The arithmetic everyone skips
A team of ten does not have ten people's worth of time. It has ten people minus holidays, minus public holidays, minus sickness, minus training, minus internal meetings, minus the administrative work that is nobody's project.
Work through it honestly and the answer usually lands somewhere between seventy-five and eighty-five per cent. Ten people become eight. That is not a rounding difference; it is two people, and plans are routinely built as though they exist.
Why it survives being obviously wrong
Everybody knows this. Ask any manager whether their team is fully available every week and they will laugh. Then look at the plan and it assumes exactly that.
It survives because the correct number is nobody's job to produce. HR knows about leave. The delivery system knows about assignments. Nothing joins them, and the person building the plan needs a number today, so they use headcount and adjust by feel. The feel is optimistic, because the alternative is telling somebody the work does not fit.
The failure is predictable, which is the interesting part
A plan built on headcount does not fail randomly. It fails in the specific weeks where leave clusters, which is school holidays and the fortnight before Christmas — the least surprising weeks in the calendar.
So the same conversation happens every year, treated as bad luck each time. It was not luck. The information was available months earlier, in a system nobody joined to the plan.
Publishing a number people recognise
The practical fix is unglamorous: take real working patterns, subtract approved leave and holidays, subtract a measured figure for non-project time, and publish what is left.
The last one is where people get uncomfortable, because measuring internal time means admitting how much there is. Teams that have never measured it usually guess ten per cent and find twenty-five. That gap is the whole argument.
One warning. Once you have an honest capacity figure, do not turn it into a utilisation target. The moment it becomes something people are judged on, it becomes something people manage, and you will be back to a number nobody believes — only now with more spreadsheets behind it.