How to use the person-months estimator
A person-month, still widely called a man-month in older estimating literature and in vendor bids, is the work one person does in one month. Three quantities lock together: base effort = team size × duration, total effort = base effort + buffer effort, and duration = total effort ÷ team size. Fix any two and the third follows.
The buffer is added on top of base effort, not hidden inside it. It is where requirement changes, defect fixing, vacation and onboarding live: real time that rarely shows up on a Gantt chart. If your duration already carries slack, set the buffer to zero rather than padding twice. Entering a fully loaded cost per person-month multiplies it by total effort to give a labor cost; leaving it blank hides that row.
One caveat worth stating plainly. The arithmetic assumes effort and schedule trade cleanly, but communication paths grow faster than headcount, and Brooks's law observes that adding people to a late project tends to make it later. Read the duration as a floor for a negotiation, not as a commitment you can hand to a client.
Frequently asked questions
Base effort is team size multiplied by duration. Adding the buffer percentage on top gives total effort, and dividing total effort back by team size gives how long the current team needs to absorb it.
Added on top. The buffer percentage is applied to the base effort derived from team size and duration. If your duration estimate already carries slack, set the buffer to 0 so it is not counted twice.
Usually not. Communication paths and onboarding overhead grow as people are added, which is the point behind Brooks's law. This tool assumes a straight proportional trade, so treat the duration as a floor rather than a promise.