🧱 Concrete Calculator

What This Calculator Does

This calculator sizes a concrete pour. Enter the slab's length and width in feet and its thickness in inches, and it returns the volume in cubic feet and cubic yards — the unit ready-mix trucks sell — plus the bag counts for 60 lb and 80 lb bagged mix. It answers the two forms of the same question every slab project asks: how many yards to order, or how many bags to haul.

Concrete is unforgiving of estimation errors in a way mulch is not: run short mid-pour and the slab gets a weak cold joint; order far over and you pay for material you cannot store. The volume math is straightforward, and doing it exactly — then adding a deliberate margin — is how professionals order.

The Volume Math of a Pour

A slab is a rectangular prism: length × width × thickness. Work in feet — a 4-inch thickness is 4/12 = 0.333 ft — so a 10 × 10 ft slab at 4 inches is 10 × 10 × 0.333 = 33.3 cubic feet. Ready-mix sells by the cubic yard (27 cu ft), making that slab 1.23 yards. Footings, curbs, and columns follow the same logic with their own dimensions; complex layouts decompose into prisms that sum, exactly like square-footage jobs.

Bagged mix translates through yield: an 80 lb bag makes about 0.60 cu ft of concrete, a 60 lb bag about 0.45 (a 40 lb bag about 0.30). The example slab therefore needs 33.3/0.6 ≈ 56 eighty-pound bags — over two tons of material to move, mix, and place by hand. That arithmetic is itself the decision tool: somewhere around 1 cubic yard, most people conclude that a ready-mix truck (or a haul-and-tow trailer) beats fifty-plus bags, and the calculator shows both numbers precisely so the crossover is obvious for your job.

Thickness, Strength, and Getting the Order Right

Thickness is the structural decision embedded in the volume. Four inches is the residential standard for patios, sidewalks, and shed slabs; driveways and anything bearing vehicles step up to 5-6 inches; heavy-duty applications go further with engineering input. Because volume scales linearly with thickness, a 6-inch driveway needs half again the concrete of a 4-inch patio of the same footprint — worth knowing before pricing. Reinforcement (wire mesh or rebar) and a proper compacted gravel base contribute as much to slab longevity as the concrete itself, and neither appears in the volume but both belong in the plan.

Order with a margin: 5-10% over the computed volume is standard, absorbing subgrade dips, form flex, and spillage. For ready-mix, communicate the yardage plus margin and confirm the mix strength (3,000-4,000 PSI covers most residential work) and any fiber or air-entrainment for freeze-thaw climates. For bagged pours, buy the extra bags — unmixed bags return or store, while a mid-pour shortage does not. And respect the clock: concrete begins setting on schedule regardless of your readiness, so forms, tools, and help should be staged before the first drop of water or the truck's arrival. The calculator handles the quantity; preparation handles everything else.

Bags or the Truck: A Practical Crossover

The bag-versus-truck decision is mostly quantity, partly logistics. Below roughly half a yard — post footings, a small pad, repairs — bags are unbeatable: buy exactly what the math says plus a couple spare, mix in a wheelbarrow or mixer, done. Between half a yard and a yard, bags remain feasible but become genuine labor; a rented mixer earns its fee. Past a yard, the fifty-bag arithmetic, the mixing time (each batch must be placed before the previous stiffens), and consistency across batches all argue for ready-mix — a truck delivers uniform, properly mixed concrete at a pace matched to finishing.

Costs compare more closely than people expect once bag counts climb, because bagged concrete carries a hefty per-yard premium; the truck's advantage grows with volume, offset by delivery fees and possible short-load charges below their minimum. Access matters too — trucks need approach room, while bags reach the fenced backyard. Whichever route, the workflow is identical: compute the true volume here, add the margin, choose the delivery form the number recommends, and pour once. A slab poured continuously from a correct order is the entire objective of this arithmetic.

Quick Tips

• Volume = L × W × (thickness ÷ 12) in feet; ÷ 27 for cubic yards.
• 80 lb bag ≈ 0.60 cu ft; 60 lb ≈ 0.45 — past ~1 yard, think ready-mix.
• 4 inches for patios and walks; 5-6 for driveways.
• Order 5-10% extra; a mid-pour shortage means a weak cold joint.

Frequently Asked Questions

How do I calculate concrete for a slab?

Length x width x thickness gives the volume: convert thickness to feet (inches / 12), multiply, and divide cubic feet by 27 for cubic yards. A 10 x 10 ft slab at 4 inches is 33.3 cu ft — about 1.23 cubic yards.

How many bags of concrete do I need?

An 80 lb bag yields about 0.60 cu ft and a 60 lb bag about 0.45. The 10 x 10 x 4-inch slab needs roughly 56 eighty-pound bags or 74 sixty-pound bags — which is why larger pours switch to ready-mix delivery.

How thick should a concrete slab be?

4 inches is the standard for patios, walkways, and shed bases; driveways carrying vehicles typically use 5-6 inches. Thickness drives volume directly, so the choice meaningfully changes the order.

Should I order extra concrete?

Yes — 5-10% over the calculation is standard practice. Subgrade irregularities and spillage consume more than the perfect-prism math, and running short mid-pour creates a cold joint that weakens the slab.

Exact, Practical, and Free

This calculator runs entirely in your browser, sizing pours exactly the moment you type. It needs no account or download and works on any device. Like every tool here, it is free to use as often as you like.

Related Tools

For more project math, the square footage calculator measures the footprint, the mulch calculator handles landscaping volume, and the percentage calculator adds the waste margin.