How to Convert Days to Hours for Project Tracking
A project manager says a task will take "3 days." Does that mean 72 hours, or 24 hours of actual work? A contractor submits an invoice for "5 days of work." Is that 40 hours or 35, depending on whether they work 7 or 8-hour days?
The conversion from days to hours seems like basic arithmetic, but in project tracking it carries ambiguity that causes real disputes. This article covers the math, the different interpretations, and a full reference table covering both calendar hours and working hours.
Use the Time Converter for any time conversion. The reference tables below cover the amounts that come up most often in scheduling and billing contexts.
The Simple Conversion: Calendar Days to Hours
One calendar day = 24 hours. Exactly.
This is the only unambiguous version of the conversion. A calendar day runs from midnight to midnight. If something takes 3 calendar days, it takes 72 hours. If a container is shipped with a 14-day lead time, that's 336 hours from dispatch to arrival.
| Calendar Days | Hours |
|---|---|
| 0.5 day | 12 h |
| 1 day | 24 h |
| 2 days | 48 h |
| 3 days | 72 h |
| 4 days | 96 h |
| 5 days | 120 h |
| 7 days (1 week) | 168 h |
| 10 days | 240 h |
| 14 days (2 weeks) | 336 h |
| 21 days (3 weeks) | 504 h |
| 30 days | 720 h |
| 31 days | 744 h |
| 60 days | 1,440 h |
| 90 days | 2,160 h |
| 365 days | 8,760 h |
| 366 days (leap year) | 8,784 h |
Calendar days matter in legal and contractual contexts. A 30-day notice period means 720 hours from the moment notice is given, regardless of weekends or holidays.
The Working Days Conversion: Where It Gets Complicated
For project work, "days" almost always means working days — and working days in hours depends on your standard working day length.
Standard working day assumptions:
- 8-hour day: the most common in office environments (9–5 or similar)
- 7.5-hour day: common in some industries; 37.5-hour week
- 7-hour day: used in some countries and sectors
- 10-hour day: common in construction, field work, and 4-day week arrangements
| Working Days | Hours (8h/day) | Hours (7.5h/day) | Hours (7h/day) |
|---|---|---|---|
| 0.5 day | 4 h | 3.75 h | 3.5 h |
| 1 day | 8 h | 7.5 h | 7 h |
| 2 days | 16 h | 15 h | 14 h |
| 3 days | 24 h | 22.5 h | 21 h |
| 4 days | 32 h | 30 h | 28 h |
| 5 days (1 week) | 40 h | 37.5 h | 35 h |
| 6 days | 48 h | 45 h | 42 h |
| 7 days | 56 h | 52.5 h | 49 h |
| 8 days | 64 h | 60 h | 56 h |
| 9 days | 72 h | 67.5 h | 63 h |
| 10 days (2 weeks) | 80 h | 75 h | 70 h |
| 15 days (3 weeks) | 120 h | 112.5 h | 105 h |
| 20 days (4 weeks) | 160 h | 150 h | 140 h |
| 22 days (~1 month) | 176 h | 165 h | 154 h |
The 22-working-day month is commonly used in payroll — it's the average number of working days in a month when weekends are excluded (approximately 261 working days per year ÷ 12 = 21.75, rounded to 22).
Why This Matters in Project Tracking
Estimating Task Duration
When a developer says "that'll take about 3 days," they mean 3 working days — roughly 24 hours of work. If that work is spread across a week due to meetings and context switching, it might appear on the calendar as 5 days, but only 24 hours of effort was applied.
Project management tools handle this differently:
- Effort-based tools (Jira, Linear): tasks are estimated in hours of work; calendar time depends on allocation
- Duration-based tools (MS Project, older tools): tasks are estimated in calendar days; hours of work depend on the resource calendar
When someone estimates "3 days" in a duration-based tool, it means 3 calendar working days with 8 hours available each — 24 hours of potential effort, assuming full allocation. If the person is allocated 50%, the task takes 6 calendar days to deliver 24 hours of work.
Billing and Contractor Invoicing
Contractors who bill by the day need to agree on day length before starting work. In professional services, a "day rate" typically assumes 7–8 hours of billable work. A consultant billing 3 days at an 8-hour day rate should invoice for 24 hours.
If the contract says "day rate" without specifying hours, disputes arise when actual hours worked differ from assumed hours. A contractor who worked 10 hours in a day may expect a 10/8 day rate (1.25 days billed). The client may expect exactly 1 day regardless. Specifying the hourly equivalent in the contract prevents this.
Common professional services day lengths by industry:
- Legal and consulting: typically 8 hours billable per day
- Creative agencies: often 7.5 hours
- IT contracting: usually 8 hours, sometimes "up to 8 hours" with overtime starting above
- Construction and trades: often 8–10 hours depending on contract
Sprint and Iteration Planning
In agile project management, sprint capacity is calculated in working hours, not calendar days. A 2-week sprint has 10 working days, but actual capacity depends on team size, daily hours, and time allocated to meetings.
A team of 4 people, each working 8-hour days with 1 hour per day in meetings, has:
- 10 days × 7 productive hours × 4 people = 280 person-hours per sprint
Expressing this as "280 hours" is clearer for capacity planning than "10 days × 4 people," because it acknowledges that not all hours in a working day are available for sprint work.
Service Level Agreements (SLAs)
SLAs often specify response and resolution times in hours or business days. "8-business-hour response time" and "1 business day" sound similar but mean different things:
- "8 business hours" from a request at 2 PM Tuesday (in an 8-hour 9–5 workday): response by 10 AM Wednesday
- "1 business day" from 2 PM Tuesday: typically by end of Wednesday, which is 4 PM or 5 PM — 1.5–2 hours later
When a request comes in at 4:30 PM on a Friday with an "8 business hour" SLA, the clock stops at 5 PM (0.5 hours used) and resumes Monday at 9 AM, with 7.5 hours remaining — meaning the response is due by 4:30 PM Monday.
Converting Between Calendar Days and Working Days
Moving between calendar days and working days requires knowing how many weekends (and holidays) fall in the period.
Quick rules of thumb:
- 5 working days ≈ 7 calendar days (1 calendar week)
- 10 working days ≈ 14 calendar days (2 calendar weeks)
- 22 working days ≈ 31 calendar days (approximately 1 calendar month)
More precisely: in any 7-day week, 5 days are working days (excluding Saturday and Sunday). So working days = calendar days × (5/7) ≈ calendar days × 0.714. Calendar days = working days × (7/5) = working days × 1.4.
A project that takes 15 working days occupies approximately 15 × 1.4 = 21 calendar days — 3 calendar weeks. This doesn't account for public holidays, which add further calendar time without adding working hours.
A Practical Decision: Which Conversion to Use When
Use calendar days (× 24) when:
- The time period includes around-the-clock processes (shipping, curing, fermentation, legal deadlines)
- Contracts specify calendar days explicitly
- You're calculating SLA windows for 24/7 support
Use working days (× 8 or agreed rate) when:
- Estimating effort for tasks done by people with standard working hours
- Billing clients for work performed
- Planning sprints, milestones, and project timelines
- Converting between hours logged and days on a timesheet
When the context is ambiguous, specify. "3 days (24 working hours)" or "3 calendar days (72 hours)" eliminates guessing. In project management particularly, the most expensive word is "days" used without qualification.
The Time Converter handles the pure arithmetic — calendar days to hours and back. The interpretation layer (which kind of "day") is always a human decision that the math can't make for you.
