How Many Weeks Between Two Dates? Calculator & Guide

You probably know there are 7 days in a week, but counting weeks between two dates gets surprisingly tricky. The standard method—total days divided by 7—often needs context for pregnancy, project planning, or payroll.

Days in a standard week: 7 · Average weeks in a year: 52.1775 · Typical pregnancy duration in weeks: 40 · Excel DATEDIF unit for weeks: “d” then divide by 7

Four key facts, one pattern: the math behind weeks-between-dates is simple, but applying it correctly requires context.

How many weeks are between two dates?

Understanding the basic calculation

• Count the number of days between the two dates, then divide by 7. The Calculator Site (utility tool with editorial oversight) explains that this method works by getting the timestamp of the start date and subtracting it from the end date.
• For example, 108 days equals 15 weeks and 3 days (The Calculator Site).
• Inch Calculator (calculator reference site) provides a six‑step manual method: identify dates, calculate days left in the start month, days in the end month, add interim months, total the days, and divide by 7.

Why rounding matters

• Planetcalc (online calculator tool) returns full weeks by dividing total days by 7 and rounding down, showing the remainder as leftover days.
• In project planning, partial weeks may be counted as full weeks (to buffer schedules) or truncated (for precise milestone tracking). The choice is a policy decision, not a mathematical one.

The implication: a one‑size‑fits‑all calculator can’t know whether you want 3 weeks 2 days or simply 4 weeks. Always check the tool’s rounding logic.

How to calculate the number of weeks between two dates in Excel?

1. Identify your start and end dates.
2. Use the DAYS function: =DAYS(end_date, start_date) to get the total days.
3. Divide by 7: =DAYS(end_date, start_date)/7 to get weeks as a decimal.
4. For whole weeks and days, use =INT(DAYS(end_date, start_date)/7) and =MOD(DAYS(end_date, start_date),7).

Using DAYS (or DATEDIF) and dividing by 7

• Both Google Sheets and Microsoft Excel support the DAYS function: =DAYS(end_date, start_date) returns the exact day count (Inch Calculator (calculator reference site)).
• To get weeks, divide by 7: =DAYS(end_date, start_date)/7 gives a decimal (e.g., 3.2857 weeks).
• =INT(DAYS(end_date, start_date)/7) returns whole weeks, and =MOD(DAYS(end_date, start_date),7) gives the remainder days.

Using WEEKNUM for business weeks

• The WEEKNUM function returns the week number of a date within a year, not the weeks between two dates. For work‑week counts, NETWORKDAYS (which counts only weekdays) can be divided by 5 (Inch Calculator (calculator reference site)).
• Pearson (educational publisher with calculator tools) offers a duration calculator that can display results in years, months, days, and optional weeks or hours, including an inclusive option that adds 1 day.

Formatting the result as weeks and days

• Combine INT and MOD in a formula like =INT((B2-A2)/7) & " weeks, " & MOD((B2-A2),7) & " days".
• The Economic Times (news and utilities) notes that their date calculator displays the difference in years, months, weeks, and days, with an option to include the last day (adding 1 day to the difference).

The pattern: Excel’s flexibility allows you to tailor the result to your specific need, but clarity about the unit (decimal weeks vs. weeks and days) is essential.

How does a week calculator by date work for pregnancy?

Pregnancy weeks vs. calendar weeks

• Pregnancy is counted from the first day of the last menstrual period (LMP), not from conception (Wikipedia (medical reference)).
• A full‑term pregnancy is 40 weeks (280 days) from LMP. Some calculators use 38 weeks from conception, which can cause a 2‑week discrepancy.

Due date estimation from LMP

• Online pregnancy week calculators add 40 weeks to LMP to estimate the due date. Wikipedia (medical reference) states that gestational age is typically reported in weeks from LMP.
• Because the LMP is a fixed calendar date, the calculator uses standard date arithmetic: LMP + 280 days.

The catch: the same date range can produce different week counts depending on whether you align with clinical guidelines or a consumer app. For medical decisions, always use the LMP‑based method recommended by the American College of Obstetricians and Gynecologists (via Wikipedia).

How many days are between two dates?

Days as the base unit for weeks

• The number of days is the essential intermediate value for week calculations. The Calculator Site (utility tool with editorial oversight) works by getting timestamps and subtracting, returning hours, days, and weeks.
• Excel’s DATEDIF and DAYS functions both return whole days between two dates (Inch Calculator (calculator reference site)).

Leap year considerations

• Leap years add one extra day every four years. Over a period of 10 years, this can shift the week count by about 0.14 weeks (Wikipedia (calendar reference)).
• For long intervals, use a tool that accounts for leap years automatically (most online calculators do; manual math must be checked).

How many months are between two dates?

Months vs. weeks: why months are not a fixed number of weeks

• Months vary in length from 28 to 31 days, so converting months to weeks requires an average: approximately 4.345 weeks per month (52.1775 weeks per year ÷ 12) (Wikipedia (calendar reference)).
• DATEDIF(start, end, "m") returns whole calendar months. If you need weeks from months, multiply the month difference by 4.345.

Using DATEDIF for months in Excel

• =DATEDIF(start_date, end_date, "m") gives the number of complete months. For weeks, multiply by 4.345 (or use the days‑route).
• For payroll or billing, a month may be treated as exactly 4 weeks (28 days). This simplifies accounting but diverges from calendar reality (The Economic Times (news and utilities) provides a date difference calculator that shows months and weeks separately).

The pattern: when you need months, remember they are not equal containers. Using weeks as an intermediate unit avoids the month‑length ambiguity altogether.

What’s confirmed and what’s still unclear

“The formula DAYS(end_date, start_date) / 7 returns the number of weeks as a decimal. For whole weeks, use the INT function on the result.”

Inch Calculator (calculator reference site)

“A full‑term pregnancy is 40 weeks from the first day of the last menstrual period. Gestational age is typically expressed in weeks.”

Wikipedia (medical reference)

For anyone scheduling a project or tracking a personal milestone, the choice is clear: decide upfront how you will treat partial weeks and which starting point your calculator uses. Without that decision, the same date range can yield different answers—and that ambiguity is where mistakes happen.

Related reading: **weeks-between-dates calculator** · **days-between-dates calculator**

For a more hands-on approach, you can use the detailed calculator and guide that walks through manual math, Excel formulas, and pregnancy calculators step by step.

Frequently asked questions