How Much Concrete Do I Need? Slabs, Driveways & Footings

For a rectangular slab, you need length × width × thickness (all in feet) ÷ 27 cubic yards. A 10-by-10-foot patio, 4 inches thick, needs about 1.23 cubic yards — so order 1.5 cubic yards after a 10% waste allowance. For bags, one cubic yard takes about 45 eighty-pound bags.

That short answer covers most patios, shed bases, and sidewalks. The trouble starts when the shape is not a neat rectangle, when the thickness changes, or when you are choosing between bags and a ready-mix truck. A small measuring error gets expensive fast: concrete is heavy, it sets on its own schedule, and running short halfway through a pour is hard to fix. This guide gives you the formula, a table of common slab sizes, a full driveway example, and the waste, bag, and ordering details you need to get the number right.

What Does a Cubic Yard of Concrete Actually Mean?

Ready-mix suppliers sell concrete by volume, and the standard unit in the United States is the cubic yard: a cube 3 feet on each side, holding 27 cubic feet (3 × 3 × 3 = 27).

That is why 27 appears in the formula: work out the cubic feet first, then divide by 27. Skip the division and you will order roughly 27 times too much — an easy mistake when a calculator answer in cubic feet gets read as cubic yards.

A cubic yard weighs roughly 4,000 pounds, depending on the mix. A full truck carries about 10 cubic yards, and one cubic yard is about two tons to mix and place by hand — which is why bags stop making sense at scale.

How Do You Calculate How Much Concrete You Need?

For any rectangular slab, the calculation has three steps. Measure the length and width in feet. Convert the thickness from inches to feet by dividing by 12. Then multiply and divide by 27.

Formula: cubic yards = (length in feet × width in feet × thickness in feet) ÷ 27.

Useful conversions: 3 inches is 0.25 feet, 4 inches is 0.33 feet, 5 inches is 0.42 feet, and 6 inches is 0.50 feet. Most patios use 4 inches; driveways are often 5 or 6 inches, and a 6-inch slab uses 50% more concrete than a 4-inch slab of the same size.

Example: a shed base 8 feet by 6 feet, 4 inches thick. Convert 4 inches to one-third of a foot: 8 × 6 × 1/3 = 16 cubic feet, and 16 ÷ 27 = 0.59 cubic yards. Add 10% for waste: 0.65 cubic yards, or roughly 30 eighty-pound bags. Double-check square footage with CalcDune’s area calculator.

CalcDune’s concrete calculator does the same arithmetic for slabs, and the concrete bag calculator turns the result into bag counts. Knowing the formula still matters, because it lets you sanity-check any number a calculator or supplier gives you.

How Much Concrete Do I Need for a Slab?

The table below covers the most common slab sizes, all at 4 inches thick. Figures are exact volume before waste — add waste, then round up.

Slab size	Use	Cubic feet	Cubic yards	Order with 10% waste
10 × 10 ft	Small patio	33.3	1.23	1.5
12 × 12 ft	Patio or shed base	48.0	1.78	2.0
10 × 20 ft	Long patio	66.7	2.47	2.75
20 × 20 ft	Large patio	133.3	4.94	5.5
20 × 24 ft	Two-car garage floor	160.0	5.93	6.5
24 × 30 ft	Workshop floor	240.0	8.89	10.0

Note how fast volume grows: doubling each side quadruples it. And the waste column is not padding — on a 20 × 24-foot garage floor, 10% waste is more than half a cubic yard.

For other shapes, split the area into rectangles, work out each one, and add the results. For a curved edge, average the widest and narrowest measurements and lean on the waste allowance.

How Much Concrete Do I Need for a Driveway?

Driveways follow the same formula, but thickness matters more, because a driveway carries vehicles daily. A 4-inch driveway suits light use, but most contractors pour driveways at 5 or 6 inches, especially for heavier vehicles or freezing winters. Check local practice first: thickness is far harder to fix after the pour than before it.

