Concrete Calculator
Estimate how much concrete your slab needs in cubic yards plus the number of 60lb and 80lb bags to buy.
Example
A 10 ft × 10 ft slab at 4 in deep is 10 × 10 × (4 ÷ 12) = 33.33 ft³. With a 10% waste factor that is 36.67 ft³ ≈ 1.36 cubic yards, or about 82 sixty-pound bags / 62 eighty-pound bags.
How it works
Volume (ft³) = length × width × depth, with depth converted from inches to feet (in ÷ 12). Cubic yards = ft³ ÷ 27. Bag counts use yields of 0.45 ft³ per 60lb bag and 0.60 ft³ per 80lb bag, rounded up.
Good to know
The Concrete Calculator turns the dimensions of a rectangular slab into the volume of concrete you actually need to buy, expressed in both cubic yards and cubic feet, plus a count of how many 60 lb and 80 lb pre-mix bags to pick up. Enter length and width in feet, depth in inches, the number of identical slabs, and a waste percentage, and the result updates instantly as you type. It is aimed at DIY homeowners and small contractors pouring patios, walkways, shed pads, footings, or fence-post bases who want a quick parts list before a trip to the hardware store or a ready-mix order.
Reach for it during planning rather than at the mixer. Use the cubic-yards figure when you are ordering ready-mix from a truck (most suppliers quote and sell by the yard, often with a minimum load), and use the bag counts when the job is small enough to mix by hand. The "number of slabs" field is handy when you have several piers or pads of the same size, so you do not have to add them up yourself.
Read the big number as your headline order quantity and treat the bag counts as already rounded up and inclusive of your waste factor, so they reflect what to actually buy rather than the bare theoretical volume. Note how the two bag sizes compare: because an 80 lb bag yields more (about 0.60 ft³ vs 0.45 ft³ for a 60 lb bag), you need fewer of them, but they are heavier to carry and mix.
A practical caveat: this assumes a uniform, level slab of constant depth. Footings, sloped subgrade, or over-dug edges can swell real usage well beyond the math, so if your forms are deep or the ground is uneven, bump the waste field above the 10% default. For anything structural, confirm depth and concrete strength against local code or an engineer.
Frequently asked questions
How many 80lb bags of concrete are in a cubic yard?
One 80lb bag yields about 0.60 ft³, and a cubic yard is 27 ft³, so you need roughly 45 eighty-pound bags per cubic yard. For 60lb bags (about 0.45 ft³ each) it is closer to 60 bags per yard.
Should I add a waste factor for concrete?
Yes. Most pros add 5-10% to cover spillage, uneven subgrade, and over-excavation. This calculator includes an adjustable overage field, defaulting to 10%, which is applied to the volume before bag counts are computed.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser. Your inputs never leave your device, and it works offline once loaded.
Is this calculator free?
Yes, completely free with no sign-up and no limits.
People also ask
How do I calculate how much concrete I need for a slab?
Multiply length by width by depth (all in the same unit) to get cubic feet, converting depth from inches by dividing by 12, then divide cubic feet by 27 to get cubic yards. This tool does that automatically and adds your chosen waste factor.
How many cubic feet are in a cubic yard of concrete?
There are 27 cubic feet in one cubic yard. The calculator shows both figures so you can match whichever unit your supplier or bag label uses.
Should I buy 60 lb or 80 lb concrete bags?
80 lb bags cover more volume per bag (about 0.60 ft³ vs 0.45 ft³), so you handle fewer bags but each is heavier. 60 lb bags are easier to lift and mix but you need more of them; the tool shows both counts so you can decide.
How thick should a concrete slab be?
Thickness depends on use: many residential patios and walkways are commonly poured around 4 inches, while driveways or load-bearing areas are often thicker. Always check local building codes and the specific requirements for your project.
How much does a cubic yard of concrete cost?
Pricing varies by region, supplier, mix, and delivery minimums, and ready-mix is often sold by the yard with a minimum load charge. This calculator estimates quantity only, not cost, so request a current quote from a local supplier.
Why does my concrete order include extra for waste?
A waste or overage allowance covers spillage, uneven or over-excavated subgrade, and form variations so you do not run short mid-pour. The calculator applies your waste percentage (10% by default) to the volume before computing bag counts.
Can this calculator estimate concrete for footings or non-rectangular shapes?
It is built for rectangular slabs of uniform depth. For footings, columns, or irregular shapes you would break the project into rectangular sections, total them, or use a shape-specific method, since this tool assumes constant depth across a length-by-width area.
How many 60 lb bags of concrete make a cubic yard?
A 60 lb bag yields roughly 0.45 ft³ and a cubic yard is 27 ft³, so it takes about 60 sixty-pound bags to equal one cubic yard. Bag yields can vary slightly by brand.
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