Week Number
What week is it? Today's ISO week number and every week of the year.
About this tool
The current ISO week number, front and centre; a date picker for any other date; and a table of every week of the year with its Monday-to-Sunday range. If your workplace plans in week numbers โ common across Scandinavia, Germany and manufacturing everywhere โ this page is the answer to "what's W37 again?".
How ISO week numbers actually work
The standard (ISO 8601, same family as the 24-hour clock) has two rules: weeks run Monday to Sunday, and week 1 is the week containing the year's first Thursday โ equivalently, the week of January 4th. The rules' odd consequences are where confusion lives: January 1st can belong to week 52 or 53 of the previous year, late December can belong to week 1 of the next, and some years (like 2026's neighbour 2032) have 53 weeks. US calendars often number Sunday-start weeks from January 1 instead, which drifts a week from ISO โ so when a delivery is promised "week 12", it's worth one message to confirm whose week 12.
Frequently asked questions
How is week 1 of the year decided?
ISO rule: week 1 contains the year's first Thursday (equivalently, January 4). So January 1-3 can fall in the previous year's week 52/53, and December 29-31 can fall in next year's week 1.
Why do some years have 53 weeks?
A year has 52 weeks and one or two leftover days; when they push an extra Thursday into the year โ years starting on Thursday, or leap years starting Wednesday โ the count reaches 53. It happens roughly every 5-6 years.
Why does my US calendar show a different week number?
Many US calendars count Sunday-start weeks numbered from January 1 rather than ISO's Monday/first-Thursday rule โ the two systems drift a week apart. Confirm which system the other party uses.
Do weeks start on Monday or Sunday?
In ISO 8601 (used here): Monday. US convention often starts Sunday โ same days, different bookkeeping.