Worked example: a driveway 12 feet wide and 30 feet long, poured 4 inches thick.

• Step 1 — convert thickness: 4 inches ÷ 12 = one-third of a foot.
• Step 2 — cubic feet: 12 × 30 × 1/3 = 120 cubic feet.
• Step 3 — cubic yards: 120 ÷ 27 = 4.44 cubic yards.
• Step 4 — waste: 4.44 × 1.10 = 4.89 cubic yards with a 10% allowance.
• Step 5 — order: round up to 5 cubic yards.

At 6 inches thick, the same driveway needs 12 × 30 × 0.50 = 180 cubic feet, or 6.67 cubic yards — 7.5 with waste. That is a 50% increase from a two-inch change, which is why guessing thickness is the costliest mistake here. Measure form depth at several points and use the deepest reading.

Measure the driveway in sections if it widens, narrows, or includes a turning area. A driveway 12 feet wide at the street and 20 feet wide at the garage is two rectangles, not one, and the apron where it meets the road counts too.

How Much Concrete Do I Need for Footings and Sonotubes?

Footings use different shapes, so the slab formula needs adjusting. A square footing still uses length × width × depth in feet, divided by 27. A round sonotube — the cardboard tube for deck and fence posts — uses the cylinder formula instead.

Round footing formula: cubic yards = (3.14 × radius² × depth) ÷ 27, with radius and depth in feet.

Take a 12-inch-diameter sonotube, 3 feet deep. The radius is 0.5 feet, so 3.14 × 0.5² × 3 = 2.36 cubic feet, or 0.09 cubic yards per tube. Six need 0.52 cubic yards, about 0.58 with 10% waste — roughly 26 eighty-pound bags, a manageable weekend job where bags beat a truck.

A square footing is simpler: 2 feet by 2 feet and 1 foot deep holds 4 cubic feet, or 0.15 cubic yards, so ten need 1.5 cubic yards before waste. Count every footing on the plan, including stairs and landings. And remember the hole is rarely as neat as the plan — sides can cave and bottoms widen, so waste allowance matters even more underground, where you cannot see the overrun until the truck is empty.

How Much Extra Concrete Should You Allow for Waste?

The standard waste allowance is 5–10%. Use 5% when forms are straight, ground is level and compacted, the shape is a clean rectangle, and you measured carefully. Use 10% — or a touch more — for uneven ground, irregular shapes, footings, or flexible forms that can bow under wet concrete.

Waste is not just spillage. The main causes are low spots in the subgrade that make the slab thicker than planned, forms flexing a fraction of an inch over a long run, and concrete left in the wheelbarrow, chute, or pump hose.

Extra is cheap insurance against a second delivery or a cold joint, where new concrete meets concrete already starting to set. With bags, buy two or three spares and return unopened ones if allowed.

How Many Bags of Concrete Do I Need?

Each bag makes a fixed volume once mixed. Typical yields are about 0.60 cubic feet per 80-pound bag, 0.45 per 60-pound bag, and 0.30 per 40-pound bag. Since one cubic yard holds 27 cubic feet, the counts work out as follows.

Volume needed	80 lb bags	60 lb bags	40 lb bags
0.5 cubic yards	23	30	45
1 cubic yard	45	60	90
2 cubic yards	90	120	180
5 cubic yards	225	300	450

Those numbers settle the bags-versus-truck question. Half a cubic yard is 23 eighty-pound bags: heavy, but doable with a mixer over a day. Five cubic yards is 225 bags — over nine tons to mix by hand, with the first batches setting before the last are mixed. As a rough guide, bags suit jobs up to about 1 cubic yard; above that, use a truck.

Check the yield on the bag you actually buy, since fast-setting and high-strength mixes vary. If your bag yields 0.55 cubic feet instead of 0.60, your count rises by about 9%. Enter your bag’s yield in the concrete bag calculator for an exact count.

What Should You Know Before Ordering Ready-Mix Concrete?

