Concrete Slab Volume Basics

Formula

ft³ = L × W × (thickness_in / 12). yd³ = ft³ / 27. order = yd³ × (1 + waste).

Units

Length and width in feet; thickness usually inches → divide by 12.

Step-by-step

1. Measure length and width of the pour area.
2. Confirm slab thickness (and thickened edges separately if needed).
3. Compute ft³, then yd³.
4. Add waste (often 5–10%) for subgrade and pump loss.

Example — 10×12 ft at 4 in

10 × 12 × (4/12) = 40 ft³ ≈ 1.48 yd³. With 8% waste ≈ 1.60 yd³ ordered.

Example — 20×24 ft garage at 6 in

20 × 24 × 0.5 = 240 ft³ ≈ 8.89 yd³. With 5% waste ≈ 9.3 yd³ — round to plant increments.

Common mistakes

• Using inches for length/width in the same product as feet.
• Ignoring thickened edges, turn-downs, or beam pockets.

Practical notes

• Ready-mix has minimum loads — small slabs may still be bag territory.
• After volume, use the bag calculator if you are not ordering a truck.

Related calculator

Use the free Concrete Volume Calculator to run the same math with your field numbers.

Related tools

• Concrete Volume Calculator — Cubic feet and yards for slabs, walls, columns, and round forms.
• Concrete Bag Calculator — How many bags of concrete for slabs, footings, and round pours.

Related guides

• How Many Bags of Concrete Do I Need? — Estimate bag count from pour volume and bag yield.
• Concrete Bags for Post Holes — Estimate bags for round post-hole footings.
• 80 lb vs 60 lb Concrete Bags — How bag size changes count for the same pour.