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Free Online Week Number Calculator

Find the ISO week number and ISO week-year.

Your input stays on this device. Processing is bounded; regex runs in a worker with a two-second timeout.

Result



How to use Week Number Calculator
Find the ISO week number and ISO week-year. ISO weeks begin Monday. Week 1 contains the first Thursday; the ISO week-year can differ from the calendar year.
  1. Set Start / calendar date.
  2. Run the tool, review the result and its stated limitations, then copy or download if needed.
Example & practical use
Find the ISO week number and ISO week-year. The example below demonstrates Start / calendar date.
Example input:
{
  "date": "2021-01-01"
}
Expected result (relevant fields):
{
  "isoYear": 2020,
  "week": 53
}
Features & result limitations
ISO weeks begin Monday. Week 1 contains the first Thursday; the ISO week-year can differ from the calendar year.
Frequently asked questions
What inputs does Week Number Calculator accept?
Start / calendar date
How should I interpret the Week Number Calculator result?
ISO weeks begin Monday. Week 1 contains the first Thursday; the ISO week-year can differ from the calendar year.
Where does Week Number Calculator process my data?
Inputs are processed locally in your browser. Submitted text is not sent to external services. A successful-run event increments an aggregate tool counter without your input.