First, confirm the mix strength. A general-purpose mix suits a patio, but a driveway or structural footing may need a higher-strength mix, and local building code or your contractor may specify it. Ordering the wrong mix cannot be fixed after delivery.

Second, ask about short-load fees and access before you book. Many suppliers charge extra for small loads — sometimes ordering slightly more is cheaper than the fee. A mixer truck needs a firm, level route, so flag slopes, soft ground, narrow gates, and overhead wires in advance.

Third, have forms built, levelled, and braced, reinforcement in place, and enough hands to screed and finish before the truck arrives. Ready-mix starts setting when water meets cement, so a delayed start wastes the concrete itself. Pour in the morning in hot weather; in cold weather, ask about cold-weather mixes and curing protection.

What Are the Most Common Concrete Estimating Mistakes?

The most common mistake is mixing up units: inches swapped with feet, or square feet read as cubic yards. Convert every measurement to feet before multiplying, and write the units beside each number.

Next, measure at several points, not just the most convenient spot, and use the largest depth reading. Slabs are rarely perfectly level underneath, and a thin corner discovered after the pour cannot be undone.

Third, do not forget a section: steps, thickened edges, footings under a slab, and the driveway apron all count. Walk the whole project and list every section before totalling.

Finally, never order exactly the calculated amount with no waste allowance — and never guess the thickness. That two-inch gap between 4 and 6 inches is worth thousands of pounds of concrete on a large pour.

The Bottom Line

Use one formula — length × width × thickness in feet, divided by 27 — then add 5–10% for waste and round up. A 10 × 10-foot patio at 4 inches needs 1.23 cubic yards, so order 1.5. A 12 × 30-foot driveway at 4 inches needs 4.44, so order 5. Jobs under about 1 cubic yard suit bags; larger ones suit a truck.

Before you order, check your numbers with CalcDune’s concrete calculator and, for bags, the concrete bag calculator. The construction calculators hub gathers related tools, and a flooring calculator or tile calculator can cost the finish on top of the slab. Measure twice, add waste, round up.

Frequently Asked Questions

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

A 10-by-10-foot slab at 4 inches thick needs 1.23 cubic yards before waste. With 10% waste that becomes 1.35 cubic yards, so round up to 1.5 — about 68 eighty-pound bags. At 6 inches thick it needs 1.85 cubic yards before waste (2.04 with waste).

How do I calculate cubic yards of concrete?

Multiply length × width × thickness, all in feet, then divide by 27. Example: a 12-by-20-foot slab at 4 inches (one-third of a foot) is 12 × 20 × 1/3 = 80 cubic feet, and 80 ÷ 27 = 2.96 cubic yards. Add 5–10% for waste, then round up.

How many 80 lb bags of concrete are in a cubic yard?

About 45 eighty-pound bags make one cubic yard, at a typical yield of 0.60 cubic feet per bag. One cubic yard also takes about 60 sixty-pound bags or 90 forty-pound bags. Check your bag’s printed yield, since mixes vary.

How much concrete do I need for a driveway?

A 12-by-30-foot driveway at 4 inches thick needs 4.44 cubic yards, so order 5 with waste. At 6 inches thick it needs 6.67, so order 7.5. Measure each section separately if the driveway widens or includes an apron.

Should I add extra concrete for waste?

Yes — 5% for a simple, level, rectangular slab, 10% for uneven ground, irregular shapes, or footings. Waste covers spillage, form flex, low subgrade spots, and concrete left in wheelbarrows or hoses. A little extra is far cheaper than a second delivery or a weak joint from a short pour.

Is it cheaper to buy bags or order a ready-mix truck?

Bags are usually cheaper up to about 1 cubic yard, where a truck’s delivery and short-load fees outweigh the bag price. Above that, a truck is usually cheaper per cubic yard and far faster — mixing dozens of bags by hand takes hours, and early batches risk setting before the pour finishes.