How to Calculate Weeks Between Two Dates

Calculating the exact number of weeks between two dates is more useful than it sounds. Whether you're tracking a pregnancy, planning a project deadline, or calculating a notice period, knowing precisely how many weeks span between two dates — and whether you're counting partial weeks, working weeks, or calendar weeks — makes the difference between a rough guess and a proper plan. Our weeks between dates calculator does the maths instantly, but understanding how it works stops you misquoting numbers later.

The Basic Calculation

Here's the core maths: take the number of days between your two dates and divide by 7. If you're starting on Monday, January 6th and ending on Friday, January 24th, that's 18 days — or 2 weeks and 4 days.

Most contexts use seven days per week, but precision matters. If you want only full weeks, you'd count 2 weeks (14 days). If you want partial weeks included, that's 2.57 weeks. The calculator handles both, but understanding the difference stops you repeating a number that technically means something different.

The formula is straightforward:

Days between dates ÷ 7 = Weeks (plus remainder)

If you're calculating backwards — "I need 6 weeks for this project, when do I start?" — flip it: multiply 6 weeks by 7 to get 42 days, then count backwards from your deadline. Adding or subtracting days from a date gets easier once you've pinned down your starting point.

Why This Matters: Common Use Cases

Pregnancy and gestation tracking — A typical pregnancy runs roughly 40 weeks from the last menstrual period (LMP). Knowing exactly how many weeks you are helps your midwife or GP monitor your health and predict your due date. The NHS uses week counts to determine the timing of scans and appointments. The NHS pregnancy calendar shows how medical professionals use week-based dating throughout your pregnancy.

Project deadlines and timelines — You're handed a deadline of March 15th. How many working weeks do you actually have? The answer depends on whether you're counting calendar weeks, working weeks (5 days), weekends, and UK bank holidays. A project that's "8 weeks away" could be 8 calendar weeks (56 days) or 8 working weeks (40 days) — a 40% difference that catches managers off guard. Calculating days between dates gives you exact day counts; divide by 5 for working days.

Notice periods and employment law — In the UK, statutory notice periods are often measured in weeks. If you're due to leave in 4 weeks, you need to know whether that means 28 calendar days or 28 working days (or something else entirely). Employment contracts can vary; some count from the date you hand in notice, others from the next working day. Our notice period calculator handles the legal ambiguities so you don't hand in notice at the wrong time.

Countdown milestones — School holidays, holidays abroad, license renewals, insurance payments. Knowing you have exactly 14 weeks and 3 days until a deadline helps with proper planning. Whether you book that holiday for week 12 or week 13 depends on knowing the precise week count rather than rounding.

Calendar, Working, and Gestational Weeks

Not all weeks are the same, and context determines what you're really measuring.

Calendar weeks — Seven consecutive calendar days. Monday to Sunday, or Sunday to Saturday. This is the simplest and most common interpretation. If today is Monday and you ask "how many calendar weeks until next Monday," it's one full week (7 days). Most people mean calendar weeks when they say "weeks."

Working weeks — Five business days, excluding weekends (and sometimes UK bank holidays). A project deadline 6 weeks away is 30 working days if you ignore bank holidays, or 28–29 if you account for them. This is where precision breaks down: bank holidays vary by region (Scotland has different public holidays than England), and some companies give additional holidays. Check the gov.uk bank holiday calendar for 2026 dates, then count manually if your workplace has extras.

Gestational weeks — Used in pregnancy dating, and they work differently than you'd expect. Week 1 of pregnancy actually counts from your last menstrual period (LMP), not from conception. That's why a "full-term" pregnancy is 40 weeks from LMP, not 40 weeks from conception. This confuses people. A doctor saying "you're 20 weeks" means 20 weeks from LMP, or roughly 18 weeks since conception. It's not an error — it's just how medical professionals count, and it helps them pinpoint your due date more accurately.

ISO weeks — Used in some business and project contexts. The ISO 8601 standard defines week 1 as the first week with a Thursday in the new calendar year. This can shift the first day of the year (January 1st might be in ISO week 1 of the next year, depending on the day of the week). Most people don't encounter this unless they're writing business reports in Europe or dealing with international project management.

For most purposes, calendar weeks or working weeks are what you need. The calculator shows you all of them.

Complications: Leap Years, Time Zones, and Midnight

Leap years — February in leap years has 29 days instead of 28, which adds one extra day every four years (with exceptions for century years). If your date range spans a February 29th, you'll have one extra day — or one fewer day if you're counting backwards. The calculator accounts for this automatically, so you don't have to.

