How we measure size, and which units you see
Summary
The Overview's How big is it? card gives the size of the asset: its length and width in plan, its height, its footprint or floor area, and how much of each kind of element it contains. These figures are measured from the 3D geometry itself, not read from properties, so they are there even when the file carries no quantities at all.
What is measured
| Figure | How it is measured |
|---|---|
| Length × width in plan | The smallest rectangle around the asset seen from above, turned to follow the asset rather than the north arrow. A bridge lying diagonally in the model still gets its real length. |
| Height | From the lowest to the highest point of all measured elements, so foundations and piles are included. |
| Footprint | The area covered in plan by upward-facing surfaces, combined on a 0.5 m grid so overlapping elements count once. |
| Floor area (buildings) | Per storey, the area covered by the tops of the floor slabs on the same 0.5 m grid (roofs and landings left out), added up over the storeys. Only for models with storeys. It does not depend on rooms being modelled. |
| Per element type | One natural measure per kind of element: area for slabs, walls, roofs, coverings, plates, stairs, ramps, curtain walls and spaces; length for beams, members, railings, pipes, ducts and cables; volume for footings, piles, columns, earthworks and generic building element proxies; a count for the rest, such as equipment and doors. |
Reinforcement, fasteners, spatial elements and alignments are not measured as elements: they say nothing about the size of the asset or would be counted twice.
Large models
To keep the analysis fast, each kind of element is measured in full up to 400 elements. Above that, 400 are picked at random and the total is scaled up to the full number. The card then says larger classes sampled and scaled up. The measurement also has a time limit; if it is reached, the card says so and the figures are a minimum. Very long sweeps, such as a kerb that follows a road for kilometres, are measured along their path instead of as a mesh.
Plausibility warnings
The card lists what looked wrong while measuring, for example:
- an element longer than 5 km, which is skipped: usually a unit or placement error;
- road courses, pavements or bridge decks sloping more than 15%;
- a model without storeys that is implausibly tall for its length;
- a partial measurement, when less than 80% of the elements could be measured.
Which units you see
IFC geometry is processed in SI units, so every size is measured in metres, square metres and cubic metres, whatever unit the file uses. What you see depends on your Default view of units in Settings:
| Setting | Sizes on the card |
|---|---|
| File default (the default) | Follows the units of the IFC file: feet, ft² and ft³ when the file declares feet, inches or yards as its length unit; otherwise metres, m² and m³. |
| Metric | Always metres, m² and m³. |
| Imperial | Always feet, ft² and ft³. |
The card states where its units come from, for example Units: metres (m, m², m³), from the units of the IFC file: the IFC file declares its lengths in millimetre. A file in millimetres or centimetres still shows metres, which are easier to read at the scale of a whole asset. When a file declares no length unit, metres are assumed and the card says so.
Elsewhere in the report the same setting applies. Alignment lengths and stations are shown in the file's own length unit under File default, because that is how the file and its designers express them.
How size is used
- Savings for infrastructure follow the size of the asset, not the number of IFC objects, and are also expressed per metre of length. See Operate & maintain savings.
- Savings for buildings are expressed per m² of floor area, when the floor area could be measured.
- The quantity check reuses the same measurements to test the quantities declared in the file. See The quantity check.
Unit checks in the audit
| Check | What it tests |
|---|---|
| Units defined (Validity) | The file declares a length unit. Without one, every number in it can be read in more than one way. |
| Units match their quantity (Consistency) | Each unit in the file's unit assignment fits the kind of quantity it is declared for, for example luminous intensity in candela rather than lumen. Each mismatch costs 30 points, down to a minimum of 20. |
| Qset room area plausibility (Consistency, models with rooms) | The average room area from the quantity sets is plausible: below 2 m² or above 2,000 m² per room points to a unit or export error. |
| Quantities match geometry (Consistency) | Declared quantities against the measured geometry, including unit errors. See The quantity check. |
Frequently asked questions
The length on the card differs from the drawing. Why?
The card measures the outer extent of everything modelled, foundations, wing walls and railings included, along the asset's own direction. A drawing usually gives a design length between two points. A large difference can also mean an object far away from the rest, which the warnings will point out.
Why is there a floor area for my building but not for another one?
The floor area needs storeys and floor slabs. A model without storeys, or one whose slabs were sampled because there are very many, shows a footprint instead.
I changed my units setting. Do I need to upload the file again?
No. The units are applied when the report is shown, so reopening the report is enough.