Quantities from a Revit model: the documented behaviours that distort them, and how I check them
Quantities from a Revit model look very precise: three decimal places, a tidy schedule, a total at the bottom. That precision belongs to the modelled geometry, not to the bill of quantities (BOQ). The software calculates by its own rules, which are not the project's methods of measurement, and it has several documented behaviours that distort quantities for anyone who does not know them. This page sums up the main ones, and how I check them. It describes my own use and experience, not a list of the software's capabilities.
Quantities from a Revit model: room volume versus room height
The example I always start with is room volume. The software's official documentation says that volume computation is not turned on by default, and that while it is off, the schedule shows "Not Computed". When you turn it on, the volume is cut at a ceiling or roof defined as room-bounding; if there is no such ceiling in the area, the software uses an upper limit set by hand, by default four metres above the level. The room height parameter shown in the schedule is the "maximum potential height", not the real height under the dropped ceiling.
In practice, a room with a dropped plasterboard ceiling that was not modelled as room-bounding gets a volume up to the structural slab, or up to the manual limit, and not up to the plasterboard. For HVAC, paint and plaster quantities that is a material difference. In my work I do not use the room height from the schedule as the ceiling height. In a few sample rooms I check the volume and the area against the actual ceiling height in the sections, and I correct the calculation method before it runs on the whole building.
Wall height versus the ceiling drop
A wall modelled up to the level above gets the area and volume of all its geometry in the schedule, including the part above a dropped ceiling. Plaster, paint and wall cladding are usually measured up to the ceiling, so the wall area from the schedule is larger than the area for the bill. The official documentation explains that the unconnected height parameter is the height as drawn, and that attaching the top of the wall to a floor, a roof or a reference plane changes the geometry without changing that parameter. The official forum also documents a case in which a wall attached to a sloped reference plane reported a volume about a third lower than the correct one, until the plane was replaced.
My way of handling it: for finishes quantities I calculate the room perimeter times the actual ceiling height, not the wall area from the schedule, and I add the top constraint and offset columns to the wall schedule, to see which walls are "attached" and which are not. The difference between what is measured and what goes into the bill is explained on the finishes quantity takeoff page.
Overlaps, joined geometry and openings
- Overlaps. Two elements that overlap and are not joined are both counted in full. On the official forum, a masonry wall with a concrete column passing through it was shown still reporting its full volume. Joining the geometry deducts the overlap from one of the elements in a fixed order, which can be switched. The software does what it was asked to do here; the question is who asked, and by which method of measurement.
- Openings. The wall area in the schedule is net: doors, windows and openings are deducted, with no minimum threshold. Common methods of measurement, by contrast, do not deduct openings smaller than a certain threshold, and do deduct larger ones. Converting between the two is a quantity surveyor's job, not the software's.
- Material takeoff. The official documentation states that when it calculates layer volumes the software "makes approximations", and that discrepancies are expected with certain profiles or joins. The volume of a material in a material takeoff schedule is not always the volume of the element.
- Rounding and totals. A column total is calculated from the exact values, not the displayed ones, so adding up the column on screen by hand does not always give the total row. That is not a bug, but it is confusing during a check.
The rule I work by. Every quantity from the model gets a line of proof: which schedule, which settings, and how it converts to the bill's method of measurement. A quantity without such a line is a candidate, not a quantity. How to check it against the drawings is explained on the quantity cross-check page.
Linked models, phases and design options
When a building is split into several files, elements from linked files go into a schedule only when you tick that option, and according to the official documentation some of their fields become read-only and cannot be filtered on. A schedule that leaves out the links is missing whole floors; a schedule that includes them twice counts them twice. The schedule's phase also decides what is counted: an element marked for demolition will not appear in the schedule of a new phase, and a temporary element may be counted. With design options, the schedule always shows the main model plus one option per set, according to a choice in the visibility settings.
In my work, before measuring a model with links, I take a screenshot of the schedule settings, count the floors and the files, and compare the total in the host schedule with the total in the same source file. More on working with several files in a Revit model is on the construction quantity surveyor page.
IFC export and rebar
In an IFC export, base quantities are exported only if you tick that option, and according to the documentation they are derived from the geometry and are "independent of measurement rules". Over the years, versions of the IFC exporter have fixed wrong quantities for foundations and floors, so the version matters. For rebar, the documentation defines the reinforcement volume as a read-only estimate, there is no built-in weight, and when length rounding is turned on the total shows only the rounded value. Each of these is a reason to check one sample by hand before relying on a thousand lines.
How I take quantities out of the schedules, and what stays outside them
This is how I work with Revit's own tools, without add-ins:
- Schedules: I use a schedule to count elements and to total area, volume and length by type and level. When "Itemize every instance" is cleared, the rows collapse into groups, and subtotals and a grand total appear.
- Material takeoff: I use it when I need the volume or area of a layer, for example concrete against fill in a floor, and I check a sample because of the approximations described above.
- Calculated values: I use them to convert to the bill's units, and I record the formula in the calculation file.
- Export: Revit saves a schedule as a delimited text file that Excel can open. I export with the column headers, and I check that the total in the file equals the total in the schedule.
Whatever is not modelled will not appear in any schedule: finishes, waterproofing and many completions are missing from an architectural model, and I calculate them separately. The software's official page: Revit on the Autodesk website. Revit and Autodesk are trademarks of their owner, which has no connection with this site.
What a professional quantity surveyor adds here
Anyone who exports a schedule and pastes it into the bill gets quantities from a Revit model by the software's rules. A quantity surveyor who comes from the field, an engineer or a registered building technician who worked on site, like Yaakov Avital, knows how the contractor will measure the same wall, and translates. For example, the wall schedule of a typical floor shows a plaster area for walls modelled up to the underside of the structural slab, while the ceiling plan has a plasterboard drop of forty centimetres in the corridors. Measuring by the schedule would put plaster into the bill that nobody will apply; a professional check flags the gap, recalculates by the ceiling height, and writes the assumption into the bill so that the designer can approve it. The quantities come with evidence, and the designers decide.
Frequently asked questions
Is this a full description of Revit?
No. It is how I use the software in quantity surveying work, and the behaviours I have met and checked against the official documentation. Autodesk's documentation describes the software itself.
Are quantities from a Revit model more accurate than a takeoff from drawings?
They are more consistent and quicker to update, but they are only as accurate as the model and its settings. A good model with a correctly set-up schedule is an excellent basis; a partial model gives numbers that look precise and are not. That is why I check them.
Can the model give quantities by the methods of measurement?
Not directly. The software measures geometry; deduction thresholds, net measurement by rules and classification into items are a layer added on top, in the calculation or with dedicated tools. That is the work of a quantity surveyor for construction projects who works from a model.
What should I check first in a model I have received?
Whether volume computation is on, the computation height of the levels, which elements are room-bounding, whether there are overlap warnings, and which files are linked. The full list of checks is described on the quantity takeoff page.
What do you do when there is no model, only DWG?
The quantities are measured from the drawings, and the document says so. The difference is in how fast it can be updated, not in the principles: there too, every quantity needs a source, a unit and a method of measurement.
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.