Concrete Slab Calculator

Fast, unit-aware estimates for slab volume, bags, thickness, area, or weight — with waste included and clear calculation steps.

Practical Guide

Concrete Slab Calculator: From Dimensions to Yards, Bags, Rebar, and Cost

This guide mirrors the calculator above so you can turn length, width, and thickness into cubic yards or cubic meters, estimate bag counts, and add realistic waste and reinforcement. We’ll cover the fastest methods, the variables that move your number, and worked examples in US and metric units.

9–13 min read Updated November 10, 2025

Quick Start

  1. 1 In Solve for, choose what you need: Volume (yd³/m³), Bag Count, or Cost. Pick a shape or Composite to sum multiple shapes.
  2. 2 Enter length, width, thickness and their units. For thickness in inches or millimeters, the calculator converts to feet/meters under the hood.
  3. 3 Set optional waste (typically 5–10%) for over-excavation, spillage, and irregular edges. If you’ll saw joints, a little extra helps avoid short loads.
  4. 4 If estimating bags, select the bag size (40/60/80 lb in the US) and the calculator will convert volume to the closest whole bags.
  5. 5 Review Calculation Steps to see the equations, unit conversions, and rounding assumptions used for the result.

Tip: Measure to the nearest inch (or centimeter) and keep a consistent convention (inside of forms). Small input changes swing the final yardage more than you’d think on large slabs.

Watch-out: Thickness is the biggest driver of volume. A 0.5 in increase on a 20×20 ft pad adds nearly 1.5 yd³—often an extra truck.

Variables & Symbols

  • \(A\) Area (ft², m²): \(A=L\times W\) for rectangles
  • \(t\) Slab thickness (ft, m)
  • \(V\) Volume \(=A\times t\) (ft³, m³)
  • \(\text{yd}^3\) Cubic yards \(=\frac{\text{ft}^3}{27}\)
  • \(n\) Bags required \(=\frac{V}{\text{yield per bag}}\)
  • \(p\%\) Waste/contingency (%)
\[ A = L\times W,\qquad V = A\times t,\qquad \text{yd}^3 = \frac{\text{ft}^3}{27} \] \[ \text{Bags (80 lb)} \approx \frac{V_{\text{ft}^3}}{0.60},\quad \text{Bags (60 lb)} \approx \frac{V_{\text{ft}^3}}{0.45},\quad \text{Bags (40 lb)} \approx \frac{V_{\text{ft}^3}}{0.30} \] \[ V’ = V \times \left(1+\frac{p}{100}\right)\quad\text{(apply waste)} \]

Choosing Your Method

Method A — Dimensions → Yards (or m³)

Best for most patios, sheds, garages, and interior slabs when you know length, width, and thickness.

  • Direct from drawings or measurements.
  • Works with mixed units (in/ft or mm/m).
  • Easily adds waste and cost.
  • Sensitive to thickness variability across the slab.
  • Irregular shapes require splitting into basic shapes.
Yardage: \(\ \text{yd}^3 = \dfrac{L\cdot W\cdot t_{\text{ft}}}{27}\).

Method B — Bags or Trucks → Coverage

Useful when you have a fixed supply (bags or a truckload) and want to see how far it will go at a target thickness.

  • Great for deciding between bags vs ready-mix.
  • Helps size pours to truck capacities and avoid short loads.
  • Requires realistic bag yield or truck volume assumptions.
  • Doesn’t include waste unless you add a margin.
Coverage: \(A = \dfrac{V}{t}\).\ \ 1 truck (US) ≈ \(8\text{–}10\ \text{yd}^3\) depending on local limits.

What Moves the Number the Most

Thickness

Going from 4 in to 5 in adds 25% more volume. Confirm grade and screed rails before ordering.

Waste allowance

Edges, spillage, and over-excavation consume 5–10% quickly—especially on irregular forms or slopes.

Shape decomposition

Breaking L-shapes into rectangles avoids undercounts. Add and subtract alcoves or block-outs explicitly.

Unit hygiene

Keep inches with feet (or mm with m). Convert once at the end; avoid mixing bases mid-calculation.

Subbase flatness

Wavy subgrade forces variable thickness; compact and screed aggregate uniformly to hold the target.

Rounding & delivery

Order to the nearest 0.5–1.0 yd³ depending on plant policy, haul distance, and weather window.

Worked Examples

Example 1 — US (Imperial): 12 ft × 20 ft × 4 in patio

  • Dimensions: \(L=12\ \text{ft},\ W=20\ \text{ft}\)
  • Thickness: \(t=4\ \text{in}=4/12=0.333\ \text{ft}\)
  • Waste: \(p=10\%\)
  • Bag yields (typical): 40 lb ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³
1
Area: \(A=12\times 20=240\ \text{ft}^2\)
2
Volume (ft³): \(V=A\times t=240\times 0.333= \mathbf{80.0\ \text{ft}^3}\)
3
Yards: \(\text{yd}^3=80/27=\mathbf{2.96\ \text{yd}^3}\)
4
With 10% waste: \(2.96\times 1.10=\mathbf{3.26\ \text{yd}^3}\)
5
Bags (no waste): 80 lb → \(80/0.60=\mathbf{134}\) bags (round up); 60 lb → \(80/0.45=\mathbf{178}\); 40 lb → \(80/0.30=\mathbf{267}\).

