Project deadlines, delivery estimates and payroll periods are almost always quoted in working days rather than calendar days, since weekends (and often public holidays) don't count toward the timeline. The problem is that counting working days by hand is more error-prone than it looks, because the number of weekends in a range depends on which day of the week the range starts and ends on, not just its total length.
Why you can't just divide total days by 7 and multiply by 5
A common shortcut is to take the total number of days, divide by 7, and multiply by 5 to estimate working days. This gives a reasonable estimate over long ranges but is frequently wrong for shorter ones, because it assumes an even distribution of weekdays and weekend days that only holds true for ranges that are exact multiples of 7 days. A 10-day range starting on a Monday contains 8 working days; the same 10-day range starting on a Saturday contains only 6, the estimate formula can't tell the difference.
The accurate method counts day by day (or uses a closed-form formula based on the day-of-week of both the start and end date), correctly handling partial weeks at either end of the range. This is precisely the calculation the Working Days Calculator runs, so you get an exact count rather than an estimate that can be off by a day or two.
Public holidays make manual counting even harder
Once you factor in public holidays, which vary by country, region, and even by company, the calculation compounds further: a working-day count now has to skip both weekends and any holiday dates that fall on what would otherwise be a working day (a holiday that falls on a Saturday typically doesn't reduce the count further, since that day was already excluded as a weekend). For accurate project planning, it's worth cross-checking your range against the raw calendar span first using the Date Difference Calculator, then applying the working-day and holiday adjustments on top.
Where this matters in practice
Working-day counts show up constantly in contracts ("delivery within 10 business days"), payroll (calculating per-day pay for a partial month), and SLAs (support response times measured in business days, not calendar days). Getting the count wrong by even a day or two in a contractual context can matter, so it's worth using a dedicated calculator rather than eyeballing a calendar, especially for ranges that span a public holiday or cross a month boundary.