How to Calculate Concrete for a Slab
Concrete is sold in two ways: pre-mixed bags (40, 60, or 80 lbs) for small projects, or by the cubic yard from a ready-mix truck for larger pours. Knowing the volume you need — and which delivery method makes sense — is the first step to a successful pour. This guide covers the formula, real-world tips, and the crossover point where bags stop making sense.
The Volume Formula
Cubic Yards = Volume (cu ft) ÷ 27
A 10 ft × 10 ft × 4 inch slab: 10 × 10 × (4/12) = 33.3 cubic feet = 1.23 cubic yards. With the standard 10% overfill, you need about 1.36 cubic yards.
Bags vs. Ready-Mix Truck
The crossover point is roughly 1 cubic yard (about 27 cubic feet). Below that, bags are practical. Above that, a ready-mix truck is faster, cheaper per yard, and produces better results because the entire slab is poured from a single batch.
- 80 lb bag yields ~0.6 cubic feet when mixed (about 45 bags per cubic yard)
- 60 lb bag yields ~0.45 cubic feet (about 60 bags per cubic yard)
- 40 lb bag yields ~0.3 cubic feet (about 90 bags per cubic yard)
Pro Tip: For slabs over 1.5 cubic yards, always order ready-mix. Mixing 70+ bags by hand is exhausting, takes hours, and risks cold joints (where partially-set concrete meets fresh concrete, creating a weak seam).
Why 4 Inches Is the Standard Thickness
Most residential slabs — patios, sidewalks, garage floors — are poured at 4 inches thick. This provides adequate strength for foot traffic and light vehicles. Driveways should be 5–6 inches to handle car weight. Post footings and structural foundations may require 8–12 inches.
Common Mistakes
1. Not Accounting for Subgrade
The ground under your slab is rarely perfectly level. Low spots, soft areas, and excavation irregularities mean you'll use more concrete than the simple L × W × D calculation suggests. The 10% overfill factor covers this.
2. Forgetting Rebar or Wire Mesh
Concrete is strong in compression but weak in tension. Rebar or welded wire mesh prevents cracking from ground movement. For a 4" slab, #3 rebar on 18" centers or 6×6 welded wire mesh is standard.
Watch Out: Concrete sets fast — you have about 90 minutes of working time in warm weather. Have all your forms, tools, and helpers ready before the truck arrives. There is no "pause" button on a concrete pour.
How Much Does Concrete Cost?
- Bags: $4.50–$6.50 per 80 lb bag (~$180–$300 per cubic yard)
- Ready-mix truck: $120–$180 per cubic yard (minimum delivery of 1 yard, short-load fees for under 5 yards)
For our 10×10×4 slab (1.36 cy): bags cost about $280, ready-mix about $200–250 delivered. The truck wins on both cost and quality for anything above 1 cubic yard.
Punch your dimensions into the calculator above, choose your bag size, and get the exact number you need.