Quantity cross-check: what to compare and how to record a gap
A quantity cross-check is there to find out whether two sources describe the same work, in the same quantity. You compare measurements from the model, from the drawings and from schedules, record the gaps and trace their cause before the result goes into the bill of quantities (BOQ). A numerical match is an important check, but it does not replace a check of the scope and of the measurement rules.
Start from a common basis for comparison
Before subtracting one number from another in a quantity cross-check, check the units, the building, the floor, the design version and the range of elements. The volume of a wall with all its layers is not the same figure as the volume of its concrete alone. A gross area and an area after deducting openings can each be right for a different purpose. The distinction is explained in the guide to net quantity, BOQ quantity and order quantity.
An IFC file exported from the same Revit model is not a fully independent source. Comparing the two can reveal a change in the export or in the settings, but a mistake shared by the model can remain in both. So for each check, choose a source that lets you test the relevant assumption: for example, a section to check a height, or a structural drawing to identify a material.
Four checks in a quantity cross-check
1. Model content and floor totals
Compare the element totals with the floor outlines and with the split by floor. A difference in slab area may come from shafts, balconies, a duplicated element or a different boundary definition. First identify what each side includes, and only then decide whether there is a mistake.
2. The data before and after export
Check whether the elements, the classifications and the units were kept between the files. A gap between Revit and IFC is a point to investigate: do not automatically replace the smaller quantity with the larger one, and do not pick the one that fits the estimate.
3. Model against drawings and specifications
Look for details that are not represented in the model, such as a thickening, a finish detail or a change of material. A contradiction goes to the designer as a question. The order of precedence between the documents is set in the project's contract documents, not by the type of file that is convenient to measure.
4. Ratios between quantities
A ratio of concrete volume to built area, or of finish area to floor area, can flag an area to check. The comparison is useful when it is defined against similar projects or floors. An unusual ratio is not proof of a mistake, nor a reason to replace a measured quantity with a general factor.
Example: a typical floor, six rows and two sources
The example is a typical floor in a residential building, entirely synthetic. Each row has a quantity from the model, a quantity from the drawings and the gap between them. Clicking a row highlights in the model the elements that went into the calculation and shows the formula.
| Item | Description | Unit | From model | From drawings | Gap | Status |
|---|---|---|---|---|---|---|
| 02.010 | B30 concrete in a 22 cm slab, net of shafts | m³ | 94.60 | 94.60 | 0.0% | Match |
| 02.020 | B30 concrete in external and core walls 25 cm thick, at net height | m³ | 39.50 | 39.20 | 0.8% | Within tolerance |
| 02.030 | B30 concrete in protected-room (mamad) walls, net of the blast-door and blast-window openings | m³ | 31.20 | 31.20 | 0.0% | Match |
| 02.040 | B30 concrete in 30/80 and 40/100 cm columns, at net height | m³ | 12.44 | 12.44 | 0.0% | Match |
| 02.050 | B30 concrete in 20/60 cm beams, the part below the slab | m³ | 5.70 | 5.75 | 0.9% | Within tolerance |
| 02.100 | Reinforcing steel for the floor elements | tonnes | Missing | Missing | — | Missing |
The example sets an internal threshold of 2%: in a geometric check, a gap below it is marked as reasonable, and a gap above it goes back for checking. The threshold is set per project; it is not a standard, and not a professional approval to accept a gap. The rebar row stays "missing": there are no bar schedules yet, so it has no quantity. It is not given a zero, nor a factor that looks reasonable.
Two overlaps to check when measuring concrete
In the wall example, a floor height of 3.10 m and a 22 cm slab give a net height of 2.88 m. When a wall and a slab are measured separately, check how the shared part is handled. The calculation shown fits the assumptions of the example only.
| Measurement basis | Calculation | m³ |
|---|---|---|
| At floor height, as in a raw export | (64 × 3.10 − 26.32) × 0.25 | 43.02 |
| At net height | (64 × 2.88 − 26.32) × 0.25 | 39.50 |
In a beam 60 cm deep overall that crosses a 22 cm slab, the part below the slab is 38 cm. If the slab already includes the shared volume, counting the full depth of the beam again can create a double count. Check how it was modelled and how it was calculated before correcting it.
| Measurement basis | Calculation | m³ |
|---|---|---|
| At the full beam depth | 75 × 0.20 × 0.60 | 9.00 |
| Only the part below the slab | 75 × 0.20 × 0.38 | 5.70 |
Record a gap so that it can be closed
For every gap, attach the item, the unit, the location, the two sources and their revisions, the quantity from each source, and the question that needs an answer. If you show a percentage difference, state which quantity the percentage was calculated from. When the basis of comparison is missing or zero, do not show a percentage as if it were an ordinary figure.
Once the answer comes, keep the explanation and the quantity chosen, together with who approved it and in which version. That way the decision can be revisited when the design changes. The detail of the cross-checks can be handed over in an accompanying quantity calculation file; the scope of the deliverable is set in the service quote.
What is left for a human check
The thickness of a wall does not by itself identify its material, and the volume of a multi-layer element is not necessarily the volume of the material the item needs. Even two identical results do not prove that all the work was included. The quantity surveyor reads the drawings and the specification and clears up what is missing with the designers.
A quantity surveyor can be an engineer or a registered building technician, like Yaakov Avital. Before a project is checked, it is worth preparing the available models, the current drawings, the specifications and the list of revisions. You can also start from drawings only, according to the material and the scope agreed. Read about preparing the material, the steps of a quantity takeoff and writing a bill of quantities.
Frequently asked questions
When does a small gap in a quantity cross-check need investigating?
When it comes from a different scope, a wrong unit or missing work, even a small percentage can matter. The threshold is chosen for the particular check and does not replace finding out the reason.
What happens until the designer answers?
The question stays open and the row is marked accordingly. If a temporary assumption was agreed, it is shown explicitly, together with its effect on the quantity.
Can quantities be cross-checked without a model?
Yes. You can compare a plan with a section, a schedule with a plan, or area totals with the overall area. The level of the check depends on the material provided and on the purpose of the work.
Have a project? Tell me briefly what the project is and what already exists. When the project is a good fit, I usually reply with focused questions and a written quote.
Contact me for a quote* Created with AI for marketing, for illustration only. Not reviewed by Yaakov Avital and not professional advice. Every project must be checked on its own by a professional quantity surveyor and an engineer.