How to Calculate Weeks Between Two Dates
Reviewed by Jerry Croteau, Founder & Editor
Table of Contents
I was staring at a calendar… and still got it wrong
I was standing in the lumber aisle doing math on my phone and nothing was adding up. I needed to know how many weeks we had before a delivery window closed, because the supplier was saying “about 6 weeks” and my calendar was saying something else and my foreman was basically just waiting for me to stop squinting at the screen.
So I did what you probably did: I counted boxes on a calendar, then I counted days, then I second-guessed whether the start day “counts,” and then I got a different answer depending on which app I used. Fun.
Weeks are simple until they aren’t.
And the annoying part is: you’re usually not asking this for trivia. You’re trying to plan payroll, a project timeline, a payment schedule, a return period, a training plan, or some “net 45” invoice thing that someone threw out in a meeting like it was obvious.
What you actually mean by “weeks between” (this is where people trip)
You can’t calculate “weeks between two dates” until you decide what kind of week answer you want. I had no idea what that meant at first. I nodded like I understood. I didn’t.
Here are the three versions that show up in real life:
- Total weeks (decimal): you want the exact span in days divided by 7. This is the cleanest for planning and estimating.
- Whole weeks: you only care about complete 7-day blocks. Great for “every 2 weeks” schedules where partial weeks don’t count.
- Calendar weeks crossed: this is the weird one—how many week boundaries you pass (like how many Mondays you hit). Payroll systems and reporting sometimes care about this, and it’ll disagree with the other two.
So yeah, if you and someone else are arguing over the “right” number of weeks, you might both be right… you’re just answering different questions.
end_date = the ending date
days = the number of 24-hour days between the two dates (based on your inclusion rules)
Notice that little parenthetical about inclusion rules? That’s the whole game.
The method I use (and what I tell my team to use)
If you want something you can do fast and not regret later, do it in this order:
- Pick your counting rule: are you counting elapsed time (exclusive) or counting days on the calendar (inclusive)?
- Convert dates to days: get the day difference using a calculator (or a date diff function).
- Divide by 7: that’s your decimal weeks.
- Then decide rounding: round up, round down, or keep the decimal.
But here’s the part people skip: step 1. They jump straight to dividing by 7 and then get mad when the answer doesn’t match the “6 weeks” someone said out loud.
Exclusive means you’re measuring elapsed time from the start date to the end date, not counting the start day as a full day. A lot of software does this. If something starts on March 1 and ends on March 8, that’s 7 days of elapsed time.
Inclusive means you’re counting calendar days “on the page.” March 1 through March 8 feels like 8 days to most humans because you’re counting both endpoints (and honestly, that’s how a lot of jobsite planning conversations go).
One sentence that saves arguments: “Are we counting the start date?”
And if you’re dealing with schedules, I also ask: “Are we counting weekends?” Because “weeks” sometimes secretly means “work weeks,” which is a whole different thing (and no, dividing by 7 won’t help you there).
A worked example (the way it comes up in real life)
Say you’ve got a material lead time from April 3 to May 20. You want to tell the customer how many weeks that is, and you don’t want to sound like you’re guessing.
- Step 1: Decide the rule. For lead time, I treat it as elapsed time (exclusive).
- Step 2: Find the day difference. April 3 to May 20 is 47 days (that’s the number you’d get from a standard date-difference tool).
- Step 3: Divide by 7: 47 ÷ 7 = 6.714… weeks
- Step 4: Rounding: if you’re ordering and you’d rather be safe, you round up to 7 weeks. If you’re reporting exact duration, keep 6.7 weeks.
That’s it. And it works!
But if someone insists it’s “almost 8 weeks,” they might be doing inclusive counting and rounding up aggressively. Or they might be counting calendar weeks crossed. Or they might just be tired.
Quick reference table (so you can pick the right flavor)
| What you need | How to calculate | Typical use | Common mistake |
|---|---|---|---|
| Decimal weeks | days ÷ 7 | Planning, estimating, timelines | Rounding too early |
| Whole weeks (complete) | floor(days ÷ 7) | “Every 2 weeks” cycles | Forgetting leftover days exist |
| Weeks rounded up | ceil(days ÷ 7) | Lead times, buffers | Overstating time on tight schedules |
| Weeks + days | (whole weeks) and (remainder days) | Clear communication (“6 weeks, 5 days”) | Mixing inclusive/exclusive rules mid-way |
That “weeks + days” row is underrated. If you tell someone “6 weeks and 5 days,” they stop arguing with you because it sounds like you actually checked.
Use a calculator when you’re in a hurry (I do)
I built ProCalc.ai because I got tired of redoing the same math in Notes apps and then wondering if I fat-fingered something. If you just want the answer without the calendar gymnastics, use these:
And yeah, I still sanity-check the result sometimes by eyeballing the calendar. Old habits.
FAQ (the stuff that always comes up)
Do I count the start date and end date?
If you’re measuring elapsed time, most of the time you count the difference between dates (exclusive of the start date as a full day). If you’re counting calendar days on a schedule, people often count both ends (inclusive). Pick one and stick to it—don’t switch halfway because the number “feels better.”
Why does my answer change depending on the app?
- Some tools do inclusive counting, some don’t.
- Some round to whole weeks automatically (which is kind of sneaky).
- If time-of-day is involved, you can get partial-day effects (like 6.9 days instead of 7).
What if I need “work weeks,” not calendar weeks?
Then you’re not really asking for “weeks between dates,” you’re asking for business days (or working days) converted to weeks. A quick-and-dirty approach is: business_days ÷ 5 = work_weeks. But you’ll need a business-day counter if holidays matter (and they usually do).
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