Week Number Calculator
Find the ISO 8601 week number (1 to 53) for any date you choose.
Date
2026-W29 — Monday
- 1
ISO weekday of the date
11 = Monday … 7 = Sunday (ISO 8601). JavaScript maps Sunday to 0, remapped to 7 here. - 2
Days to shift to Thursday
4 − 1 = 3 - 3
Days from ISO year start to that Thursday
196 - 4
ISO week number
⌈(196 + 1) ÷ 7⌉ = 29
Formula
How this is calculated
You enter a single calendar date. The calculator returns its ISO 8601 week number, the ISO week year it belongs to, the weekday name, and the day-of-year. Under ISO 8601 a week runs Monday (day 1) to Sunday (day 7), and week 1 is defined as the week containing that year's first Thursday — equivalently the week containing January 4.
To find the week, the date is first converted to UTC so daylight-saving shifts cannot change the calendar day. Its ISO weekday is computed, then the date is moved forward to the Thursday of the same week (adding 4 minus the weekday number). That Thursday determines the ISO week year. The week number is then the count of whole weeks between January 1 of that ISO year and the Thursday, rounded up.
Because early-January or late-December days can land in a week belonging to the adjacent year, the reported ISO week year may differ from the calendar year. An ISO year has 53 weeks when it begins on a Thursday, or on a Wednesday in a leap year; otherwise it has 52. The result is exact integer date arithmetic with no rounding of the date itself.
Examples
| Input | Result |
|---|---|
| 2026-01-01 (Thursday) | Week 1 of ISO year 2026 |
| 2027-01-01 (Friday) | Week 53 of ISO year 2026 |
About this calculator
The week number calculator returns the ISO 8601 week number for any date. Under ISO 8601, weeks start on Monday and week 1 is the week containing the year's first Thursday — equivalently, the week containing January 4. Years have either 52 or 53 ISO weeks.
Because the first and last days of a calendar year can belong to a week in the neighbouring year, the calculator also reports the "ISO week year", which may differ from the calendar year. For example, the last days of December can fall in week 1 of the next ISO year, and early January can fall in week 52 or 53 of the previous one.
Frequently asked questions
Weeks run Monday to Sunday, and week 1 is the week that contains the first Thursday of the year (equivalently, the week containing January 4).
Days at the very start or end of a calendar year can belong to a week assigned to the adjacent year, so late December may be week 1 of next year and early January may be week 52 or 53 of last year.
Yes. An ISO year has 53 weeks when it starts on a Thursday, or starts on a Wednesday in a leap year; otherwise it has 52.
Also known as
TG we-Calculate Editorial Team. (2026). Week Number Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/week-number-calculator
TG we-Calculate Editorial Team. "Week Number Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/week-number-calculator.
TG we-Calculate Editorial Team, "Week Number Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/week-number-calculator
@misc{wecalculate_week_number_calculator, title = {Week Number Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/week-number-calculator}}, year = {2026}, note = {TG we-Calculate} }
Did this calculator help you?