How to prepare a computo metrico estimativo step by step

Opening the bill before these are in place almost always produces a document that has to be redone. The time saved at the start is lost several times over at the end.

The first decision concerns the layout of the document rather than the quantities. A bill is structured by work categories following the logical sequence of construction, from excavation through to finishes, and within each category by building element or by floor where the project calls for it.

This structure is not cosmetic. It determines the readability of the summaries, the ability to apportion work stage percentages under a lump sum contract, and the ease with which the direttore dei lavori, the works supervisor, will locate items during valuation. Changing it halfway through costs far more than deciding it at the outset.

It is also worth fixing at this stage the item numbering convention and the decision to keep the prezzario code alongside each item, a choice that makes the estimate traceable and verifiable by third parties. That choice is what makes the estimate updatable rather than rebuildable.

Every quantity must be derived from the drawings through an explicit calculation, not entered as a single aggregate figure. The difference is substantive: an explicit calculation is verifiable, can be updated when the design changes and is defensible at verification stage.

Established practice writes dimensions out line by line, stating sizes and the number of elements, and lets the total be the result of the operation. An example for a wall. The total then remains reconstructible from the raw data.

Deductions must be set out line by line rather than absorbed silently into the total, because they are where challenges concentrate. The rules determining when a deduction applies are not universal: they depend on the measurement criteria of the prezzario adopted, which should be consulted before measuring.

With quantities settled, rates follow. Each item must be located in the prezzario by reading its full description, not just its heading. An item whose heading matches the operation but whose description includes or excludes work other than that intended produces a divergence that no check on the total will reveal.

Three checks are worth making on every item: that the unit of measurement in the prezzario matches the one used in the take off, that the territorial scope of the prezzario corresponds to the location of the works, and that any work excluded from the description is measured elsewhere. They cost seconds and prevent the commonest divergences.

No prezzario covers the entire field of possible operations. Where an item is missing, or where an existing item does not match the real conditions, the rate is determined by analysis. The method is codified: elementary prices are applied to the quantities of materials, labour, plant hire and transport needed to produce one unit of the operation, then the percentages prescribed for overheads and for the contractor's profit are added.

The analysis must be documented and attached, not only because the law requires it in public works, but because it is the only way to make the rate contestable on technical grounds rather than on impressions. It is the only way to make the rate arguable on technical grounds.

The oneri della sicurezza derive from the health and safety plan and must be kept separate from the measured works. In public works they are not subject to discount, and merging them with ordinary items deprives them of the protection the law affords. The separation must be built into the structure at stage 1, not extracted after the fact.

Three deliverables derive from the bill and should be produced together rather than days apart: the summary by work category, the labour incidence schedule and the works amount to be carried into the quadro economico, the project cost framework. If the three do not reconcile, the error is in the bill and should be traced immediately.

The final step is a cross check that runs back up the documentary chain. It verifies that every quantity traces back to a drawing.

The plausibility check deserves a qualification. It does not validate the bill, which remains right or wrong depending on quantities and rates, but it intercepts gross errors: a category weighing twice what it normally weighs signals a unit error or a double count far more often than it signals a design peculiarity.

The route from drawing to bill line closes on the amount. Taking the wall measured at stage 2, with a total of 68.28 square metres, the valuation reads as follows.

Three observations on this line, which hold for any other. The quantity is not rounded relative to the stage 2 calculation, because rounding is an operation to perform once and at the end. The extra for reveals appears as a standalone item because the openings were deducted in this example, and its length derives from the perimeter of the two openings, namely twice 1.40 plus 1.30 and therefore 5.40 metres each: had the prezzario prescribed measurement without deductions, that extra would already be included and repeating it would be a double count. The unit values are illustrative and should not be taken as market references.

Where the item does not exist in the prezzario, the same line is built from the bottom up. The procedure is that of price analysis.

The two final percentages are not at the preparer's discretion in public works: their level is set by law, and the procedure is covered in the dedicated page on price analysis. Their level is set by law, and the procedure is covered separately.

A few layout choices make a difference to how long the work takes and how maintainable it is. They must be made when the document is opened, not halfway through.

It is worth keeping one column for the prezzario code and one for the edition used, because months later traceability of the source is worth more than speed of drafting. It is worth keeping dimension columns separate from the subtotal, so that the calculation stays visible and editable. It is worth numbering items with a structure reflecting the work categories, so that inserting a forgotten item does not force the whole document to be renumbered.

What should be avoided is the practice of grouping several operations into one line to shorten the document. The bill is not a summary deliverable: summaries serve that purpose, and are produced downstream.

A figure typed in place of an explicit calculation is the first. It makes the bill impossible to update or verify, and at the first design change it forces the take off to be redone.

An item applied without reading its notes is the second, and produces silent divergences that surface only during valuation. The divergences stay silent until valuation.

A bill prepared before the design is stable is the third, and the most expensive: every change to the drawings feeds through to the quantities, and the document progressively diverges from the design it is meant to describe. Every change to the drawings feeds through to the quantities.

Copying a previous bill and adapting it to the new project is the fourth. It saves time in the first few hours and costs far more at verification, because it carries over items nobody rereads.