Construction

Concrete Calculator

Estimate the volume of concrete needed for any slab, footing, or pour. Results in cubic feet, cubic yards, and pre-mix bags.

Enter slab dimensions to estimate concrete volume and bags needed.

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Concrete Volume Formula

Concrete volume is calculated from the slab dimensions. Depth is entered in inches and converted to feet.

Volume (ft³) = Length × Width × (Depth ÷ 12)

Volume (yd³) = ft³ ÷ 27

Bags = ⌈ ft³ ÷ yield per bag ⌉

Example: 10 ft × 10 ft × 4 in = 33.3 ft³ = 1.23 yd³ ≈ 56 × 60 lb bags

Concrete Buying Tips

Pre-mix concrete bags (40, 60, or 80 lb) are ideal for small projects under 1 cubic yard. For larger pours, ordering ready-mix concrete from a local supplier is more economical and avoids the labour of mixing dozens of bags.

Always add 5–10% to your calculated volume when ordering. Uneven subgrades and minor over-pour are inevitable. Running out of concrete mid-pour creates a cold joint — a structural weak point — so it is always better to have a little extra.

Frequently asked questions

How do I calculate how much concrete I need?
Multiply the length (ft) × width (ft) × depth (in inches, converted to feet by dividing by 12) to get the volume in cubic feet. Divide by 27 to convert to cubic yards. For pre-mix bags, divide the cubic feet volume by the yield per bag (0.45 ft³ for a 60 lb bag or 0.60 ft³ for an 80 lb bag). Always round up to the nearest whole bag.
How many bags of concrete are in a cubic yard?
One cubic yard (27 cubic feet) requires approximately 45 bags of 60 lb pre-mix concrete or 34 bags of 80 lb pre-mix. Ready-mix concrete delivered by truck is sold by the cubic yard and is more cost-effective for pours larger than about 1 cubic yard. For smaller DIY projects, pre-mix bags are more practical.
What is the standard concrete slab thickness?
Standard residential concrete slabs are typically 4 inches thick for patios, sidewalks, and garage floors with light use. Driveways carrying vehicle loads should be at least 4–6 inches. Structural slabs and those exposed to freeze-thaw cycles are often poured at 6 inches. Footings are usually 8–12 inches deep depending on frost depth and load.
How many cubic yards is a typical 10×10 concrete slab?
A 10 ft × 10 ft slab poured at 4 inches (0.33 ft) deep is 100 × 0.33 = 33.3 cubic feet, or about 1.23 cubic yards. At that volume, you would need approximately 56 sixty-pound bags or 42 eighty-pound bags of pre-mix concrete. Add 5–10% extra for spillage and uneven subgrades.
Should I add extra concrete to my estimate?
Yes — always order 5–10% more concrete than your calculated volume. Uneven subgrades, spillage during pouring, and slight measurement errors can consume the extra margin. For ready-mix deliveries, concrete companies typically recommend over-ordering by at least half a yard to avoid running short mid-pour, since partial loads cost nearly as much as full ones.

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