Quantity take-off

A quantity take-off, called metraj in Turkey, is the item-by-item calculation of the quantities that make up a building, taken from the drawings. Items such as excavation volume, concrete and reinforcement, wall, plaster, paint and finish areas and the number of windows and doors are listed in cubic metres, square metres, metres, tonnes or units. The result underpins cost estimates, tender documents, purchasing plans and progress payments.

How it is done

A take-off is prepared from the construction drawings and details, following a set order and set rules. For each item, it is stated clearly where the dimensions come from; deductions for openings, overlaps and waste are all defined in the calculation. On public projects in Turkey, items are matched to the unit price definitions of the relevant institutions, known as poz. The calculation must be traceable, since later checks and disputes rely on it; someone else should be able to reach the same result from the same drawings.

Take-offs from a digital model

With BIM, much of the quantity work can be extracted from the model automatically. That only works if the model is built for it: walls with the right layers, slabs at their true thickness, objects in the right categories. Models prepared for visualization often contain only visible surfaces and are not enough for a take-off on their own. Separate models, or clearly separated layers, for the two purposes are the safer approach.