Long term data centre or network room capacity planning may seem impossible in the face of evolving IT technology and business requirements. Nevertheless, data centre facilities have a lifetime that may span many generations of IT equipment, so planning – or lack of planning – can have a large impact on the effectiveness of investments. Many unnecessary costs can be avoided with simple planning strategies, and even uncertainty itself can be incorporated into a plan. This paper shows a simple and effective way to develop a capacity plan for a data centre or network room.