Concrete Calculator

Pick slab, footing, or column, enter the dimensions, and get the concrete you need in cubic yards, cubic feet, and 60-lb or 80-lb bags — always rounded up, with a 10% ordering margin shown alongside.

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How the concrete math works

Concrete is ordered by volume, so every pour comes down to the same move: convert all your dimensions to feet, multiply them into cubic feet, and divide by 27 to get cubic yards — the unit ready-mix plants and most suppliers quote. The trap is mixed units: slab thickness and footing widths are measured in inches, so they get divided by 12 first. This calculator handles the conversions and always rounds the yardage up, because in concrete work the only unforgivable estimate is a low one.

The formulas

Slab: L × W × (T ÷ 12)   ·   Footing: L × (W ÷ 12) × (D ÷ 12)   ·   Column: π × (dia ÷ 24)² × H × qty

All results are in cubic feet; divide by 27 for cubic yards. L, W, and H are in feet; slab thickness T, footing width W and depth D, and column diameter are in inches. For columns, dia ÷ 24 converts the diameter in inches to a radius in feet.

Worked example

A 10 × 10 ft patio slab, 4 inches thick: 10 × 10 × (4 ÷ 12) = 33.33 cu ft, which is 33.33 ÷ 27 = 1.2346 → 1.24 cu yd rounded up. With a 10% waste margin you'd order about 1.36 cu yd — in practice, ask the plant for 1.5.

In bags, that same slab is 75 sixty-lb bags (at 0.45 cu ft each) or 56 eighty-lb bags (at 0.60 cu ft each) — over two tons of material to haul and mix. This pour is squarely in ready-mix territory.

Never order exact — the cold-joint rule

Order 5-10% more than the calculated volume, on purpose and every time. Subgrades dip, forms flex outward under the weight, and wheelbarrows and chutes keep their share. The calculator shows the exact number and a +10% row so you can see both — the padding is a decision you make, not something hidden in the math. Here's why it matters more than the cost of the extra half yard: if you run short mid-pour, the first batch starts curing before the make-up load arrives, and the seam between them — a cold joint — is a permanent weak plane that cracks first and lets water in. Extra concrete costs a few dollars; a cold joint is forever. Over about one cubic yard, skip the bags entirely and order ready-mix: 45+ bags means hours of mixing, inconsistent water ratios, and the last batch poured an hour after the first — which is its own cold-joint risk.

Frequently asked questions

How much concrete do I need for a 10x10 slab?

A 10 × 10 ft slab at the standard 4 inches thick is 33.33 cubic feet — 1.24 cubic yards. Order about 1.4 yards to cover an uneven subgrade and spillage; ready-mix companies sell in quarter-yard increments, so ask for 1.5.

How many bags of concrete make a cubic yard?

A 60-lb bag yields about 0.45 cubic feet and an 80-lb bag about 0.60, so one cubic yard (27 cubic feet) takes 60 of the 60-lb bags or 45 of the 80-lb bags. That's 3,600 lbs of material either way — which is why bags stop making sense fast.

Should I order ready-mix concrete or buy bags?

The usual break-even is about one cubic yard. Below that, bags are cheaper and simpler; above it, mixing 45+ bags by hand is brutal work and the batch-to-batch consistency suffers. Ready-mix trucks charge a short-load fee for small orders, so right around a yard, price both.

How much extra concrete should I order?

Add 5-10% over the calculated volume. Subgrades are never perfectly flat, forms bow slightly, and some concrete always stays in the truck or wheelbarrow. Running short is far more expensive than a little extra — a pour that stops and restarts creates a cold joint, a permanent weak line.

How thick should a concrete slab be?

4 inches is standard for patios, sidewalks, and shed slabs. Go to 5-6 inches for driveways or anything that carries vehicles, and thicken the edges. Going from 4 to 6 inches adds 50% more concrete, so measure your real thickness rather than guessing.

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