How to use it
- Enter the slab length in feet.
- Enter the slab width in feet.
- Enter the thickness in inches.
- Click Calculate to see cubic yards, bags, and cubic feet.
The Concrete Foundation Calculator tells you exactly how much concrete you need for a slab or footing. Just enter the length, width, and thickness of the pour. It instantly shows the volume in cubic yards (what ready-mix companies sell), the number of 80-lb bags (for DIY mixing), and cubic feet (for comparing with mixer capacities). This matters because over-ordering wastes money and under-ordering causes delays and weak cold joints. The result uses the standard yield: one 80-lb bag makes about 0.6 cubic feet. You can read the numbers directly — if it says 5.3 bags, you'll need to buy 6. For larger jobs, the cubic yard figure tells you how much to order from a ready-mix supplier. No guesswork, no shortfall. Just accurate quantities for a smooth, professional pour.
Pro tips
- Add 10% waste allowance to account for spillage, uneven subgrade, or residue in bags.
- Round bag count up to the nearest whole bag — never round down.
- For large slabs, order ready-mix by the cubic yard and confirm minimum delivery amounts.
- Double-check thickness in inches — a 4-inch vs 6-inch slab changes volume by 50%.
Real-life examples
- A homeowner planning a 20x30 ft patio slab and wanting to know how many bags to haul.
- A contractor pricing a concrete foundation for a 40x24 ft garage addition.
- A DIYer building a 6x6 ft shed base and comparing bagged vs delivered concrete costs.
Case study
Before, Mark guessed he needed 20 bags for his 10x10 ft slab — he ran out mid-pour. After using this calculator, he saw the exact need: 4 inches thick gives 3.7 cubic yards, which is 167 bags. He ordered ready-mix instead and finished in one day, no gaps.