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- In BIMpedia
A quantity is a promise about money. The estimator multiplies it by a rate, the contractor is paid by it, and the client budgets on it. It comes from three places at once: the drawings (how big), the specification (what material, and how it's measured) and, more and more, the model.
| Term | Meaning |
|---|---|
| Quantity | How much of something: m³ of concrete, tonnes of steel, m² of finish |
| Measurement rule | Says what is counted and what is deducted (a standard method, or the specification's measurement clause) |
| Pay item / bill item | The line a quantity is priced and paid under |
Follow one requirement through
Take concrete. The specification gives each class (say C32/40 for car park slabs, C28/35 elsewhere) and how concrete is measured. The drawings give the sizes. The model can then produce volumes, but only if:
- Each element carries its class as a property, so the take-off can group by it.
- The geometry follows the measurement rule: for example, no deduction for reinforcement, or deductions only for openings above a stated size.
- Items the model doesn't hold are added from schedules and rules: reinforcement from bar schedules, formwork from surface areas, finishes from the room data.
What the model gives and what it doesn't
| From the model, if properties are right | Usually from elsewhere |
|---|---|
| Concrete volume by element, class and level | Reinforcement mass (bar schedules) |
| Areas of walls, slabs and finishes | Formwork (by rule, from surfaces) |
| Counts of doors, units, fittings | Temporary works, plant, preliminaries |
| Lengths of ducts, pipes, cable trays | Allowances for waste and laps |
The take-off from the model sums every slab as "Concrete: 412 m³". The specification has a more durable class for the car park, priced 15% higher. The estimator applies one rate, and the shortfall appears as a claim when the supplier delivers the stronger mix.
The coordinator checks that every slab carries its class and exposure class, and runs the take-off grouped by class: 268 m³ C28/35 and 144 m³ C32/40. Each line goes to its own bill item, and the tender prices both.
A small structure has:
- 2 slabs, each 30 m × 20 m × 0.25 m, specified C32/40, exposure XD3;
- 24 columns, each 0.40 m × 0.40 m × 3.50 m, specified C40/50.
- Give the concrete quantities by class, in m³.
- Convert the total to cubic yards.
- Which property must every element carry for the model to produce these two lines?
Key takeaways
- Every specified class or grade that is priced differently needs its own quantity line.
- Measurement rules (the specification's measurement clause, or a standard method) decide what is counted and how.
- Model quantities are geometry; they become priced quantities only once grouped by the right properties and adjusted to the rules.
- Reinforcement, formwork and finishes are often measured from schedules and rules, not from the model.