How to Calculate Weeks Between Dates

Weeks as a unit

A week is a convenient unit because it has a fixed length of seven days and a rhythm that people recognise. Many plans run in weeks: training programmes, school terms, project sprints, medical follow-ups and delivery schedules. Turning a date range into weeks makes it easier to plan.

The calculation is a division. Count the days between the two dates, divide by 7, and the whole number is the number of full weeks. The remainder is the number of extra days.

The calculation step by step

From Thursday 22 October 2026 to Friday 1 January 2027 there are 71 days. Divide by 7: 71 = 10 × 7 + 1, so the range is 10 weeks and 1 day. Written as a decimal, it is about 10.1 weeks.

Another example: from Tuesday 1 September 2026 to Friday 18 December 2026, a typical school term, there are 108 days. 108 = 15 × 7 + 3, so the term is 15 weeks and 3 days.

Because the remainder is the weekday shift, it tells you how the end weekday relates to the start. A remainder of 1 means the end date is one weekday later than the start, as with Thursday and Friday above.

Weeks and days, or decimal weeks

You can state a result as weeks and days, such as 10 weeks and 1 day, or as a decimal, such as 10.14 weeks. Weeks and days are more precise and easier to check. Decimals are handy for charts and rates, such as an amount per week.

Avoid rounding the weeks without saying so. A pregnancy at 12 weeks and 6 days is nearly 13 weeks, but is still described as 12 weeks in most medical contexts, which count only completed weeks. The convention depends on the field.

A year has 52 weeks and 1 day, or 52 weeks and 2 days in a leap year. Fifty-two weeks is 364 days, which is why a yearly event moves forward one weekday from year to year.

Elapsed weeks and ISO week numbers

The weeks between two dates are not the same as the difference in their ISO week numbers. ISO weeks are fixed calendar blocks from Monday to Sunday, and a date near the end of a week is in the same block as one near its start.

For example, Thursday 22 October 2026 is in ISO week 43, and Friday 1 January 2027 is in ISO week 53 of 2026. The week numbers differ by 10, which matches the 10 full weeks, but only because the two days sit at similar positions in their weeks. A range from Sunday to the following Monday spans two ISO weeks while lasting one day.

Use elapsed weeks for durations and ISO weeks for schedules. They answer different questions, and mixing them leads to off-by-one errors.

Common uses

• Training plans: a 12-week programme starting on Monday 26 October 2026 ends on Sunday 17 January 2027, and the next one starts on Monday 18 January 2027.
• Project sprints: six two-week sprints starting on Monday 26 October 2026 finish on Sunday 17 January 2027.
• Medical and baby milestones: age in weeks, or weeks since an appointment.
• Courses and terms: the number of teaching weeks between the first and the last session.

In each case the useful number is the weeks and days between two dates, and the useful date is the one you get by adding a number of weeks to a start date.

Adding and subtracting weeks

To add weeks, multiply by 7 and add the days. Twelve weeks after 22 October 2026 is 84 days later, which is 14 January 2027. The weekday stays the same, a Thursday, and that is a useful check.

To subtract, do the same in reverse. Forty weeks before a date is 280 days before it, which is exactly the length used in due-date calculations. Weeks are the one unit where the date arithmetic and the weekday logic fit together perfectly.

A planning example with weeks

Suppose a course has a first session on Tuesday 1 September 2026 and a last session on Friday 18 December 2026, with a two-week break in October. The term runs for 15 weeks and 3 days, so 13 weeks and 3 days remain for teaching once the break is removed. Working in weeks makes that subtraction easy, because the break is a whole number of weeks.

Another example is a savings plan. If you want to set aside a fixed amount each week until 1 January 2027, starting on Thursday 22 October 2026, you have 10 full weekly payments and one extra day. Dividing the target by 10 gives the weekly amount, and a calculator confirms the number of weeks before you commit to the figure.

In both cases the weeks-and-days form is the one to read. It tells you at once whether you have a clean number of weeks, and what is left over when you do not.

Using the calculator

On the days-between tab, the row Weeks and days shows the range as weeks and a remainder, together with the total days, workdays, hours and minutes. Enter the two dates and read the line.

On the Add or subtract tab, type a number in the Weeks field to move a date forward or backward by whole weeks. The result shows the new date, its weekday and its ISO week.