Week Calculator

Weeks between two dates, with the remainder kept

Subtract the earlier date from the later one to get a day count, then divide by seven. The whole part is the number of full weeks and the remainder is the leftover days. A span of 293 days is 293 / 7 = 41.857, so 41 weeks and 6 days, not 42 weeks.

That remainder is the part people drop, and dropping it changes the answer to a planning question. A project that needs 41 weeks and 6 days does not fit in a 41 week window. Keeping the days also makes the result checkable against a calendar, because 41 x 7 + 6 = 293.

The count is end minus start, so the start date is included and the end date is not. That convention makes a Monday to the following Monday exactly 7 days, and it makes the same date twice zero. If you need a count that includes both ends, add one day before dividing, and say which convention you used.

How this page differs from the week number and weekday pages

Three existing pages answer three narrow questions. Week number 2026 takes one date in 2026 and returns its ISO week, the Monday and Sunday around it, and how many weeks are left in the year. Day of the week calculator takes one date and returns the weekday, or lists every date in a month that falls on a chosen weekday.

This page measures a span and moves a date. It answers how many weeks sit between two dates, what date is 12 weeks from now, how many of those days are workdays, and which ISO week each endpoint belongs to. The weekday pages do not count, and the week number page does not add or subtract.

The overlap is the ISO week number, which appears here as context for the two endpoints rather than as the main answer. If the question is what week it is today, the week number page is the shorter route. If the question is how long a stretch is or when a deadline lands, this page does the arithmetic. The days between dates calculator covers the same spans in days when weeks are not the unit you need.

ISO 8601 weeks and the year boundary

ISO 8601-1:2019 defines a week as seven days starting Monday, and week 1 as the week containing the first Thursday of the year, which is the same as the week containing January 4. A year has 52 or 53 ISO weeks, and the last days of December can belong to week 1 of the next year while the first days of January can belong to week 52 or 53 of the previous one.

That is not a bug in the calendar, it is the rule doing its job. The ISO week year is built from whole weeks, so it starts on a Monday and ends on a Sunday, which means it does not line up with January 1.

The practical effect shows up in payroll and scheduling software. A weekly report labelled by calendar year splits the week of December 28 across two files, and a system that keys on ISO week keeps it in one. When two reports disagree about the last week of December, check which week numbering each one uses before hunting for a data error.

Adding and subtracting weeks keeps the weekday

A week is exactly seven days, so adding weeks moves the date but never the weekday. October 21, 2026 is a Wednesday, and every whole number of weeks from it is also a Wednesday. That property is why weekly meetings, subscription renewals and payroll cycles stay on the same day.

Subtraction works the same way. Twelve weeks before October 21, 2026 is July 29, 2026, a Wednesday. The calculator handles both directions with the add or subtract switch, and it reports the weekday of the result so a mismatch is visible immediately.

Adding days is different. Adding 84 days and adding 12 weeks give the same date, but adding a month does not, because months run 28 to 31 days. A monthly renewal on the 31st disappears in February, while a 4 week renewal does not. If a schedule has to stay on the same weekday, count in weeks.

Workdays and weekend days inside a span

The workday count treats Monday to Friday as workdays and Saturday and Sunday as weekend. It does not know about federal holidays, which is why a 12 week span that contains Thanksgiving and Christmas still reports the same 60 weekdays as one that contains neither.

Counting starts on the start date and stops before the end date, matching the day count. Ten consecutive days beginning on a Monday contain 8 weekdays and 2 weekend days; the same ten days beginning on a Saturday contain 6 weekdays and 4 weekend days. The start weekday decides the split.

For overtime and pay purposes, the legal workweek is a fixed and regularly recurring 168 hour period, and the Department of Labor fact sheet notes that it can begin on any day and at any hour. A Sunday to Saturday workweek is as valid as Monday to Friday, so a payroll week and the weekday count here can legitimately differ by a day at the edges.

Worked example: a 12 week project from October 21, 2026

A team signs a 12 week contract on Wednesday, October 21, 2026 and wants the delivery date. Twelve weeks is 12 x 7 = 84 days, and 84 days after October 21 is January 13, 2027. The result is a Wednesday, the same weekday as the start, and the span crosses New Year without any adjustment.

Break the 84 days into working time. The span holds 60 weekdays and 24 weekend days. Remove six public holidays inside the window, which is plausible for late November through early January in the United States, and 54 working days remain. At 7.5 productive hours a day that is 405 hours, or about 10.1 weeks of a 40 hour week, which is the number to put against the estimate.

The ISO weeks at each end are useful for reporting. October 21, 2026 sits in week 43 of 2026, and January 13, 2027 sits in week 2 of 2027, so the project touches 12 ISO weeks plus the partial weeks on either side. A status report written as week 43 to week 2 is unambiguous to anyone using ISO numbering, and confusing to anyone counting calendar weeks from January.

Leap years, daylight saving and the seven day assumption

Every count on this page uses calendar dates, not clock time, so daylight saving changes nothing. The day a clock jumps forward is still one calendar day, and a span that crosses a spring transition is not 23 hours short in this arithmetic. Software that subtracts timestamps instead of dates does hit that problem, which is one reason date math belongs in date types.

Leap years add a day, and that day can change a weekday count. February 29, 2028 is a Tuesday, so a span that includes it gains a weekday, and a 52 week count starting January 1, 2028 ends on December 30 rather than December 31. Leap years follow the 4, 100, 400 rule: divisible by 4, except centuries, unless divisible by 400. 2000 was a leap year and 2100 will not be.

The 52 week assumption is the other place counts drift. A year is 52 weeks and 1 day, so a biweekly payroll runs 26 checks in most years and 27 in some. A weekly rent of $400 is $20,800 over 52 weeks and $20,400 over 51, which is why a 52 week year and a 365 day year are not interchangeable when money is attached.