Quantity take-off (QTO) is measuring the quantities of work and materials on a project: how many doors, how many square metres of wall finish, how many cubic metres of concrete. Model-based QTO reads these quantities from the model's elements rather than measuring them from drawings by hand. It is faster and updates with the design, but only as reliable as the model.
At a glance
| Users | Cost managers, quantity surveyors, estimators, contractors, procurement |
| Sources in IFC | Quantity sets (Qto_), geometry, counts of elements |
| Grouped by | Classification (Uniclass, UniFormat, NRM), type, location, level |
| Tools | Dedicated QTO and cost software, schedules from authoring tools, IFC schedule tools |
| Governed by | Measurement rules (for example NRM in the UK) |
What a model can measure well
| Quantity | Example | Condition |
|---|---|---|
| Counts | Doors, windows, light fittings, pumps | Each item is a separate element of the right type |
| Lengths | Pipes, ducts, beams, skirting | Elements split sensibly, no duplicates |
| Areas | Slabs, walls, ceilings, finishes | Faces modelled consistently, openings handled |
| Volumes | Concrete, excavation, insulation | Solids accurate and not overlapping |
What the model does not tell you
- Items below the modelled level of detail: fixings, sealants, small fittings.
- Temporary works, preliminaries, waste and laps.
- Work described in specifications but not modelled (finishes, coatings).
- Items modelled for another purpose that do not follow measurement rules (a wall modelled through slabs gives a different area than measurement rules require).
A model-based take-off is therefore combined with traditional measurement, and the report says what came from the model and what did not.
Conditions for reliable quantities
- Agreed scope: the BEP or LOIN says which elements are modelled for quantities.
- Consistent classification on every element that is measured.
- Modelling rules that match measurement: how walls meet slabs, how layers are modelled, how openings are cut.
- No duplicates across discipline models.
- Locations: levels and zones assigned, so quantities can be split for packages and sequencing.
Common mistakes
- Taking schedule totals at face value without checking what is included.
- Generic elements (a "wall" with no build-up) used for materials quantities.
- Comparing quantities between model revisions without noting scope changes.
- Mixing gross and net areas without saying which.
Where to read more
National measurement standards define how quantities are reported. Cost management guidance from professional bodies (such as RICS) covers model-based measurement.