Ready-mix may be more economical above ~2 yd³. If your supplier has a 3 yd³ minimum, the 10% waste conveniently clears it.

Example 2 — Metric: 6.0 m × 4.0 m × 100 mm garage slab

  • Dimensions: \(L=6.0\ \text{m},\ W=4.0\ \text{m}\)
  • Thickness: \(t=100\ \text{mm}=0.10\ \text{m}\)
  • Waste: \(p=7\%\)
1
Area: \(A=6.0\times 4.0=24.0\ \text{m}^2\)
2
Volume: \(V=A\times t=24.0\times 0.10=\mathbf{2.40\ \text{m}^3}\)
3
With 7% waste: \(2.40\times 1.07=\mathbf{2.568\ \text{m}^3}\)
4
US conversion: \(1\ \text{m}^3=1.30795\ \text{yd}^3\Rightarrow \mathbf{2.568\times 1.308=3.36\ \text{yd}^3}\)

For bagged mix, check your local bag yield (often stated in liters of concrete per bag) and divide \(V\) by that yield.

Thickness, Reinforcement & Variations

Use this table to align thickness, reinforcement, and jointing with typical use-cases. Always follow local codes and engineer-of-record guidance.

Use-CaseTypical ThicknessReinforcement / Notes
Walkways / Pads4 in (100 mm)WWR mesh (e.g., 6×6-W1.4/1.4) or fiber; joints 2–3× thickness (ft) → 8–12 ft panels.
Patios / Sheds4–5 in (100–125 mm)Fiber or #3 rebar at 18–24 in o.c.; compact subbase to limit differential settlement.
Garages4–6 in (100–150 mm)#3–#4 rebar grid at 16–24 in o.c.; thicken edges at door openings.
Driveways (light vehicles)5–6 in (125–150 mm)Higher psi mix (e.g., 3500–4000); control joints match wheel paths and geometry.
Hot tubs / Point loads6–8 in (150–200 mm)Engineer slab or pier pads for concentrated loads; verify soil bearing at supports.
Slopes / RampsAs specifiedVolume increases with slope if thickness measured perpendicular to surface; calculate by area × thickness along the slope.
  • Joint spacing rule-of-thumb: 2–3× slab thickness in inches, expressed in feet (e.g., 4″ → 8–12 ft).
  • Place reinforcement in the upper third of slab depth for crack control unless design dictates otherwise.
  • Consolidate and finish minimally; avoid adding water at the surface—use proper curing to reach design strength.
  • Document assumptions (thickness, waste %, bag yields, truck size) on quotes and tickets.

Buying, Logistics & Practicalities

Choosing Mix & Delivery

  • Strength: Common specs: 3000–4000 psi (20–28 MPa) for slabs-on-grade; ask for air-entrainment in freeze–thaw climates.
  • Bag vs Ready-mix: Bags suit small pads and tight access; ready-mix is cost-effective above ~2–3 yd³.
  • Truck capacity: Order in whole or half yards; coordinate multiple trucks for large pours.

Site Prep & Placement

  • Excavate to allow subbase + slab; compact aggregate base uniformly (e.g., 4–6 in for most slabs).
  • Set forms to finished elevation; use screed rails to hold thickness.
  • Strike off, bull float, edge, and joint at proper timing; begin curing as soon as bleed water dissipates.

Sanity Checks

  • Does yardage from dimensions match supplier dispatch within a few percent?
  • Will your joint layout keep panels roughly square and within spacing rules?
  • Did you include waste for edges, pump priming, and irregular excavation?

Codes, specs, and acceptance criteria vary by jurisdiction. Use this calculator and guide for planning; follow the governing specification and engineer-of-record for design decisions.

Frequently Asked Questions

How many cubic yards do I need for a 4" slab?
Use \( \text{yd}^3 = \dfrac{L\cdot W\cdot (t/12)}{27} \) with \(L,W\) in feet and \(t\) in inches. Example: 12×20 ft at 4″ is \(12×20×(4/12)/27=2.96\ \text{yd}^3\) (add waste).
How many 80 lb bags are in a cubic yard?
About 45 bags. One 80 lb bag yields ≈0.60 ft³; \(27/0.60 \approx 45\) bags per yard. For 60 lb bags it’s ≈60, and for 40 lb bags ≈90.
What waste percentage should I add?
Most small slabs use 5–10% to cover edge irregularities, spillage, and over-excavation. Larger or complex pours may need more.
Should I use rebar, mesh, or fiber?
For crack control, fiber or welded wire mesh is common; for higher loads or poor soils, rebar grids are used. Placement and jointing matter as much as the reinforcement type.
How far apart should control joints be?
A practical rule is 2–3× slab thickness in inches, expressed in feet (e.g., 4″ slab → 8–12 ft). Keep panels as square as practical.
What’s the difference between slab thickness and subbase thickness?
Slab thickness is concrete depth. Subbase thickness is compacted aggregate beneath the slab; it improves support and drainage but doesn’t count toward concrete volume.
Is it okay to add water on site?
Only within mix design limits and with the supplier’s guidance. Excess water reduces strength and increases shrinkage; consider plasticizers instead.
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