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Project Layout & Design — Concrete Slab Calculator

Quickly estimate concrete volume, weight, bag counts, basic reinforcement, and cost for slab-on-ground projects. Designed for builders, engineers, and homeowners, this calculator helps you plan pours, order ready-mix, and avoid shortfalls.

Concrete slab calculator

Overall slab dimension along X (feet or meters depending on unit).

Overall slab dimension along Y (feet or meters depending on unit).

Slab thickness (commonly 4 in / 10 cm for light-duty residential).

Typical allowance is 5–10% to cover spillage, over-excavation, and edge bevels.

For crack control only; follow ACI/local code and engineer for structural needs.

Results

Concrete Volume

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Estimated Cost

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Excl. delivery & labor

Bag Counts (40/60/80 lb)

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Approximate total per size

Weight

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Includes waste allowance

Reinforcement (est.)

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Mesh sheets / rebar length

Ready-Mix Order

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Round up for truck order

Computation log

Authoritative data source & methodology

Primary references:

  • NRMCA — National Ready Mixed Concrete Association, Concrete in Practice (CIPs), yield and ordering guidance.
  • ACI — American Concrete Institute, ACI 318 (Structural Concrete) and ACI 302.1R (Guide for Concrete Floor and Slab Construction).

Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.

The formula explained

\[ V = L \times W \times T \] \[ T_\text{ft} = \begin{cases} \frac{T_\text{in}}{12} & \text{(imperial)}\\[4pt] \frac{T_\text{cm}}{100} & \text{(metric)} \end{cases} \quad V_\text{yd}^3 = \frac{V_\text{ft}^3}{27} \quad V_{\text{m}^3} = V_\text{m}^3 \] \[ V_\text{net} = V \times \left(1 + \frac{w}{100}\right) \] \[ \text{Bags}_{80} \approx 45 \times V_\text{yd}^3 \quad \text{Bags}_{60} \approx 60 \times V_\text{yd}^3 \quad \text{Bags}_{40} \approx 90 \times V_\text{yd}^3 \] \[ \text{Weight} \approx \rho \times V_\text{net} \quad \text{with } \rho \approx \begin{cases} 150\ \text{lb/ft}^3 & \text{(2400 kg/m}^3) \end{cases} \]

Glossary of variables

SymbolDescriptionUnits
LSlab lengthft or m
WSlab widthft or m
TSlab thicknessin or cm (converted)
wWaste allowance%
VConcrete volume (gross and net)ft³ / yd³ / m³
ρFresh concrete density (approx.)150 lb/ft³ ≈ 2400 kg/m³

How it works: a step-by-step example

Example: 20 ft × 12 ft × 4 in slab, 7% waste, $140/yd³

  1. Convert thickness: \(T_\text{ft} = 4/12 = 0.333\) ft.
  2. Volume: \(V_\text{ft}^3 = 20 \times 12 \times 0.333 \approx 79.92\ \text{ft}^3\).
  3. Yards: \(V_\text{yd}^3 = 79.92/27 \approx 2.96\ \text{yd}^3\).
  4. Waste: \(V_\text{net} = 2.96 \times 1.07 \approx 3.17\ \text{yd}^3\).
  5. Cost: \(3.17 \times 140 \approx \$444\) (excl. delivery/labor).
  6. 80-lb bags: \(45 \times 2.96 \approx 133\) (yard-basis, before waste; round up).

Frequently asked questions

Should I order by yard or by bag?

For small pads or remote sites, bags can work. For most slabs over ~1 yd³ (0.76 m³), ready-mix is more consistent and faster.

What slump or admixtures should I specify?

Follow ACI and supplier recommendations. Slump depends on placement method; admixtures can improve workability and set time.

Will reinforcement stop cracking?

It controls crack width; joints and curing are equally important. Always place reinforcement at the correct depth and provide control joints.

Do I need vapor barrier or base?

Per ACI 302.1R, many slabs need compacted granular base and in some cases a vapor retarder; check climate and use case.

How accurate are bag conversions?

Bag yields vary by brand and mix. Calculator uses standard approximations; verify with your supplier’s yield charts.

Tool developed by Ugo Candido. Content verified by CalcDomain Editorial Board.

Last accuracy review: October 27, 2025

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