Types of estimates

An estimate predicts what a piece of work will cost before it is built. Estimates get more detailed as design develops, and each stage uses a different method.

A preliminary or approximate estimate is made at the idea stage, using rates per square metre of plinth area, per cubic metre of building volume, per bed for a hospital, or per kilometre of road. It supports budgeting and approval in principle.

A detailed estimate follows the drawings. Each item, such as excavation, concrete, steel, brickwork, plaster and so on, is measured in the units of the measurement standard (IS 1200 in India), entered in a bill of quantities and multiplied by an analysed rate. Contingencies and work-charged establishment are added. Revised and supplementary estimates handle changes later.

Measurement is the backbone. Long wall and short wall, or centre-line methods, are used for walls and foundations; earthwork uses cross sections; steel comes from the bar bending schedule. Rates come from schedules of rates or a rate analysis of labour, material and plant.