The families of unit
Four families, with distinct implications. They cover length, area, volume and number.
The unit of number, counting identifiable elements. It is the safest, leaving little room for interpretation.
The unit of length, used for linear works.
The unit of area, the most widespread and the most prone to divergence, because of deductions.
The unit of volume, used for bulk works.
The lump sum is a case apart, covered in the article on measured and lump sum, since it measures nothing but values a whole.
| Family of unit | Example of use | Point to watch |
|---|---|---|
| Length | Services, skirtings, kerbs | How corners are measured |
| Area | Finishes, renders, roofs | Rules for deducting openings |
| Volume | Earthworks, concrete | Bulking and compaction |
| Number | Equipment, fittings | Definition of the unit counted |
The question of deductions
This is where the gaps open up.
Two surveyors measuring the same wall may arrive at different areas, not through error but because they do not deduct the same voids.
The cases involved are always the same: openings, recesses, returns, intersections and crossings of works.
A rule must decide, and it must be the same for every contractor invited.
That rule may come from the applicable technical clauses, or be stated in the particular clauses where the reference document does not provide for it.
What is unacceptable is that it should be implicit, since each contractor will then apply its own.
Why the rule outranks the unit
The central point of this article.
The unit names the quantity; the rule names the method.
Two schedules expressed in the same unit may carry different quantities for the same works, if the measurement rules differ.
A unit price therefore has meaning only against a rule, exactly as a price per square metre has meaning only against a reference area, a principle recalled in the guide on construction costs.
Practical consequence: state the applicable rule in the schedule, or refer to it expressly.
The cases that measure badly
Four situations where the unit chosen often misrepresents the work. They produce variances at the final account.
Small-scale works, where set-up time outweighs material cost, and where a unit of area distorts the price.
Works at height or in constrained areas, where the same quantity does not represent the same labour.
Repairs and junctions with existing fabric, whose quantity is uncertain by nature.
Dispersed works, spread across the site, where movement weighs more than execution.
In these four cases, a measured item produces erratic prices. An item by number, a lump sum, or an item with an express allowance give more reliable results.
What must be written
Three elements, systematically.
The unit adopted.
The applicable measurement rule, by reference or by statement.
The treatment of particular allowances, where the work involves any.
What this means for a professional
Four rules.
Never leave the measurement rule implicit, the unit alone being insufficient.
Check that the rule is the same for every contractor invited.
Choose the unit according to what drives cost, not by habit.
Move from measurement to number or lump sum where quantity does not represent the work.
This article sets out a method of professional orientation. It reproduces no content from normative documents.