January 2027 Week Numbers
Updated November 6, 2026 · Edited by the JanuaryCalendar.com team
ISO week 1 of 2027 is the Monday-to-Sunday week that contains January 4, 2027; any January days before that Monday belong to the last week of 2026. January 2027 begins on a Friday. The table below gives the ISO week numbers and date ranges for every January from 2020 to 2040.
- Standard
- ISO 8601
- Weeks start
- Monday
- Week 1 rule
- Week containing January 4
- January 1, 2027
- Friday
- US style
- Sunday start, week with Jan 1 is week 1
January week numbers 2020–2040
The table lists the ISO weeks that overlap January in each year, with the January dates each week covers. A week numbered 52 or 53 at the start of January belongs to the previous ISO year.
| Year | January 1 is in | ISO weeks in January |
|---|---|---|
| 2020 | Week 1 | W1: Jan 1–5 W2: Jan 6–12 W3: Jan 13–19 W4: Jan 20–26 W5: Jan 27–31 |
| 2021 | Week 53 of 2020 | W53 (2020): Jan 1–3 W1: Jan 4–10 W2: Jan 11–17 W3: Jan 18–24 W4: Jan 25–31 |
| 2022 | Week 52 of 2021 | W52 (2021): Jan 1–2 W1: Jan 3–9 W2: Jan 10–16 W3: Jan 17–23 W4: Jan 24–30 W5: Jan 31 |
| 2023 | Week 52 of 2022 | W52 (2022): Jan 1 W1: Jan 2–8 W2: Jan 9–15 W3: Jan 16–22 W4: Jan 23–29 W5: Jan 30–31 |
| 2024 | Week 1 | W1: Jan 1–7 W2: Jan 8–14 W3: Jan 15–21 W4: Jan 22–28 W5: Jan 29–31 |
| 2025 | Week 1 | W1: Jan 1–5 W2: Jan 6–12 W3: Jan 13–19 W4: Jan 20–26 W5: Jan 27–31 |
| 2026 | Week 1 | W1: Jan 1–4 W2: Jan 5–11 W3: Jan 12–18 W4: Jan 19–25 W5: Jan 26–31 |
| 2027 | Week 53 of 2026 | W53 (2026): Jan 1–3 W1: Jan 4–10 W2: Jan 11–17 W3: Jan 18–24 W4: Jan 25–31 |
| 2028 | Week 52 of 2027 | W52 (2027): Jan 1–2 W1: Jan 3–9 W2: Jan 10–16 W3: Jan 17–23 W4: Jan 24–30 W5: Jan 31 |
| 2029 | Week 1 | W1: Jan 1–7 W2: Jan 8–14 W3: Jan 15–21 W4: Jan 22–28 W5: Jan 29–31 |
| 2030 | Week 1 | W1: Jan 1–6 W2: Jan 7–13 W3: Jan 14–20 W4: Jan 21–27 W5: Jan 28–31 |
| 2031 | Week 1 | W1: Jan 1–5 W2: Jan 6–12 W3: Jan 13–19 W4: Jan 20–26 W5: Jan 27–31 |
| 2032 | Week 1 | W1: Jan 1–4 W2: Jan 5–11 W3: Jan 12–18 W4: Jan 19–25 W5: Jan 26–31 |
| 2033 | Week 53 of 2032 | W53 (2032): Jan 1–2 W1: Jan 3–9 W2: Jan 10–16 W3: Jan 17–23 W4: Jan 24–30 W5: Jan 31 |
| 2034 | Week 52 of 2033 | W52 (2033): Jan 1 W1: Jan 2–8 W2: Jan 9–15 W3: Jan 16–22 W4: Jan 23–29 W5: Jan 30–31 |
| 2035 | Week 1 | W1: Jan 1–7 W2: Jan 8–14 W3: Jan 15–21 W4: Jan 22–28 W5: Jan 29–31 |
| 2036 | Week 1 | W1: Jan 1–6 W2: Jan 7–13 W3: Jan 14–20 W4: Jan 21–27 W5: Jan 28–31 |
| 2037 | Week 1 | W1: Jan 1–4 W2: Jan 5–11 W3: Jan 12–18 W4: Jan 19–25 W5: Jan 26–31 |
| 2038 | Week 53 of 2037 | W53 (2037): Jan 1–3 W1: Jan 4–10 W2: Jan 11–17 W3: Jan 18–24 W4: Jan 25–31 |
| 2039 | Week 52 of 2038 | W52 (2038): Jan 1–2 W1: Jan 3–9 W2: Jan 10–16 W3: Jan 17–23 W4: Jan 24–30 W5: Jan 31 |
| 2040 | Week 52 of 2039 | W52 (2039): Jan 1 W1: Jan 2–8 W2: Jan 9–15 W3: Jan 16–22 W4: Jan 23–29 W5: Jan 30–31 |
For a month view of the weeks, see the January 2027 calendar.
