How Much Concrete Do I Need? Slabs, Footings and Post Holes

To figure out how much concrete you need, multiply length × width × thickness in feet to get cubic feet, then divide by 27 to get cubic yards. A 10 × 12 ft patio slab 4 inches thick needs 10 × 12 × (4 ÷ 12) = 40 ft³, which is 40 ÷ 27 = 1.48 cubic yards. Add 5 to 10 percent for waste, then round up to your supplier’s ordering increment or to whole bags.

The concrete volume formula

Thickness is almost always given in inches, so convert it first: 4 in = 0.333 ft, 5 in = 0.417 ft, 6 in = 0.5 ft. A cubic yard is 3 × 3 × 3 = 27 cubic feet, which is why you divide by 27, not by 3.

Example 1: a patio slab

A 10 ft × 12 ft patio, 4 inches thick.

Seventy-four 80 lb bags weigh nearly 6,000 pounds and take hours to mix by hand. At this size, compare the delivered price of ready-mix.

Example 2: a driveway

A 24 ft × 20 ft two-car driveway, 5 inches thick.

Ready-mix is ordered in yards, typically in quarter- or half-yard increments, and a standard truck carries about 9 to 11 yards, so this pour fits in one load.

Example 3: a continuous footing

A footing 2 ft wide and 1 ft deep running 60 ft around a garage.

Footings are dug into soil, which is rarely perfectly straight, so a 10% allowance is the minimum here.

Example 4: fence post holes

Twelve holes, each 10 inches across and 36 inches deep, for 4 × 4 posts (actual size 3.5 × 3.5 in). Each hole is a cylinder, minus the post.

The cylinder formula comes from how to calculate volume. The concrete calculator handles slabs, footings, columns, tubes, curbs and steps.

Bag yields

Yields vary slightly by brand and mix, so use the figure printed on the bag. High-strength and fast-setting mixes may yield a little less.

Bags or ready-mix?

The break-even point is usually somewhere around one cubic yard, but price is only part of the decision.

• Labor. Each 80 lb bag must be lifted, opened and mixed. Forty-five bags per yard is a long, heavy day even with a rented mixer.
• Consistency. Hand-mixed batches vary in water content, and a large slab poured in batches can develop cold joints where fresh concrete meets concrete that has begun to set.
• Access. A ready-mix truck needs a firm path close to the forms; otherwise you need wheelbarrows or a pump truck, which adds cost.
• Minimum orders. Suppliers often charge a short-load fee below a few yards, which can make a one-yard delivery surprisingly expensive.

For post holes and small pads, bags win. For anything poured in one continuous slab over about a yard, get a ready-mix quote.

How thick should it be?

The International Residential Code (IRC) sets the 3½-inch minimum for concrete floor slabs on the ground and specifies minimum compressive strengths by use and climate, from 2,500 psi for protected interior slabs up to 3,500 psi for garage floors, porches and steps in severe-weathering regions. The American Concrete Institute’s residential standard, ACI 332, covers similar ground in more detail. These figures are starting points; your local building department, plans or engineer set the actual requirements.

What else to plan for

• Base material. Most slabs sit on 4 inches of compacted gravel. For the 10 × 12 patio, that is another 40 ft³, about 1.5 yd³ of gravel; the gravel calculator converts it to tons.
• Reinforcement. Rebar or welded wire mesh controls cracking; the rebar calculator estimates bar lengths for a grid.
• Weight. Normal concrete weighs about 145 to 150 lb per cubic foot, roughly 4,000 lb per cubic yard. A pickup’s payload is often under a yard of wet concrete or bags.
• Uneven ground. Measure depth at several points. A slab that averages 4½ inches instead of 4 needs 12.5% more concrete.
• Curing. Keep fresh concrete moist and protected from freezing. Design strength is specified at 28 days, though light foot traffic is usually fine after a day or two.

Converting units

The patio’s 1.63 yd³ is about 1.25 m³. The cubic feet to cubic yards converter and cubic yards calculator handle the arithmetic for any dimensions.

Common mistakes

• Leaving thickness in inches when multiplying by feet, which overstates volume twelvefold.
• Dividing by 3 instead of 27 to get cubic yards.
• Ordering the exact amount with no allowance for waste or uneven grade.
• Forgetting the post’s volume in post holes, or the hole’s flare at the bottom.

To work out the floor area first for irregular slabs, see how to calculate square footage. For walls built from block instead of poured concrete, the concrete block calculator counts blocks and mortar.