How to use this concrete slab calculator
- Enter your project dimensions and choose US or metric units.
- Check the product coverage or yield on its packaging. Optional prices are in USD.
- Select Calculate to see your estimate. If you change measurements, calculate again.
- Save the valid result to your project board, copy its link or print it.
The calculation explained
Slab volume in cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12. Divide by 27 to convert to cubic yards. A purchase allowance can then be applied before estimating bag counts.
Worked example
A 16 × 12 ft slab, 4 inches thick, requires 16 × 12 × (4 ÷ 12) = 64 cubic feet, or about 2.37 cubic yards before a waste allowance.
What to check before you buy
This is a volume estimator, not a slab design tool. The selection of thickness, reinforcement, subbase and concrete strength depends on use, soil and building requirements. Bag yields are approximate; check product-specific packaging.
These figures are estimates, not final purchase orders, professional installation instructions or engineering advice. The product packaging and site conditions take precedence over default values. When in doubt, confirm with a qualified contractor, supplier or relevant professional.
Frequently asked questions
How many bags do I need?
The tool provides an approximate bag count using common 40 lb, 60 lb and 80 lb yield assumptions. A product label can specify a different yield.
Does concrete volume include waste?
Yes. The waste percentage is applied to theoretical slab volume before computing estimated bags and cubic yards.
Can I design a load-bearing foundation with this?
No. Structural design, local permits and reinforcement requirements require professional assessment.
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Metric worked example
A 4 × 3 m slab at 10 cm thickness requires 1.2 m³ before allowance, or 1.32 m³ with 10% extra. This is volume planning, not structural design.