Concrete Calculator

Frequently Asked Questions

How do I calculate concrete volume?

Volume (cubic yards) = length × width × thickness in feet, divided by 27. So a 10×10×0.33 ft slab = 33 ÷ 27 ≈ 1.22 cubic yards.

How many bags of concrete do I need?

An 80 lb bag yields about 0.6 cubic feet. So 1 cubic yard (27 ft³) needs about 45 bags. For larger pours, ready-mix delivery is usually cheaper.

Why add a waste factor?

Spillage, uneven excavation, and over-pouring at edges typically waste 5-10%. Running short mid-pour creates a cold joint, so order extra.

What is the typical thickness for concrete slabs?

Patios and walkways: 4 inches. Driveways and garage floors: 4-6 inches. Heavy vehicle areas: 6-8 inches with rebar reinforcement.

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This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

Ordering concrete comes down to one number - volume - and getting it wrong is expensive in both directions: too little means a cold joint when the truck leaves mid-pour, too much means paying for material you shovel into a waste pile. Choose your shape (slab, footing, or column/cylinder), pick your units, and enter the dimensions in feet (with optional inches) or in meters; this tool computes the volume in cubic feet, cubic yards, and cubic meters, then converts that into the number of 60 lb and 80 lb pre-mix bags you would need. Cubic yards is the unit ready-mix trucks deliver in, while bag counts matter for small DIY pours, so the calculator gives you both at once. Bag counts are always rounded up - you cannot buy a fraction of a bag.

Formula / Method

For a slab or footing, volume = length × width × depth. For a column or cylinder, volume = π × r² × height, where r is half the diameter. Inch inputs are divided by 12 to convert to feet before the multiplication. Cubic yards = cubic feet ÷ 27 (since a cubic yard is 3 ft × 3 ft × 3 ft = 27 ft³). Bag counts use standard yields: a 60 lb bag makes about 0.45 ft³, so bags = ⌈cubic feet ÷ 0.45⌉ (about 60 bags per cubic yard); an 80 lb bag makes about 0.60 ft³, so bags = ⌈cubic feet ÷ 0.60⌉ (about 45 bags per cubic yard).

Worked Example

A patio slab 10 ft × 10 ft, 4 inches thick. Convert the depth: 4 ÷ 12 = 0.333 ft. Volume = 10 × 10 × 0.333 = 33.3 ft³ = 33.3 ÷ 27 ≈ 1.23 cubic yards. In bags that is roughly 33.3 ÷ 0.45 ≈ 74 sixty-pound bags, or 33.3 ÷ 0.60 ≈ 56 eighty-pound bags. At over a cubic yard, ordering ready-mix from a truck is faster and cheaper than mixing 50-plus bags by hand. A 12-inch-diameter post hole 3 ft deep is π × 0.5² × 3 ≈ 2.36 ft³ ≈ 0.087 cubic yards.

Common Mistakes

Mixing units - entering length in feet but thickness in inches without converting; this tool handles it only if you set the unit selector correctly.

Using diameter as radius for columns - the radius is half the diameter, and forgetting that quadruples the volume.

Not adding waste - uneven subgrade, spillage and edge overpour mean you should order about 10% extra.

Forgetting 1 cubic yard = 27 cubic feet, not 3 - a linear-vs-cubic slip that understates the order ninefold.

Rounding bag counts down - always round up; running short mid-pour ruins the job.

Tips & FAQ

Bags or ready-mix? For pours under about one cubic yard, bags are practical; above that, ready-mix delivered by the cubic yard is faster, more consistent and usually cheaper. How much extra should I order? Add roughly 10% to the calculated volume for subgrade variation, spills and edge overpour. What is a standard mix ratio? About 1 part cement : 2 parts sand : 3 parts gravel by volume for general-purpose concrete. How long does it take to cure? Concrete gains about 70% of its design strength in the first 7 days and reaches full strength at 28 days; keep it moist and above 50°F during curing to prevent cracking. Why both bag sizes? An 80 lb bag yields more per bag (fewer bags, less mixing) but is heavier to handle; 60 lb bags are easier to lift but you buy more of them - the calculator shows both so you can weigh effort against trips to the store. Is this an engineering spec? No - it is a material estimate; structural pours need a designed mix and rebar per local code.

Concrete Calculator

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