Quantity measurement on site: method, criteria and the errors that cost most
Cubicar means measuring and quantifying the items of a job in order to value them. It is done at two different moments for two different purposes: when pricing, to determine what it will cost; and during execution, to determine what was built and what gets claimed. The same word, two exercises that are almost never done to the same criteria, and that is where the trouble starts.
The two moments
| Pricing measurement | Progress measurement | |
|---|---|---|
| When | Before building | During execution |
| Source | Drawings and specifications | What was actually built |
| What for | To determine the price | To determine the claim |
| Typical error | Omitting items | Measuring to different criteria than the price |
The second row is what generates most of the arguments. If you priced the render deducting openings and on site you measure gross area, your numbers are not comparable and somebody is going to lose money.
The measurement criterion is settled beforehand, not afterwards
This is the most important part of the article.
Every item needs an explicit measurement criterion, and that criterion has to be agreed before the first measurement. Concrete questions that need answering:
- Openings: are they deducted? Above what area?
- Laps and junctions: measured once or twice?
- Thicknesses: nominal or actual?
- Excavation: in bank or loose? With bulking?
- Rebar: theoretical or actual weight? Are laps counted?
- Finishes: developed or projected area?
If the contract’s technical specifications do not define it — and often they do not, in enough detail — agree it in writing with the ITO before you start and record it in the libro de obras. Half an hour of conversation at the start saves weeks of argument later.
Without a prior agreement, every measurement becomes a negotiation, and the contractor’s position is always weaker once the work has been executed.
How to measure well
Four practical rules.
By area, always. Never measure an item as a global total. Measure it by area, grid line, level or identifiable stretch. That is what makes verification possible, what avoids duplication between periods and what makes a cumulative record feasible.
With a sketch, even a hand-drawn one. A diagram with the dimensions noted and the arithmetic visible. It is what turns a number into something checkable, and it is the first thing the ITO asks for.
The same day the work is executed. A measurement taken three weeks later is a reconstruction. And whatever got covered up can no longer be measured.
With the ITO’s acceptance at the time. A measurement signed the day it was taken is worth far more than the same measurement presented at month end.
The cumulative record, which is where control is lost
On jobs of any duration, the problem is not measuring one period well. It is not losing the thread between periods.
The tool is a live sheet where each area appears exactly once, with:
- Total quantity per the design
- Cumulative quantity executed
- Cumulative quantity claimed
- Balance
With that, two things become impossible: claiming the same area twice, and losing sight of a balance left unclaimed. The second is more common than anyone admits: jobs that finish without claiming quantities they did execute, because nobody kept the cumulative record.
The six most expensive errors
Measuring to different criteria than the price. Said already and worth repeating: it is the error that moves the most money.
Measuring as a global total with no area. It prevents verification, allows duplication and makes a cumulative record impossible.
Not measuring what is about to be covered up, before covering it. Rebar, services, waterproofing. Afterwards there is no way.
Confusing physical progress with progress on the item. A wall built but not rendered is not a finished wall. If the item is “wall, finished”, partial progress needs an agreed criterion for how it is weighted.
Measuring extra work inside a contract item. It ends in an observation and contaminates the credibility of the whole payment application.
Not documenting design changes. If the design changed and you measure what was built while the contract refers to the original drawings, you need the change to be documented before you measure the difference.
A concrete case
Masonry wall, item in square metres.
Stretch on grid line 3 between grid lines B and D, level 2. Length 8.40 m, height 2.60 m.
Gross area: 8.40 × 2.60 = 21.84 m²
Window opening: 1.50 × 1.20 = 1.80 m²
Area to be measured: 20.04 m²
And the three things that make that measurement undisputed:
- The criterion for deducting openings is agreed and recorded in the libro de obras
- There is a sketch of the stretch with the dimensions noted
- The ITO accepted the stretch on the 14th, before it was rendered
Without those three, the same number is an assertion.
Frequently asked questions
- Is it any use measuring from the BIM model?
- It is very useful for pricing measurement, with the caveat that the quality of the result depends entirely on how the model was built. For progress measurement, the model tells you what should have been built, not what was built. They are complementary inputs, not substitutes.
- Who should measure, the contractor or the ITO?
- Usually the contractor submits and the ITO reviews. What is worth avoiding is each side measuring separately to different criteria: that guarantees a discrepancy. Measuring together on site, even by sampling, resolves more than it costs.
- How do I measure a lump-sum item?
- Under a lump sum, payment tends to be tied to milestones or to percentages of progress on the whole item, not to unit measurement. Even so it is worth measuring internally: it is the only way to know whether you are winning or losing on that item.
- What do I do if I find a difference between the design and what was built?
- Record it in the libro de obras before measuring, and settle with the ITO how it is to be treated. Measuring over an unresolved difference is storing up a problem for closeout.
- How much detail is too much?
- The practical test: measure in enough detail for the ITO to verify on site without having to ask you anything. More detail than that is rarely paid for; less always costs.
Chilean, designing for Latin America. Field research surfaces what actually matters to clients, and that becomes products non-technical people adopt on their own — legal, education, accounting — and that show up in productivity from week one.
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