How ISO 8601 week numbers work
ISO 8601, the international standard for dates, numbers weeks with these rules:
- Weeks run from Monday to Sunday.
- Week 1 is the week that contains the year's first Thursday. Two equivalent tests: it is the week containing January 4, or the first week with at least four days in the new year.
- Week 1 therefore starts on a Monday between December 29 and January 4.
- Each week belongs entirely to one ISO year, so the ISO year can differ from the calendar year for a few days at either end.
In ISO notation, a date is written as year, week and weekday: 2027-W01-1 is the Monday of week 1 of 2027, and weekday 7 is Sunday.
Why January 1 can be in week 52 or 53
If January 1 falls on a Friday, Saturday or Sunday, the days before the first Monday belong to the last week of the previous ISO year. That week is week 53 if the previous year had 53 ISO weeks, and week 52 otherwise. If January 1 falls on a Monday, Tuesday, Wednesday or Thursday, it is already in week 1, and week 1 starts on that Monday or in the last days of December.
This catches people out in reports and file names. A "week 1" sales report may not include January 1, and a date as early as December 29 can belong to week 1 of the next year. When sorting or grouping data by week, pair the week number with the ISO year rather than the calendar year.
Which years have 53 weeks
Most years have 52 ISO weeks. A year has 53 when it starts on a Thursday, or when it is a leap year that starts on a Wednesday. Put another way, a year has 53 weeks if January 1 or December 31 is a Thursday. Week 53 runs from the last Monday of December into the first days of the next January.
Between 2015 and 2050, the 53-week years are 2015, 2020, 2026, 2032, 2037, 2043 and 2048, usually five or six years apart. Over the full 400-year cycle of the Gregorian calendar, 71 years have 53 weeks.
US week numbers and spreadsheet formulas
Many US calendars and planners use a different system: weeks start on Sunday, and the week containing January 1 is week 1, however few days it has. Under this system week 1 can be a single day, and a leap year that starts on a Saturday ends in week 54.
- Excel: =WEEKNUM(A1) uses the US system by default. =WEEKNUM(A1, 2) starts weeks on Monday but keeps the January 1 rule. =ISOWEEKNUM(A1) or =WEEKNUM(A1, 21) returns the ISO week.
- Google Sheets: WEEKNUM and ISOWEEKNUM work the same way as in Excel, including return type 21.
- Python: date.isocalendar() returns the ISO year, week and weekday; use .week (Python 3.9 and later) or index [1] for the week. The strftime codes %U and %W count weeks differently, placing days before the first Sunday or Monday in week 0.
The two systems often disagree, especially when January 1 is a Friday or Saturday, and on Sundays, which start a US week but end an ISO week. Before comparing week numbers from two sources, check which rule each uses.
Where week numbers are used
Week numbers are part of everyday life in much of northern Europe, including Sweden, Norway, Denmark, Finland, Germany and the Netherlands, where meetings, vacations and deliveries are often scheduled by week number. ISO numbering is the norm there.
Businesses worldwide use week numbers for manufacturing production plans, shipping and logistics schedules, and weekly sales reporting. Many retailers follow a 4-5-4 fiscal calendar with its own week numbers and an occasional 53rd week, so a retail "week 1" may not match ISO week 1. Most calendar apps, including Outlook, Google Calendar and the iPhone Calendar app, have a setting to show week numbers. For planning by working day rather than by week, see working days in January.
Frequently asked questions
When does ISO week 1 of 2027 start?
It starts on the Monday of the week that contains January 4, 2027. The table above shows the exact January dates of week 1.
What week number is it today?
Use =ISOWEEKNUM(TODAY()) in Excel or Google Sheets for the ISO week, or =WEEKNUM(TODAY()) for US-style numbering. Most calendar apps can also display week numbers.
Do ISO weeks start on Sunday or Monday?
Monday. ISO 8601 weeks run Monday through Sunday, while US calendars commonly start weeks on Sunday, which is one reason the two week numbers can differ.
How many weeks are in a year?
A calendar year is 52 weeks plus 1 day, or plus 2 days in a leap year. In ISO numbering, a year has either 52 or 53 full weeks.
What is week 53?
Week 53 is the extra ISO week in years that start on a Thursday, and in leap years that start on a Wednesday. It covers the last days of December and the first days of the next January.
Why does Excel show a different week number?
Excel's WEEKNUM uses the US rule by default: weeks start on Sunday and the week containing January 1 is week 1. Use ISOWEEKNUM, or WEEKNUM with return type 21, for ISO weeks.