Midnight and time zones — If you're calculating between "Monday at 9 AM" and "Friday at 5 PM," you have 4 days and 8 hours, not quite 5 full days. The calculator works with dates, not times, so it assumes midnight-to-midnight. If you need precision to the hour or are working across time zones, convert your dates to your local timezone first, then work with the date portion.

Partial weeks — The most confusing bit. "How many weeks until Christmas?" If Christmas is 3 weeks and 2 days away, do you say "3 weeks" or "3.3 weeks"? The answer depends on your context. A due date 20 weeks away is usually 20 full weeks (you'll probably deliver within a few days either side), but a project deadline 3 weeks away often means "3 weeks plus however many extra days you've got." The calculator shows you both numbers.

When to Use the Calculator vs. Manual Counting

You can count weeks on your fingers: pick the start date, count forward in 7-day chunks until you hit or pass your end date. This works fine for a few weeks.

You should use the calculator when:

• You're counting across months (4+ weeks), where calendar transitions get confusing
• You need to account for working weeks or bank holidays
• You're calculating backwards (deadline minus duration)
• You're planning a year of key dates and want consistency
• You're tracking something medical or legal where mistakes are expensive
• You're doing the calculation more than once (copy-paste is faster than recounting)

The calculator also handles edge cases: if your start date is later than your end date, it tells you so. If you're counting across multiple years, leap-year maths are handled for you. You get three numbers — full weeks, remainder days, and decimal weeks — so you can use whichever makes sense for your situation.

Weeks in Different Contexts

Finance and payroll — Payroll often runs on a four-weekly or five-weekly cycle (28 or 35 days), not a calendar week. A "4-week payroll period" means 28 days, even if that spans three calendar weeks. This matters when you're calculating annual salary from a weekly or monthly rate.

School terms — UK school terms typically run for six weeks at a time, then break for two. Counting weeks from term start to end helps with holiday planning (or realizing your child has seven school weeks until the summer break). Back-to-school planning is easier when you know exactly how many weeks you have to prepare.

Retail and business — Some businesses measure quarters as 13 weeks (91 days). A "13-week rolling average" smooths out weekly fluctuations and gives you a clearer trend.

Retirement and life milestones — If you're counting down to retirement or tracking age across events, weeks are a useful middle ground between days and months. A project that's "52 weeks away" is easier to visualize than "365 days" but more precise than "about a year."

Frequently Asked Questions

Q: How many weeks is 90 days? A: 90 days ÷ 7 = 12.86 weeks, or 12 full weeks and 6 days. If you need only full weeks, that's 12 weeks. The calculator shows all three: full weeks, days remainder, and decimal weeks.

Q: Can you count weeks across months? A: Yes. Weeks don't align with calendar months — a week that starts on a Thursday will span two months. The calculator handles this without flinching. Just enter your start and end dates.

Q: Do bank holidays reduce the number of working weeks? A: Yes, in the context of working weeks. If your project spans a bank holiday (like Easter or Christmas), you have fewer working days. A 4-week project with one bank holiday built in is really 3 weeks and 4 days of work, not 4 weeks. You need to count manually or use a deadline calculator that flags holidays.

Q: What's the difference between gestational weeks and calendar weeks? A: Gestational weeks in pregnancy count from your last menstrual period (LMP), not from conception. So "40 weeks gestational" is roughly 38 weeks after conception. Medical professionals use gestational weeks because LMP is easier to pinpoint than the exact moment of conception.

Q: How do you calculate weeks backwards from a deadline? A: Multiply the number of weeks by 7 to get days. If your deadline is May 1st and you need 6 weeks, that's 42 days — count back 42 days from May 1st to find April 19th as your start date. This is useful for notice periods and project planning.

Q: Do leap years affect week calculations? A: Only if your date range includes February 29th. A leap year adds one extra day to February, which shifts all later dates by one day. If you're calculating from January 1st to December 31st in a leap year, you have 52 weeks and 2 days (366 days); in a non-leap year, it's 52 weeks and 1 day (365 days).

Q: What if I need weeks but also days, hours, and minutes? A: Our calculator shows weeks and days. For hours and minutes, check our exact age calculator or time difference calculator, which break down time to the minute.

Q: Can I use this for age, retirement, or other time spans? A: The weeks calculator works for any date range. For specific contexts, we have dedicated calculators: retirement countdown, age calculator, and back-to-school countdown. These handle context-specific quirks (like whether to count leap days in age, or whether to include graduation day).