What unit and formula concrete is quantified in
Concrete is always measured in cubic meters (m³), because it is a material poured to fill a volume. The base formula is that of volume: for prismatic elements (footings, pedestals, columns, beams) it is length × width × height; for flat, constant-thickness elements (slabs and floor slabs) it is area × thickness.
The practical rule is to break the structure down into simple geometric bodies, compute the volume of each and add them up. All dimensions are taken off the structural drawing in meters, so the result comes out directly in m³. It pays to separate the concrete by strength (f'c) and by element, because the unit price and the pour differ between a floor slab of f'c=100 kg/cm² and a slab of f'c=250 kg/cm².
Step by step to quantify the concrete on the project
The order matters: you work from the bottom up, following how it is built, so you neither forget elements nor count them twice.
- 1Identify the elements on the structural drawing
Separate the foundation (footings, grade beams, pedestals), the vertical structure (columns, concrete walls) and the horizontal structure (beams, slab), plus the floor slabs. Each type goes on its own line of the takeoff sheet.
- 2Take the dimensions of each element
Read the length, width and depth (height) of each element off the drawing in meters. For columns and beams, count how many identical pieces there are so you do not calculate them one by one.
- 3Calculate the volume of each element
Apply length × width × height for prismatic elements and area × thickness for slabs and floor slabs. Multiply by the number of identical pieces.
- 4Deduct openings and avoid double-counting
In slabs and walls, subtract the openings (stairwell void, ducts, doors and windows in concrete walls). Where a beam rests on a column or the slab sits on the beams, the overlap is counted only once.
- 5Add up by strength and apply the waste factor
Group the net volume by f'c, add it up and apply the waste factor (3% to 5%). That is the volume of concrete to order or budget.
Formulas by element (footings, columns, beams, slab, floor slab)
Each element has its own geometry. These are the formulas that cover most of a reinforced-concrete project:
- Isolated footing: length × width × depth. Example: 1.20 m × 1.20 m × 0.30 m = 0.432 m³ per footing.
- Grade beam or strip footing: section (width × depth) × length. Example: 0.30 m × 0.60 m × 40 m = 7.20 m³.
- Pedestal or column: (section width × length) × height × number of pieces. Example: 0.40 m × 0.40 m × 3.00 m = 0.48 m³ per column.
- Beam: (width × depth) × length. Deduct the part embedded in the slab if the depth is measured up to the top of the slab, so you do not double-count.
- Solid slab: plan area × thickness, minus the openings (stairwell voids, ducts). Example: 80 m² × 0.12 m = 9.60 m³ before deducting openings.
- Floor slab (slab on grade): area × thickness. Example: 100 m² × 0.08 m = 8.00 m³. The floor slab is usually of lower strength than the structure.
Deducting openings, double-counting and the waste factor
The gross volume is almost never the real volume. Three adjustments make the difference between ordering too much and coming up short.
Deducting openings: in a slab with a 4.0 m² stairwell void, that gap is not poured. If the slab is 0.12 m thick, you deduct 4.0 × 0.12 = 0.48 m³. The same applies to MEP ducts and to door or window openings where there are concrete walls.
Double-counting: where two elements cross, the concrete in the intersection exists only once. If you add the full volume of the column and the full volume of the beam framing into it, you count the joint twice. Common practice is to measure the beam between column faces (clear span), not center to center, and to measure the slab subtracting the area taken up by the beams if these are quantified separately.
Waste factor: for waste in transport, concrete left in the mixer or pump, and overconsumption from ground-level and pour variations, you add a percentage to the net volume. On site, 3% to 5% is common; on slabs poured over uneven ground or imprecise formwork it can reach 5% or more. The waste factor is applied at the end, on the volume already deducted.
Full worked example
Let's take a suspended floor slab from a house and work out its concrete volume from start to finish.
Data: a solid slab of 8.0 m × 10.0 m (80 m² in plan), 0.12 m thick, with a stairwell void of 2.0 m × 2.0 m (4.0 m²). The concrete is f'c=250 kg/cm² and a 4% waste factor is used.
Gross slab volume: 80 m² × 0.12 m = 9.60 m³. Stairwell deduction: 4.0 m² × 0.12 m = 0.48 m³. Net volume: 9.60 − 0.48 = 9.12 m³.
4% waste factor: 9.12 × 1.04 = 9.48 m³. You order about 9.5 m³ of f'c=250 concrete for this slab. If on top of that you pour 6 beams of 0.25 m × 0.40 m × 8.0 m = 0.80 m³ each (4.80 m³) and those beams were not deducted from the slab area, you would have to subtract their volume from the slab or measure the slab only between beams so as not to double-count the joint.