Governing Formula
A slab's concrete volume is its plan area multiplied by a uniform thickness. Rectangular slabs use the
single product L × W × H; L-shaped slabs are decomposed into two overlapping-free rectangles —
the main body L × W plus a leg L_leg × W_leg — before multiplying by the
thickness.
V = A_plan × H Where:
-
L= Main rectangle length [m] -
W= Main rectangle width [m] -
H= Slab thickness [m] -
L_leg= Leg length (L-shape only) [m] -
W_leg= Leg width (L-shape only) [m] -
A_plan= Slab plan area (L×W for rect; L×W + L_leg×W_leg for L-shape) [m²] -
V= Slab concrete volume (A_plan × H) [m³]
Derived Equations:
V = L × W × H V = (L × W + L_leg × W_leg) × H V_total = V × n V_ready = V_total × (1 + wastage/100) How the Calculation Works
The calculator computes the plan area for the selected shape, multiplies by the thickness to get the theoretical volume of one slab, then scales by the slab count for identical elements. A wastage margin is added for formwork tolerances, undulations, spills, and truck losses before the ready-mix order quantity is shown.
For site-mixed concrete the volume converts to a premix bag count using the industry approximation of 0.033 m³ of compacted concrete per 50 kg bag (about 30 bags per m³) — the same convention used across the concrete toolkit.
Worked Engineering Example
Design Scenario: L-Shaped Ground Slab
A slab plan is 6 m × 4 m with a 2 m × 1.5 m leg extension and a design thickness of 150 mm, ordering one pour with a 5% wastage margin.
- Compute plan area:
A_plan = 6 × 4 + 2 × 1.5 = 27 m² - Compute theoretical volume:
V = 27 × 0.15 = 4.05 m³ - Add wastage (5%):
V_ready = 4.05 × 1.05 = 4.25 m³ - Premix bags (if site mixing):
N_bags = ceil(4.25 / 0.033) = 129 bags of 50 kg
Assumptions & Limitations
Engineering Assumptions:
- Uniform slab thickness across the entire plan area, including the leg.
- L-shape decomposition assumes the leg does not overlap the main rectangle.
- Wastage is applied linearly to the total theoretical volume.
Design Limitations:
- Does not account for reinforcement, formwork, blinding, or finishing quantities.
- Non-prismatic slabs (varying thickness, steps, pits) must be split into prismatic sub-elements.
- For structural-critical pours, aggregate moisture and admixture dosages must be adjusted by the batching plant.
Frequently Asked Questions
What is an L-shaped slab?
An L-shaped slab is a ground or suspended slab whose plan footprint forms an "L" — typically to follow a building or partition profile. It is handled by splitting the plan into the main rectangle plus a leg rectangle, computing each prism volume, and summing.
How many 50 kg bags of concrete are in 1 m³?
About 30 bags. Each 50 kg premix bag yields roughly 0.033 m³ of compacted concrete, so 1 / 0.033 ≈ 30 bags per m³. Actual yield depends on mix design, water addition, and compaction.
Do I measure to the outside or inside face of the formwork?
Measure to the outside faces of the finished slab for quantity purposes. If formwork is set on a plumb line, subtract formwork thickness; otherwise ordering against the outer dimensions is the safe, standard practice.
Engineering Disclaimer
Engineering Note: This calculator provides simplified quantity estimates for preliminary planning and educational use. Final concrete supply quantities should be confirmed with the ready-mix supplier, which accounts for mix shrinkage, aggregate moisture, slump, and local batching practice.
Technical References
- ACI 211.1: Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete, American Concrete Institute.
- ACI 117: Specification for Tolerances for Concrete Construction and Materials, American Concrete Institute.
- EN 206: Concrete – Specification, Performance, Production and Conformity, European Committee for Standardization.