Weeks Calculator
Weeks from a date, weeks between dates, and which week number we are in.
Week numbers follow ISO 8601, so weeks run Monday to Sunday and week 1 is the one holding the first Thursday of January.
Weeks are how people actually plan. Projects run for six weeks, notice periods are four, a course is twelve. Calendars, on the other hand, are built out of months, so anything measured in weeks needs converting before it means a date.
Work forwards from today or from any date to find where a number of weeks lands. Work backwards from two dates to see how many whole weeks sit between them and how many days are left over. Or look up a week number, which is everyday vocabulary across much of Europe and quietly baffling everywhere else.
Week numbers here follow ISO 8601, so weeks run Monday to Sunday and week one is the week containing the first Thursday of January. That is why some years have fifty three weeks and why the first of January is occasionally still in last year's final week.
How weeks are counted
Adding weeks adds seven days for each week, so the result always lands on the same weekday you started from. Fractions of a week are rounded to the nearest day, so 2.5 weeks is 18 days.
Counting between two dates takes the days in between, divides by seven, and shows the whole weeks plus the days left over.
A worked example
Twelve weeks from Monday 28 September 2026 is Monday 21 December 2026, in ISO week 52. Three months from the same date would be 28 December, a week later, because those three months add up to 91 days, exactly 13 weeks.
From Thursday 1 January to Wednesday 30 September 2026 is 272 days, which is 38 weeks and 6 days.
Mistakes with weeks
Treating a month as four weeks. Only February in a common year is exactly four weeks. Every other month is four weeks and two or three days, and over a year that adds up to an extra month.
Mixing numbering systems around New Year. Under ISO 8601, 1 January 2027 falls in week 53 of 2026, while systems that start week 1 on 1 January, common in the US, call the same day week 1 of 2027. Say which system you mean when a week number goes into a contract or a schedule.
Counting the end week twice, or not at all. A plan from week 40 to week 44 covers five weeks if both are included and four if they are not. Decide which before you share it.
Six weeks is thirty working days, and they are already partly spoken for.
Planning in weeks hides how little time is really in them. PomoTodo tracks the hours those weeks actually contain, so the schedule you set is built on what happened rather than what you hoped.
Free plan tracks unlimited focus time across unlimited tasks. No card required.
More tools
Add or subtract days from a date, or count days between two.
Exact age in years, months and days, plus the next birthday.
Working days between dates, or when a deadline lands.
Days, hours and minutes until whatever you are waiting for.
Questions
What date is a number of weeks from today?
Enter the number of weeks and the date appears, along with the day of the week and which week number it falls into. Switch the direction to count backwards instead.
How many weeks are in a year?
Fifty two weeks and one day, or two days in a leap year, which works out at about 52.18 weeks. Counted as ISO weeks a year has either 52 or 53, and 53 happens roughly every five or six years.
What week number is it?
The current week is shown at the top of the page along with the dates it covers. You can also look up the week for any other date, or go the other way and see which dates a given week number covers.
Why do week numbers sometimes disagree?
Because there is more than one convention. This uses ISO 8601, which is standard across Europe and starts weeks on Monday. Some systems in the United States start the week on Sunday and count week one differently, which can put the two a week apart in January.