1. What quantities and budgets are
The mediciones y presupuestos document breaks the works down into elementary units (the partidas, or work items), quantifies each one (the medición, or quantity takeoff) and assigns it a unit price (usually broken down into its components). The product of quantities by prices, aggregated by chapters, builds the PEM (presupuesto de ejecución material, the direct works budget); on top of it operate overheads and industrial profit to form the contract budget, and the full chain up to the operation's total cost.
Three distinct functions coexist in the same document. It is a design instrument: it forces the works to be defined down to item level, and its gaps reveal the design's. It is a procurement instrument: contractors compete on its quantities and prices and the contract is signed on them. And it is a monitoring instrument: its items are the reference against which execution is measured in each certificate and every change is valued.
2. The budget's structure
The Spanish budget is organised in a stable hierarchy: chapters (earthworks, foundations, structure, masonry, services, finishes...), sub-chapters within them when the works require it, and inside them the work items, each with its code, description, unit of measure, quantity and price. The economic levels (PEM, contract, total cost) are built on that base; their exact definitions and usual percentages are developed in PEM and PEC: definition and differences.
Two elements deserve special attention. The item description is contractual: what is not described is not budgeted, and ambiguous descriptions are the raw material of site disputes. And lump-sum items (partidas alzadas, global amounts without quantities) are a legitimate tool for the unmeasurable and a risk when they replace definition: their regime is covered in the dedicated analysis.
The full development of this branch is in the structure of the works budget. It sets out the chapter structure, the lump sums and the measurement rules.
3. The takeoff: the method
Measuring means applying explicit measurement rules to the design drawings: what is measured (gross or net, deducting openings above which size, by unit or by metre), from which document and with which conventions. The rules are not a technical detail: two correct takeoffs of the same works under different rules differ by significant percentages, and a unit price only makes sense together with the rule it will be measured under.
The formal product is the bill of quantities (estado de mediciones) of the execution design, organised in the same chapter-and-item structure as the budget. Its classic pathologies (omissions, duplicates, mixed rules, quantities inherited from previous projects) have recognisable symptoms and detection methods developed in frequent takeoff errors.
The full branch: quantity takeoff in construction. That page covers the measurement rule, the takeoff order and traceability back to the model.
4. Prices: how they are formed
The canonical Spanish unit price is the precio descompuesto (unit price breakdown): the sum of materials (with waste allowances), labour (trade output multiplied by hourly cost), machinery, auxiliary means and the site indirect cost percentage. This transparency is the system's strength: it allows a price to be audited, adjusted to the project's real conditions, and new prices to be built with the same logic.
Price databases (regional and professional reference databases) publish standard breakdowns updated periodically. They are the budget's starting point, never its destination: their outputs and material prices are adapted to the area, the size of the works and its execution conditions. Labour outputs are the breakdown's most sensitive and least verified component; and new rates (precios contradictorios, prices agreed on site for unforeseen units) are built with the same structure, which gives the breakdown convention its second function: being the rulebook for changes.
The full branch: construction pricing, with the step-by-step breakdown example and the overview of price databases. The three pages are best read in that order, from principle to worked example.
5. Economic monitoring of the works
The budget does not end its life when the contract is signed: it becomes the monitoring reference. Every month (or the agreed period), the progress certificate (certificación de obra) measures what has been executed item by item, values it at contract prices in the valued statement and generates the contractor's right to payment. Certification discipline (really measuring, not spreading the budget into twelfths) is the first line of cost control.
On that base, deviation control is organised: comparing certified and committed amounts against the budget, identifying each deviation's origin (quantity, price, scope change) and acting while decisions remain open. Change orders (modificados) are valued at contract prices where they exist and with new rates where they do not, and their documentation during the works determines how hard the final account (liquidación final) will be, the economic close where general measurement, revisions and pending claims are resolved.
The full branch: economic monitoring of the works, with progress certificates as the central piece. It explains committed cost, the projection to completion and the final account.
6. From budget to contract
The same budget can be contracted under different price formulas, and the choice distributes the risks. At unit rates, what is actually executed is paid at the agreed prices: quantity risk stays on the developer's side and the bill of quantities is the living reference of the whole project. At a fixed price, the contractor takes the quantity risk on a design that must genuinely be closed: the formula works exactly to the extent that the design is.
In both cases, tender comparison is done on the common bill of quantities: an item-by-item comparison table, homogenisation of exclusions and variants, detection of anomalous prices (low to win, high on the items the contractor expects to grow). A lower total with unbalanced prices can be the most expensive offer in the tender.
The full branch: from budget to works contract. That page covers the move from technical document to contractual commitment.
7. Summary
Quantities and budgets are a system, not a document: an item structure, explicit measurement rules, auditable price breakdowns, certificates that really measure and a contract whose price formula distributes risks knowingly. Every weak link is paid for at the next one: an ambiguous description becomes a new rate, an inherited quantity a deviation, a complacent certificate a conflictive final account. The good news is the inverse: rigour at the origin pays off throughout the works.
Explore the branches of this guide
| Branch | Content |
|---|---|
| Structure | Levels, chapters, work items and lump sums |
| Takeoff | Rules, bill of quantities, frequent errors |
| Pricing | Breakdowns, databases, outputs, new rates |
| Monitoring | Certificates, deviations, change orders, final account |
| Contract | Price formulas and tender comparison |
Note: this guide describes the usual practices and conventions in Spain in 2026; each contract's specific regime and, in public works, the applicable procurement rules prevail.