Concrete Calculator
Work out how many cubic yards of concrete, or how many 80 lb and 60 lb bags, a slab needs from its length, width, and thickness.
Estimates are for planning. Confirm quantities against your measured site and product packaging before ordering, and follow local building codes.
How to measure for a concrete slab
Grab a tape measure and get three numbers: length, width, and thickness. Length and width go in feet. Measure the actual formed area, not the rough yard space, because an inch here and there changes the order more than you’d think.
Thickness goes in inches. For a patio or walkway that’s typically 4 inches. Measure down from the top of your forms to the compacted gravel base, and check it in several spots. If the base dips to 4.5 inches in places, use the deeper number. Concrete fills the hole you actually dug, not the one on paper.
For odd shapes, break the area into rectangles, calculate each one, and add them up. An L-shaped patio is just two rectangles. A circle is close enough to a square at 0.785 × diameter × diameter. When in doubt, round dimensions up to the nearest half foot.
One more thing before you measure: a slab needs a compacted gravel base underneath, usually around 4 inches, plus reinforcement (wire mesh, fiber, or rebar) per your local code. Dig deep enough for both the gravel and the concrete.
The formula and what it assumes
The calculator multiplies length in feet by width in feet by thickness converted to feet (inches divided by 12), then divides by 27 to convert cubic feet into cubic yards. So a 10×10 patio at 4 inches is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, or 1.23 cubic yards.
Then it adds your selected waste percentage, 10% by default. That covers uneven subgrade, spillage, and the concrete left coating the mixer and wheelbarrow. With the buffer, that 10×10 patio needs about 1.36 cubic yards.
For bagged concrete, the math uses the yields printed on the bags: an 80 lb bag makes 0.60 cubic feet of concrete, and a 60 lb bag makes 0.45 cubic feet. The calculator divides your total volume (waste included) by the per-bag yield and rounds up. The same patio takes 62 of the 80 lb bags.
The formula assumes a uniform rectangular slab. It doesn’t account for thickened edges, footings, or post holes, so calculate those separately and add them in.
Buying tips
US stores sell concrete mix mainly in 80 lb, 60 lb, and 50 lb bags. The 80 lb bag is the workhorse and runs roughly $5 to $7 at big-box stores depending on the mix. High-early-strength and crack-resistant blends cost more than plain all-purpose mix.
Here’s the threshold that matters: at around one cubic yard, stop buying bags and call a ready-mix plant. One yard means hauling and hand-mixing 45 of the 80 lb bags, about 3,600 pounds, and the concrete from your first batch starts setting while you’re still mixing batch twenty. Ready-mix runs roughly $125 to $175 per cubic yard delivered as of 2026, though small orders carry a short-load fee, often $40 to $60 per yard, since trucks hold about 10 yards and plants price partial loads accordingly.
Pros order a quarter yard more than the calculation, schedule the truck for early morning, and have the forms, base, and crew ready before it arrives. Most plants charge standby time if the truck waits on site, so don’t plan to screed as you learn.
If you do go with bags, rent a mixer. A drum mixer turns a miserable two-day wheelbarrow job into a steady afternoon, and consistent water ratios across batches give you stronger, more uniform concrete.
A note on curing
Concrete reaches working strength over days, not hours. Keep foot traffic off for 24 to 48 hours, vehicles off for about a week, and keep the surface damp for the first few days. Slabs also need control joints cut or tooled in, spaced roughly 8 to 12 feet apart for a 4 inch slab, so the inevitable cracks happen on your lines instead of across the middle.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
45 bags. Each 80 lb bag yields 0.60 cubic feet of mixed concrete, and a cubic yard is 27 cubic feet, so 27 ÷ 0.60 = 45 bags. With 60 lb bags (0.45 cubic feet each) you'd need 60 bags. That's over 3,500 pounds of material either way, which is why most people switch to ready-mix delivery at the one-yard mark.
How thick should a concrete slab be?
4 inches is the standard for patios, walkways, and shed slabs. Driveways that carry passenger vehicles are usually 4 inches too, but bump to 5 or 6 inches if trucks or RVs will park on it. Footings and structural slabs are governed by local code, so check with your building department before you pour.
Why does the calculator add 10% waste?
Subgrades are never perfectly flat. A gravel base that's half an inch low across a 10×10 slab swallows more than 4 cubic feet of concrete, and spillage during placement adds more loss. Running short mid-pour creates a cold joint, which is a structural weak line, so the 10% buffer is cheap insurance.
Do I need rebar or wire mesh in my slab?
For most residential slabs on grade, reinforcement requirements come from your local building code, not a national rule. Common practice is welded wire mesh or fiber-reinforced mix for patios and #3 or #4 rebar grids for driveways. A compacted gravel base of about 4 inches under the slab matters just as much as the steel.