What Is a Person-Day? The Definition and How to Convert It to Person-Months and Effort
Shusaku Yosa
A quotation says five person-days, but how many hours of work that actually means differs from company to company. And if you read it as "five person-days means five people can finish it in a day," you are almost certain to be wrong. This article covers what one person-day means, how to convert it, and the limits built into the unit itself.
What a Person-Day Is: The Work One Person Does in a Day
A person-day is a unit expressing the amount of work one person completes in one day. Five person-days of work means the volume that would take one person five days to get through.
What matters is that this is a unit of quantity, not of duration. Five person-days of work will not necessarily finish in five days, and it can equally finish in three. How many days it takes depends on how many people work on it and how they proceed.
The difference from effort and duration
These get conflated, so it is worth setting them out.
- Effort: the total amount of human work a task requires. Person-days and person-months are units for expressing it.
- Duration: the length on the calendar from start to finish. It includes waiting time.
"Five person-days of work, finished within two weeks including the client's review time" is what it looks like to hold effort and duration apart. Mix the two and you end up with an estimate that was correct while the delivery date still slips.
One Person-Day Is Not Always Eight Hours
The first thing to check when working with person-days. One person-day is generally taken as eight hours, but that is a convention rather than a rule.
Contractual working hours differ by company. Some work eight hours, others seven hours forty-five minutes, others seven. Between a company that treats a person-day as eight hours and one that treats it as seven, the same "ten person-days" differs by ten hours of actual work.
When dealing with an outside vendor, ask at the quotation stage how many hours they convert a person-day into. It sounds like a detail, but the gap compounds on larger engagements.
Converting to person-hours
There is a finer unit, the person-hour: the amount of work one person does in one hour.
1 person-day = 8 person-hours (taking a day as eight hours)
Listing tasks that take half a day produces decimals like 0.5, which gets hard to read. Where an engagement has many short tasks, accumulating in person-hours and converting to person-days at the end is easier to handle.
Converting to Person-Months: Why 20 Days
Quotations often use person-months, so being able to move between the two is useful.
1 person-month = roughly 20 person-days
The figure of 20 comes from the number of working days in a month. Excluding weekends leaves 20 to 23 days depending on the month, and once public holidays are taken out the average lands at around 20.
So one person-month means one person spending every working day of a month on that work. As covered below, this premise often does not match reality, which is why conversion needs care.
Conversion table
- 0.5 person-days = half a day (4 hours)
- 1 person-day = 8 hours
- 5 person-days = one week of working days
- 20 person-days = 1 person-month
- 240 person-days = 12 person-months (1 person-year)
When using person-months, confirm the other side is also converting at 20 days. Some companies calculate at 21, or at actual working days.
The Biggest Trap: Work Is Not Always Divisible
Because person-days multiply, it is tempting to think that ten person-days means ten people can do it in a day. In almost every case, that does not hold.
Some work cannot be divided
Design work and decisions on direction do not go faster with more people. If anything, they slow down as the effort of aligning opinions increases.
What divides easily is work where the same task repeats many times: producing pages in volume, entering data, running tests. Conversely, work that has to hold a single coherent thread through it cannot be shortened by adding people.
Adding people adds work
More people means new work appears: handover, training, aligning how things are done, reconciling differences between outputs. None of it was in the original estimate.
Adding people partway through a running project is the sharper case: until the new members become productive they consume the existing members' time, so in the short term things actually slow down. That is why adding headcount close to a deadline does not work.
A practical guide
Treating two to three times as the ceiling for what adding people can compress is the safe assumption. For ten person-days of work, two people over five days or three over four days is the realistic range; ten people in one day will not happen.
Turning Person-Days into a Schedule
Person-days are a quantity, so they cannot go straight onto a calendar. Convert in this order.
- Identify the tasks and assign person-days: break each down to roughly 0.5 to 3 person-days.
- Decide the team: how many people will work on it. Handle indivisible tasks separately here.
- Apply a utilization rate: accounting for other projects and meetings, 70 to 80 percent is realistic.
- Add waiting time: waiting on reviews and approvals is not effort, but it does land on the schedule.
Take 20 person-days of work handled by one person as an example. Converting at 75 percent utilization gives about 27 working days, or a little over five weeks. Add three rounds of waiting on review and it stretches by another week or so.
Set the deadline by treating 20 person-days as 20 working days and you have locked in nearly a month of slippage on day one.
What to Check on a Quotation
When you receive a quotation written in person-days, check these three points.
- Hours per person-day: eight, or something else?
- Team and utilization: how many people, and at what level of availability, are assumed?
- Your own side's work: the effort your team spends on reviews and supplying assets is not included.
The third gets overlooked. Watch only the vendor's person-days and the time your side spends reviewing and writing copy never enters the plan, and your own team becomes the bottleneck.
Frequently Asked Questions
Person-days or person-months?
Split by scale. Engagements finishing in a few weeks suit person-days; those running several months or longer suit person-months. Person-months are coarse, though, so asking for the breakdown in person-days makes the reasoning visible.
How do you count a less experienced member?
A person-day is a unit of work volume, so it does not inherently carry skill differences. In practice, applying a coefficient when a less experienced member owns the task is one approach: "normally five person-days, but 1.5 times at 7.5 with this team."
Our estimates always fall short
You are probably counting only the main work. Requesting reviews, making corrections, explaining things. None of these produce a deliverable, so they drop out of estimates easily, and together they can amount to 20 to 30 percent of the main work.
What about a fixed price instead of person-days?
Where the scope is clear, a fixed price makes budgeting easier. But fix the price while the scope stays vague and the vendor either loads in risk and quotes high, or the conversation turns to extra costs partway through. Either way, settling the scope comes first.
Managing Person-Days in Practice
An estimate in person-days only means something once it sits beside the person-days actually spent. Produce estimates without recording actuals and next time's accuracy will not improve.
When the estimate lives in a schedule, the actuals in timesheets, and the budget in finance's files, though, every reconciliation means re-keying. Xtrategy manages campaign schedules alongside budget and KPIs on a single screen.
Summary
- A person-day is the work one person does in a day. A unit of quantity, not duration.
- One person-day is not always eight hours. Ask vendors what they convert at.
- 1 person-month is roughly 20 person-days, based on working days in a month.
- Person-days multiply, but real work is not always divisible. Two to three times is the compression ceiling.
- To reach a schedule, apply 70 to 80 percent utilization and add waiting time.
- On a quotation, check the hourly conversion, the team and utilization, and your own side's work.
Person-days are a convenient unit, but the ease of multiplying them is itself the trap. Start by confirming how many people will do the work as routinely as you confirm how many person-days it takes.