Model quantity takeoff: which elements are measured, in which unit, and how to make sure the number is right
A model quantity takeoff is calculated from the elements themselves: every wall, slab, column and door in the model knows its own dimensions, and the software adds them up by type. But the model does not know the methods of measurement, the waste or whatever was not modelled, so what comes out of it is a physical quantity that has to be translated into a quantity in the bill of quantities (BOQ). I have experience in taking off quantities from Revit models for residential buildings, public buildings and private houses, and in cross-checking them against the drawings.
Model quantity takeoff by element: what is measured, and in which unit
The table sums up the main elements. The unit is the one customary in bills of quantities in Israel, and on each project it is settled finally by the bill and the specification.
| Element | Unit in the bill | What to take from the model | Watch out for |
|---|---|---|---|
| Masonry walls | m² | Length times height, by type and thickness | Openings and junctions according to the methods of measurement |
| Concrete walls | m³ | Volume, by thickness | Formwork is usually included in the price of the concrete |
| Slabs and floors | m² by thickness, or m³ | Area and thickness | Large openings according to the rules |
| Columns | m³ | Cross-section times height | The height is set so that the junction with the slab is counted once |
| Beams | m³ | Cross-section times length | The part inside the slab is measured once |
| Foundations and piles | m³, or m by diameter | Volume, or length | Excavation and earthworks in separate items |
| Stairs | m³ or m² | Volume of the flight and the landing | A flight modelled as a floor may go into the wrong item |
| Doors and windows | Each | A count by type and size | Opening size versus leaf size |
| Room finishes | m² and m | Floor area, perimeter and height | Needs enclosed rooms in the model |
| Building services | m and each | Pipe and duct lengths, a count of fittings | Only with an up-to-date MEP model |
In a model quantity takeoff, the unit is chosen before the run: m² are never added to m³, and each line in the bill groups elements of the same unit and the same item. An element that was not assigned to an item does not disappear; it stays on a separate list until a decision is made about it.
Example: a wall with a window and a door, from the model to the bill
A block wall 6.00 m long and 2.80 m high, with a 1.50 by 1.20 m window and a 0.90 by 2.10 m door. The model returns the wall area according to the way the wall was built, usually after the openings have been cut out of it. The project's methods of measurement may say otherwise, for example that an opening up to a certain size is not deducted. So check the rules, set out on the methods of measurement page, rather than assume them.
| Step | Calculation | m² |
|---|---|---|
| Gross area | 6.00 × 2.80 | 16.80 |
| Window | 1.50 × 1.20 | 1.80 |
| Door | 0.90 × 2.10 | 1.89 |
| Net area in the model | 16.80 − 3.69 | 13.11 |
| In the bill, if the rules deduct only the door | 16.80 − 1.89 | 14.91 |
The flow in short: 16.80 gross → 13.11 in the model → 14.91 in the bill, under the rule in the example. The difference, 1.80 m², is about 12% of the quantity in the bill. Over 400 such walls in a building, that is 720 m². The three quantities, physical, bill and order, are explained on the physical quantity, bill quantity and order quantity page.
What the model does not know
- Methods of measurement: what is deducted, what is measured separately, and how quantities are rounded.
- Waste: material lost in cutting, breakage and offcuts. It goes into the price or into the order quantity, not into the bill quantity.
- Whatever was not modelled: waterproofing, fill, manholes, connection details and ancillary works.
- Overlaps: a beam inside a slab, or a wall that runs into a column, is counted twice if the joins in the model have not been fixed.
- Finishes without rooms: a model without enclosed rooms does not know how much flooring there is in a flat.
- Version: a model gets updated. A quantity without a version date is a quantity without a source.
Whatever is missing from the model is marked as missing and measured from the drawings, not recorded as zero. That list goes along with the quantities to everyone who uses them, so that they know what is included and what is not.
Three checks against the drawings
- Floor area: the area from the model against the area on the plan. Every difference needs an explanation.
- Opening count: doors and windows from the model against the architect's list of openings.
- Order of magnitude for concrete: floor area times an equivalent thickness. A 500 m² floor with an equivalent thickness of 0.30 m gives about 150 m³, and a result double that needs checking.
More ways to cross-check are on the quantity cross-check page, and how to prepare the model in advance is on the BIM model enrichment page.
The rule: what comes out of the model is a physical quantity. The bill quantity is the same quantity after the methods of measurement, and the order quantity is the same quantity after waste. Each one is right for its own use.
A quantity surveyor who works with a model starts by checking what was drawn and what was not, because what was not drawn will not get into any schedule.
Frequently asked questions
Can you rely on a model quantity takeoff without checking it?
No. The model is as accurate as whoever built it, and a check against the drawings is part of the work, not an extra.
What do you do when the model is partial?
You combine the two: whatever was modelled is measured from the model, the rest from the drawings, and every line states where it comes from.
In what format do the quantities come?
Usually an Excel file with a line for each item, with a breakdown by floor and element beside it. More on the work itself is on the quantities from a Revit model page.
What is the difference between a model quantity takeoff and a takeoff from drawings?
On drawings you measure lines and areas and calculate from them; in a model the element already knows its own volume and area. Both ways need methods of measurement and a check. How to work from 2D drawings, and how to link them to the model, is explained on the AutoCAD quantity takeoff page.
Who does the calculation?
A quantity surveyor for construction projects who works with both models and drawings, so that the two can be cross-checked.
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.