Capacity planning is the spreadsheet behind every "yes" and "no" to new work. It takes each billable role available hours, multiplies by the target utilisation, and subtracts committed work to produce a single number: how many hours the agency can sell this month.
For agencies, the value is not the spreadsheet but the conversation it forces. Without capacity planning, sales sells whatever the pipeline brings, delivery scrambles, and the agency oscillates between burnout and bench-time. With it, the new business and resourcing teams trade in the same currency.
Capacity planning is only as good as the inputs. Optimistic utilisation targets and ignored time-off mean the forecast collapses by week three. Build in margin (75% of theoretical, not 95%), revisit every two weeks, and trust the model more than the gut feel of any individual team lead.