Concrete Calculator (Cubic Yards & Bags)
Enter the length, width, and thickness of your slab and this calculator returns the volume in both cubic feet and cubic yards, adds a waste allowance, and tells you exactly how many bags of ready-mix you'd need at your chosen bag size. Use it to size a ready-mix truck order or to decide how many 60-lb or 80-lb bags to load in the cart.
Slab dimensions
Waste & bag size
60-lb bag ≈ 0.45 ft³ · 80-lb bag ≈ 0.60 ft³
Concrete needed
1.23 yd³
33.33 ft³ before waste · 36.67 ft³ with 10% waste
Bags at 0.45 ft³
82
Cubic feet (padded)
36.67
Volume & bag detail
Bags needed by bag size
How it works
Volume is built from the slab's footprint and depth. Thickness is entered in inches and converted to feet (thicknessInches ÷ 12), then volume in cubic feet is length × width × that thickness. Cubic yards — the unit ready-mix suppliers price by — is simply the cubic-foot figure divided by 27, since a cubic yard is 3 ft × 3 ft × 3 ft. A 10 ft × 10 ft pad at 4 inches works out to about 33.3 cubic feet, or roughly 1.23 cubic yards.
The waste percentage pads the volume before the bag count is figured, because real pours lose material to spillage, uneven subgrade, over-excavation, and forms that aren't perfectly true. A 5–10% allowance is typical for a flat slab on well-prepared ground; add more for footings, sloped sites, or hand-mixing. The bag count uses the padded volume, so the waste buffer flows straight through to how many bags you buy.
Bags are counted from each bag's yield — the cubic feet of cured concrete one bag makes. A standard 60-lb bag of ready-mix yields about 0.45 ft³ and an 80-lb bag about 0.60 ft³, though exact yields vary by brand and are printed on the sack. The calculator divides your waste-inclusive volume by the yield and rounds up, since you can't buy a fraction of a bag, and shows the count for both common bag sizes so you can compare.
Frequently asked questions
How much extra concrete should I order?+
A common rule of thumb is to add 5–10% to your calculated volume. Concrete can't be topped up once a pour starts to set, so running short means a cold joint or a second delivery — both far worse than a little leftover. The overage covers spillage, a subgrade that's slightly low in spots, forms that bow outward under the weight of wet concrete, and the material left clinging to the chute or mixer. For footings and trench work where the excavation width is hard to control, many contractors budget 10% or more. This tool applies whatever waste percentage you enter to the volume before counting bags.
Should I use bags or order a ready-mix truck?+
The practical crossover is around one cubic yard. Below that, mixing bags by hand or in a portable mixer is usually cheaper and avoids short-load fees. Above roughly a cubic yard — which is about 60 sixty-pound bags or 45 eighty-pound bags — the bag route becomes a lot of heavy lifting and mixing, and a ready-mix truck delivering consistent, professionally batched concrete is generally faster and more reliable. Ready-mix suppliers price by the cubic yard and often charge a short-load fee under a minimum (frequently 1 yard), so check that threshold before deciding. This calculator gives you both the cubic-yard figure for a truck order and the bag count for the DIY route.
How thick should my slab be?+
Thickness depends on the load, not just the area, so this is a sizing aid rather than engineering advice. As a general guide, 4 inches is typical for patios, walkways, and shed floors; 5–6 inches is common for driveways and slabs that carry vehicles; and heavier loads or poor soils call for a thicker slab with reinforcement designed by a professional. Local building codes, frost depth, and soil conditions all matter, and reinforcement (rebar or mesh) and a proper compacted base are as important as the depth. Confirm the right thickness for your project with your local code officials or a structural engineer before you pour.