Person-Day & Person-Month Calculator
Convert between units of work effort (person-hours, person-days and person-months). You can also find the effort from a number of people and a duration.
What person-hours, person-days and person-months are
These are units of work effort, used in software development, design and other project work. The same units are also written "man-hours", "man-days" and "man-months".
- Person-hour: the work one person does in one hour
- Person-day: the work one person does in one day
- Person-month: the work one person does in one month
- Person-year: the work one person does in one year (1 person-year = 12 person-months)
How the conversion works
- Person-days = person-hours ÷ hours per person-day
- Person-months = person-days ÷ person-days per person-month
- Person-days = people × days of work (when the duration is given in months: people × months × person-days per person-month)
The assumptions (how a person-day and a person-month are counted)
The starting values are 1 person-day = 8 hours and 1 person-month = 20 person-days (160 hours). How these are counted differs between companies, industries and contracts (for example, 1 person-day = 7.5 hours and 1 person-month = 21 person-days). No law or standard fixes them. In a quote or a contract, check the basis that was agreed, and change the two fields above to match it.
Examples (1 person-day = 8 hours, 1 person-month = 20 person-days)
- 80 person-hours = 10 person-days = 0.5 person-months
- 3 people for 2 months = 6 person-months (120 person-days, 960 person-hours)
- 5 people for 12 days = 60 person-days = 3 person-months
- With 1 person-day = 7.5 hours and 1 person-month = 21 person-days, 1 person-month = 157.5 person-hours
More people do not make the schedule shorter in proportion
A person-month measures the amount of work. For example, 6 person-months of work is the same amount of work whether it is "1 person for 6 months" or "3 people for 2 months". But adding people does not necessarily shorten the time to finish in proportion to the headcount.
- Some work cannot be split up and has to be done in order.
- More people means more meetings, handovers and coordination.
- Someone who joins needs time to get up to speed.
Use person-months as a guide for estimating, and decide the mix of people and duration from how the work will actually proceed.
Things to know
- Efforts, number of people and durations can be 0 or more, with decimals (up to 6 decimal places). A number of people such as 0.5 is accepted.
- Hours per person-day must be greater than 0 and at most 24, and person-days per person-month greater than 0 and at most 31.
- To convert to an hourly or monthly wage, use the Hourly, Daily & Monthly Pay Converter. To count days between dates, use the Date Difference Calculator. To turn total hours into full-time equivalents, use the FTE Calculator.
Frequently asked questions
How many person-days is 1 person-month?
The starting value here is 20 person-days (8 hours × 20 days = 160 hours). It differs between companies and contracts (for example 21 person-days), so change the field to match your actual agreement.
How many person-months is 3 people for 2 months?
6 person-months (3 × 2). Use "Effort from headcount and duration".
If I double the number of people, will the duration be halved?
The amount of work is the same, but the duration does not necessarily halve, because some work cannot be split and people have to coordinate.
How do I convert person-hours to person-days?
Divide by the hours per person-day (8 by default). 80 person-hours is 80 ÷ 8 = 10 person-days.