Quantity takeoff from IFC (QTO) — base quantities, area & volume, vs cost estimating

Instead of scaling off drawings by hand, you pull “wall area,” “concrete volume,” and “door count” straight from the BIM model — that’s QTO (Quantity Take-off). IFC can carry these quantities. But they’re often missing, or net and gross get mixed up, and the pitfalls are predictable. Get the basics right and the model becomes a solid front end to estimating.

What QTO is (pulling quantities from the model)

QTO is the work of counting up the quantities needed for design and construction. Traditionally it was hand-measured off drawings; in BIM the elements themselves carry dimensions and volume, so quantities can be aggregated from the model. If there are 100 walls, the model tells you their total area and volume.

The advantages over hand takeoff are clear. It follows changes (stretch a wall and the quantity updates), it’s hard to miss (counted per element), and each quantity traces back to its element. The caveat, as below, is that “model quantity” is not the same as “estimate.”

How IFC carries quantities

In IFC an element is linked to an IfcElementQuantity (a group of quantities), which holds individual quantities. Quantities are typed.

IFC also defines standard quantity sets per element type. A wall has Qto_WallBaseQuantities (length, width, height, side area, volume, etc.); a slab has Qto_SlabBaseQuantities (in IFC2x3 these are collectively “base quantities”). When quantities are exported to this standard, the receiving side can aggregate “which wall, which quantity” mechanically.

Quantity ≠ estimate

This is the most important point. What you pull from the model is a quantity (4000 mm, 9.6 m²) — not an estimate (cost). An estimate only emerges once you apply rates, productivity, waste factors and labour to the quantities, and the model alone doesn’t decide those.

So QTO is a powerful input to estimating. The more accurately and quickly you can produce quantities, the better and faster the estimate — but not by turning model quantities directly into money. Sharing that expectation prevents mistakes.

Common pitfalls

• Mixing net and gross. Is it the net quantity (openings deducted) or gross (before deduction)? Pick per use and align sender and receiver.
• Quantities not exported. If the IFC has no quantities, there’s nothing to take off. The export must include base quantities (see IFC export tips).
• Unit mismatch. Areas and volumes blow up under a unit mismatch. Sanity-check against reality (a ~2.7 m wall, etc.) — see common IFC problems.
• Dependence on modelling quality. Quantities faithfully reflect how the model was built. A rough model gives rough quantities — quantity confidence ties to LOD.
• Vendor-specific quantities. When custom quantities mix in alongside the standard Qto_*, it’s unclear which to aggregate. Prefer the standard.

Checking and using them in practice

First, in a viewer or property panel, click an element and confirm quantities are present. If you can see the standard quantity set (Qto_WallBaseQuantities, etc.), it’s ready to aggregate. If quantities are empty, start by having them included on the export side.

When aggregating, the key is to decide and standardise the takeoff basis (net/gross, deduction rules) up front. If the basis drifts, the same model yields different quantities that won’t reconcile. Grouping elements by type and part with property sets and classification makes it easier to keep the aggregation granularity consistent.

Summary

• QTO is pulling quantities from the model. IFC can carry per-element length, area, volume, count and weight (the standard base quantities).
• Model quantities are an input to estimating, not the estimate (rates, productivity) itself.
• The classic pitfalls are net/gross, opening deduction, units and missing export. Standardise the takeoff basis first.
• Quantity confidence is proportional to how far the model is developed (LOD).

Quantity takeoff is where BIM’s “follows changes” strength shows most plainly. Leaning on quantities that trace to elements cuts both the effort and the misses. The basics of properties and classification are in property sets and classification, and the big picture in what is IFC.