The principle: no quantity without a rule
Every quantity depends on a prior convention: what is measured and how. A partition can be measured gross or deducting openings above a certain size; an excavation, at theoretical profile or with slopes; painting, by visible surface or by developed surface. Two correct takeoffs under different rules differ by significant percentages, and a unit price is only interpretable together with its rule: the contractor who knows openings are not deducted tenders a different price from the one who assumes they are.
The practical consequence is a single rule: measurement rules are declared in writing, item by item or by reference to a known source, before anyone puts prices in. The full development (rule families, available sources, how to agree them) is in measurement rules.
The document: the bill of quantities
The bill of quantities (estado de mediciones) of the execution design organises the quantities in the same chapter-and-item structure as the budget, with detailed measurement lines (length, width, height, units, subtotals) that allow every quantity to be checked against the drawings. That detail is not bureaucracy: it is what makes the takeoff auditable and what allows, on site, execution to be measured under the same rules under which the design was measured.
Its role in procurement is central: it is the common document on which offers are compared and, in unit-rate contracts, the living reference of the whole project. Content, structure and uses are developed in the design bill of quantities.
Quality control
Takeoff pathologies are recurrent and detectable: omissions (whole items or areas of the building), duplicates (the same work in two chapters), mixed rules within the same document, quantities inherited from a previous project that nobody has re-measured, and mismatches between quantities and drawings after a design revision. Each has recognisable symptoms (ratios per m² out of range, chapters with anomalous weights, suspiciously round quantities) and verification methods proportionate to the risk. The complete repertoire is in frequent takeoff errors.
| Element | What it fixes | Risk if missing |
|---|---|---|
| Measurement rule | What is measured and what is deducted in each item | Two legitimate and different measurements of the same work |
| Bill of quantities | Takeoff ordered by chapters and items | Quantities that cannot be reconstructed when comparing offers |
| Traceability to source | Link between each quantity and the drawing or model | No way to update after a design change |
| Quality control | Cross checking quantities and spotting omissions | Missing items that return later as new rates |
The three analyses in this branch
Measurement rules: conventions and sources. What is measured, how deductions work and why the rule is part of the price.
The design bill of quantities. The document: content, measurement lines, role in procurement.
Frequent takeoff errors and how to avoid them. The classic pathologies, their symptoms and the checks that detect them.
The essentials
The takeoff is a chain of three links: declared rules, a detailed and auditable document, and systematic quality control before contracting. Each link protects the document's next use: clear rules make offers comparable, detailed measurement lines make rigorous certification possible, and prior control prevents errors from becoming deviations once the contract is signed. The economic counterpart of these quantities, the prices, is developed in the construction pricing branch.
Note: usual conventions and practices in Spain in 2026; each contract's regime prevails.