avoid off-by-one errors.