Project Time Calculator: Estimate Duration, Finish Date and Team Size
Enter the effort, the team and the working hours, and the calculator returns how many working days the project takes and the date it finishes.
Quick answer
Project duration in working days = (effort in hours × (1 + buffer)) ÷ (people × hours per day × availability). For example, 320 hours with 2 people at 8 hours a day takes 20 working days, or 4 weeks. Add a start date and the calculator skips weekends and holidays to give the finish date.
Which calculator do I need?
"How long will it take, and when does it end?" Stay on this page and use the first tab.
"How many people do I need?" Use the second tab on this page.
"I only have rough, uncertain estimates." Use the PERT calculator for expected time and the chance of meeting a deadline.
"My tasks depend on each other." Use the critical path calculator to find the tasks that set the end date.
Every number the calculator shows has a plain meaning. Here is what each output tells you and what to do with it.
Mode A: effort to duration
Working days: how many days of actual work the project needs for the team you described. The exact figure is rounded up, because a project that needs 30.8 days does not finish on day 30.
Calendar weeks: working days divided by the number of working days in your week. It is a quick way to talk about the plan ("about six weeks"), not a date.
Finish date: the last working day of the project, found by counting forward from your start date and skipping non-working days and the holidays you list. If the start date is itself a working day, it counts as day 1.
Calendar days elapsed: the number of calendar days from the start date to the finish date, both included. This is the number a client sees on a calendar.
Team capacity: hours of useful work the whole team delivers per working day. Compare it with your effort to sanity-check the plan.
Planned effort: your effort estimate plus the buffer. This is the figure actually divided by capacity.
Estimated cost: planned effort multiplied by the hourly rate. It covers only the hours on this plan, so add tools, travel and other costs yourself.
Mode B: deadline to team size
People needed: the smallest whole number of people who can deliver the planned effort inside the working days you have. The exact value (for example 2.44) is shown so you can see how close the next person is.
Output per person: hours of useful work one person produces in the window. If it looks too high, your availability or hours-per-day input is optimistic.
Mode C: quick three-point estimate
Expected time (TE): the weighted average of your three estimates. It leans toward the most likely value but still respects the pessimistic tail.
Standard deviation and variance: how uncertain the task is. A wide gap between optimistic and pessimistic means a large spread.
80%, 90% and 95% ranges: the span around TE that should contain the real outcome that share of the time, assuming the usual bell-curve approximation.
Project time formulas
The calculator uses three small formulas. All of them run in your browser, and you can reproduce every result with a pocket calculator.
Mode A: effort ÷ capacity → duration
daily capacity E = people × hours per person per day × (availability % ÷ 100)
The symbol ⌈ ⌉ means "round up to the next whole number". If you enter effort in person-days or person-weeks, it is first converted to hours using your hours per day and days per week.
Mode B: deadline → team size
people needed = ⌈ planned effort ÷ (working days available × hours per day × availability % ÷ 100) ⌉
Mode C: quick three-point estimate
TE = (O + 4M + P) ÷ 6 standard deviation σ = (P − O) ÷ 6 variance = σ²
O is the optimistic estimate, M the most likely and P the pessimistic. The ranges shown are TE ± z × σ with z = 1.282 (80%), 1.645 (90%) and 1.960 (95%).
Symbols used
Symbol
Meaning
effort
Total work in hours, not elapsed time
buffer %
Extra effort added for risk and rework
availability %
Share of each day a person spends on this project after meetings, support and admin
E
Productive hours the whole team delivers per working day
Worked examples
Each example uses numbers you can load into the calculator above; the results match the tool exactly.
Example 1 (simple): a two-person build
A project needs 320 hours of work. Two people work on it, 8 hours a day, fully available, with no buffer. They start on Monday 2 November 2026 and work a five-day week.
Daily capacity E = 2 × 8 × 100% = 16 hours
Duration = 320 ÷ 16 = 20 working days
Calendar weeks = 20 ÷ 5 = 4 weeks
Finish date: counting 20 working days from Monday 2 November gives Friday 27 November 2026 (26 calendar days)
These are the calculator's default inputs, so you see this result when the page loads. Only the start date differs: the tool defaults to today, and the button below sets it to 2 November 2026.
Example 2 (realistic): four people, part-time, with buffer, cost and holidays
A website project is estimated at 600 hours. Four people work 7 productive hours a day, but only 80% available because of other duties. You add a 15% buffer, the blended rate is $60 per hour, and the team is off on 26 and 27 November 2026. Start: Monday 2 November 2026.
