How to Find the Dates of a Week Number

Start with the Monday of week 1

To turn a week number into dates, you first need the start of week 1 of that year. Week 1 is the week that contains 4 January, and a week starts on Monday, so the Monday of week 1 is the Monday on or before 4 January. If 4 January is itself a Monday, that is the start. If it is a Wednesday, step back two days to the Monday.

For 2026, 4 January is a Sunday. The Monday on or before it is 29 December 2025, so week 1 of 2026 starts on Monday 29 December 2025 and ends on Sunday 4 January 2026. For 2027, 4 January is a Monday, so week 1 of 2027 starts exactly on Monday 4 January 2027.

Add whole weeks

Once you know the Monday of week 1, the rest is simple multiplication. The Monday of week n is the Monday of week 1 plus 7 times (n minus 1) days. The Sunday of that week is six days later.

Example: week 10 of 2027. Week 1 starts on Monday 4 January 2027. Nine weeks later is 63 days, which is Monday 8 March 2027. The week runs to Sunday 14 March 2027. Check it with the calculator: the second tab shows exactly these seven days for week 10 of 2027.

Example: week 47 of 2026. Week 1 starts on 29 December 2025. Forty-six weeks is 322 days later, which is Monday 16 November 2026, and the week ends on Sunday 22 November 2026.

The first and last weeks of a year

Weeks 1 and 52 or 53 are the ones that cross New Year, and they cause most of the confusion. Week 1 of 2026 contains the last three days of 2025: Monday 29 December, Tuesday 30 December and Wednesday 31 December. Week 53 of 2026 starts on Monday 28 December 2026 and ends on Sunday 3 January 2027, so it contains the first three days of 2027.

That is why a date and its week-year can disagree. Friday 1 January 2027 is in week 53 of 2026, not week 1 of 2027. Week 1 of 2027 starts only on Monday 4 January. If you plan by week number, treat the first days of January as possibly belonging to the old year.

Does the year have that week?

A week number from 1 to 52 exists in every year. Week 53 exists only in the long years in which 1 January is a Thursday, or in which a leap year starts on a Wednesday. 2026 has a week 53, and 2027 does not. If a schedule says "week 53 of 2027", it contains a mistake, and a good calculator says so instead of making up a date.

Entering week 53 in the calculator for 2027 returns a message that 2027 has only 52 ISO weeks. For 2026 it returns Monday 28 December 2026 to Sunday 3 January 2027. The same check is worth building into any form or import that accepts week numbers.

Choosing a day inside the week

A week number is a range of seven days, and many plans need one day in it. The ISO format has a weekday digit for that, from 1 for Monday to 7 for Sunday. The code 2026-W47-3 means Wednesday of week 47 of 2026, which is 18 November 2026. The code 2026-W47-5 is Friday 20 November 2026.

The calculator lists all seven dates with their weekday names, so you can read the one you need. For a delivery "in week 47", a common reading is any working day from Monday 16 to Friday 20 November. If it matters, ask which day is meant.

Checking your result

Two quick checks catch most mistakes. First, the start must be a Monday and the end a Sunday. If your result starts on another weekday, you probably used a different rule, such as weeks starting on Sunday. Second, week 1 must contain 4 January. If 4 January is not in your week 1, the start is wrong.

A third check is the length of the year. The Monday of week 1 of the next year must be exactly 52 or 53 weeks after the Monday of week 1 of this year. From 29 December 2025 to 4 January 2027 is 371 days, which is 53 weeks, and that is why 2026 has 53 weeks.

Worked examples for 2026

Week 12 of 2026 starts on Monday 16 March and ends on Sunday 22 March 2026. Week 27 starts on Monday 29 June and ends on Sunday 5 July 2026, so it contains the turn of the month and the half-year. Week 36 starts on Monday 31 August and ends on Sunday 6 September 2026, which makes it a typical week for the start of an autumn term.

Notice that weeks 27 and 36 both start in a different month than they end. This is normal. A week number does not belong to one month, and a monthly report that groups by week number needs a rule for the weeks that cross the boundary.

Planning with a range of weeks

Many plans cover a range such as weeks 10 to 14. The start is the Monday of week 10 and the end is the Sunday of week 14, so the range is five full weeks, which is 35 days. In 2027 that is Monday 8 March to Sunday 11 April 2027.

If the range crosses New Year, count carefully. A plan from week 50 of 2026 to week 3 of 2027 passes through week 53, so it lasts 7 weeks, not the 6 you would get by assuming a 52-week year. The second tab shows the dates of each week, so you can check the start and the end.