Concrete Slab Calculator

This concrete slab calculator returns the exact volume you need to pour a slab, pad, or floor: cubic yards, cubic feet, and the matching bag count in 40-lb, 60-lb, and 80-lb mixes. It works the same way whether you're pouring a patio slab, a shed pad, a garage floor, or a driveway — enter length, width, and thickness in the calculator above and the math below shows exactly how those numbers come together.

How to Calculate Concrete for a Slab

The formula is the same for any rectangular pour: length × width × thickness, converted to cubic yards because that's how ready-mix is sold by volume.

1. Convert thickness from inches to feet: divide by 12. A 4-inch slab is 4 ÷ 12 = 0.333 ft.
2. Multiply length × width × thickness (all in feet) to get cubic feet.
3. Divide cubic feet by 27 — there are 27 cubic feet in a cubic yard — to get cubic yards.

Worked example, a 20×20 slab at 4 inches: 20 × 20 × 0.333 = 133.3 cubic feet ÷ 27 = 4.94 cubic yards. That's how you figure concrete for a slab of any size — plug your own length, width, and thickness into the calculator above, or work it by hand with this three-step formula. Add 10% for waste (see below) before you order.

Thickness matters more than people expect: going from 4 inches to 6 inches adds 50% to your concrete volume for the same footprint. The IRC sets a 3.5-inch minimum for interior residential slabs (R506.2.3), and 4 inches is the standard for patios and most garage floors, so confirm your required thickness before you lock in an order.

Concrete Slab Yardage Chart

Cubic yards needed at three common thicknesses, calculated as length × width × (thickness ÷ 12) ÷ 27. Use this table to sanity-check the calculator or plan a pour without re-entering numbers.

Figures are bare volume; add 10% waste before you place an order (the section below covers exactly how much).

How Many Bags of Concrete for a Slab

One cubic yard is 27 cubic feet, and each bag size yields a fixed volume: an 80-lb bag yields about 0.60 cu ft, a 60-lb bag about 0.45 cu ft, and a 40-lb bag about 0.30 cu ft. Divide 27 by the yield and you get bags per cubic yard:

• 80-lb bags: 45 per cubic yard
• 60-lb bags: 60 per cubic yard
• 40-lb bags: 90 per cubic yard

Bagged mix stops making sense above roughly 1 cubic yard (about 45 bags of 80-lb mix) — past that, hauling, mixing, and pouring bag by bag takes far longer than a ready-mix truck, and the pour is more likely to show cold joints between batches.

Bag counts above are bare volume, rounded up to the next whole bag; add roughly 10% more bags to match the waste factor used for ready-mix orders.

Concrete Pad vs Slab vs Floor

'Pad,' 'slab,' and 'floor' all use the same math — length × width × thickness ÷ 27 — with different typical thicknesses baked in by convention:

• Pad (AC unit, shed base, mailbox post): usually 4 inches, sometimes less for a small equipment pad.
• Slab (patio, walkway, general-purpose pour): 4 inches is standard.
• Floor (garage, workshop, basement): 5–6 inches to carry vehicle or heavy-equipment loads.

A concrete pad calculator and a concrete floor calculator return the exact same number as this one for the same dimensions — the only thing that changes between a shed pad and a garage floor is the thickness you enter.

Measuring an Irregular Slab

Most pads are simple rectangles, but an L-shaped patio, a stepped foundation, or a round fire pit base need a different approach. Split an L- or U-shaped pour into two or more rectangles, run each through the same length × width × thickness ÷ 27 formula, and add the results together. For a triangular section, use (base × height ÷ 2) for the area before multiplying by thickness; for a circular pad, use π × radius². Where thickness varies — a sloped apron or a thickened edge — measure the average depth across the pour so you don't under-order.

Slab Material Calculator: Beyond the Concrete

A full slab material calculation covers more than the pour itself:

• Gravel base: most slabs sit on 4–6 inches of compacted gravel for drainage and a stable subgrade. As a rule of thumb, a 4-inch gravel base needs about the same cubic yardage as a 4-inch concrete slab of the same footprint — a 10×10 pad needs roughly 1.23 cubic yards of gravel. Size it exactly with the gravel calculator.
• Rebar or mesh: wire mesh handles most 4-inch residential slabs; driveways and garage floors typically call for rebar on a grid. Run your dimensions through the rebar calculator.
• Vapor barrier: a 6-mil (or thicker) poly sheet under the slab blocks ground moisture — size it to your footprint plus overlap at the seams.

Order all three alongside your concrete so the crew isn't waiting on a delivery mid-pour.

Waste Factor and Ordering

Add 10% to your bare volume before ordering — it covers spillage, an uneven subgrade, and form overfill, and it's the standard allowance contractors use. For the 20×20 example above, 4.94 cubic yards becomes 5.43 cubic yards ordered.

Ready-mix plants sell by the quarter yard, so round your padded total up — 5.43 becomes 5.5 cubic yards. Most plants also charge a short-load fee below a minimum order (commonly 3–10 cubic yards, depending on the plant), so it's worth combining nearby pours or checking the minimum before you schedule a small delivery. Confirm truck access, too — a standard ready-mix chute reaches about 18 feet from the curb, so a pad set further back may need a pump or a wheelbarrow relay, which adds time to the pour.

Next Steps

Once you have your yardage, the concrete cost calculator turns that volume into a full material-and-labor number. For more on the math behind this page, see how many bags of concrete you need, how to calculate concrete, and what is a yard of concrete.