Planned effort = 600 × 1.15 = 690 hours
Daily capacity E = 4 × 7 × 0.80 = 22.4 hours
Duration = 690 ÷ 22.4 = 30.80, rounded up to 31 working days
Calendar weeks = 31 ÷ 5 = 6.2 weeks
Cost = 690 × $60 = $41,400
Finish date: skipping weekends and the two holidays, day 31 falls on Wednesday 16 December 2026 (45 calendar days)
Example 3: how many people for a deadline (mode B)
480 hours of work must be done in 30 working days. People work 8 hours a day at 90% availability, and you want a 10% buffer.
Planned effort = 480 × 1.10 = 528 hours
Output per person = 30 × 8 × 0.90 = 216 hours
People = 528 ÷ 216 = 2.44, rounded up to 3 people
A second case: 1,200 hours in 25 working days at 6 hours a day, 75% availability and a 20% buffer needs ⌈1,440 ÷ 112.5⌉ = ⌈12.8⌉ = 13 people. When the answer is that large, the schedule is probably the thing to challenge.
A task could take 4 days at best, 6 days most likely and 14 days at worst.
TE = (4 + 4 × 6 + 14) ÷ 6 = 42 ÷ 6 = 7 days
σ = (14 − 4) ÷ 6 = 1.67 days
90% range = 7 ± 1.645 × 1.67 = 4.26 to 9.74 days
Notice that TE (7 days) is longer than the most likely value (6 days). The long pessimistic tail pulls the expected time up, which is exactly why a single "most likely" number tends to be optimistic.
Get the effort in hours. If you estimated in person-days, multiply by hours per day. If in person-weeks, multiply by hours per day and days per week.
Add the buffer. Multiply the effort by 1 plus the buffer as a decimal. A 15% buffer on 600 hours gives 600 × 1.15 = 690 hours.
Work out daily capacity. Multiply people by productive hours per day by availability as a decimal: 4 × 7 × 0.80 = 22.4 hours.
Divide and round up. 690 ÷ 22.4 = 30.80, so 31 working days.
Convert to weeks. Divide the working days by your working days per week: 31 ÷ 5 = 6.2 weeks.
Count forward on a calendar. Start on the start date, count only working days, skip each holiday, and stop on the last day. That date is the finish date.
Assumptions, limits and common mistakes
Assumptions and limits
Effort is treated as a fixed total that can be split evenly across the team. Real work has dependencies; for those, use the critical path calculator.
The team size and availability are constant for the whole project. If people join or leave, calculate each phase separately.
Your working week is the first N days of the week (5 means Monday to Friday, 6 adds Saturday). Custom patterns such as Sunday to Thursday are not modelled.
Holidays are the dates you type. The tool does not know any country's public holiday calendar.
Dates are plain calendar dates with no time zone, and the cost uses a single blended hourly rate.
Common mistakes
Ignoring availability. Nobody spends eight productive hours a day on one project. Meetings, support and context switching are why 70 to 85% is a more honest figure than 100%.
Assuming twice the people means half the time. Handover, coordination and onboarding mean added people deliver less than a full share, especially late in a project.
Mixing effort and duration. 320 person-hours is not 320 hours of calendar time. Effort divided by capacity gives duration.
Skipping the buffer. A plan with zero risk allowance is a best-case plan. If you are unsure, estimate the uncertainty with the PERT calculator first.
Starting on a weekend. If your start date is a Saturday, day 1 is the next working day.
Frequently asked questions
How do I calculate project completion time?
Divide the planned effort (effort plus buffer) by the team's daily capacity (people × productive hours per day × availability) and round up. The result is the number of working days; count that many working days from the start date to get the completion date.
How do I calculate a project end date from effort?
Work out the number of working days as above, enter a start date, and list any holidays. The calculator counts forward from the start date, skipping weekends and holidays, and returns the date of the last working day.
How many people do I need to finish a project by a deadline?
Use the deadline mode. People needed = planned effort ÷ (working days available × hours per day × availability), rounded up. If the answer is much larger than your real team, shorten the scope or move the deadline rather than just adding people.
What buffer percentage should I use?
There is no universal figure. Many teams add 10 to 20% for work they understand well and more for new or uncertain work. Treat that as a rule of thumb, not a standard. A better method is to estimate each task with optimistic, likely and pessimistic values and let the PERT calculator quantify the risk.
Does adding people always shorten a project?
No. The formula assumes work divides evenly, which is the best case. In practice, extra people add communication and onboarding overhead, and some tasks cannot be split at all. Check the dependencies in the critical path calculator.
What is the difference between effort and duration?
Effort is the amount of work, measured in person-hours. Duration is the elapsed working time. The same 320 hours of effort lasts 40 days for one person and 20 days for two people working in parallel, assuming the work can be shared.
How do I convert hours into working days?
Divide the hours by the productive hours in a working day. For a quick conversion without the rest of the project inputs, use the hours to working days calculator.
Is my data stored or sent anywhere?
No. All calculation happens in your browser. Nothing you type is sent to a server or saved. Copying the link only encodes your inputs in the URL so you can share the same result.