How to Calculate Yards of Concrete: Formula & Examples

The Cubic Yard Formula

Cubic yards of concrete equal length times width times thickness, all in feet, divided by 27:

Length (ft) × Width (ft) × Thickness (ft) ÷ 27 = Cubic Yards

There are 27 cubic feet in one cubic yard (3 × 3 × 3 = 27)

Concrete is sold by the cubic yard, so the divide-by-27 step turns cubic feet into the unit the plant quotes. Measure inside the forms, not outside them. If you would rather keep thickness in inches, use L (ft) × W (ft) × T (in) ÷ 324; the 324 is 27 × 12.

Convert Inches to Feet Before You Multiply

Most people measure slab thickness in inches, but the formula requires feet. A 4-inch slab is 0.333 feet (4 ÷ 12), NOT 4 feet. Using 4 instead of 0.333 will give you a result that is 12 times too much concrete.

Quick conversion: divide your inches by 12. So 4" = 0.333 ft, 6" = 0.5 ft, 8" = 0.667 ft.

Quick-Reference Coverage Table

Instead of running the formula every time, use this table to see how many square feet one cubic yard of concrete covers at common thicknesses:

Thickness	Thickness (ft)	Sq Ft per Cubic Yard
2"	0.167 ft	162 sq ft
3"	0.250 ft	108 sq ft
4" (most common)	0.333 ft	81 sq ft
5"	0.417 ft	65 sq ft
6"	0.500 ft	54 sq ft
8"	0.667 ft	40.5 sq ft
12"	1.000 ft	27 sq ft

The 81 Shortcut for 4-Inch Slabs

At 4 inches thick, one cubic yard covers exactly 81 square feet, so square footage divided by 81 gives cubic yards. A 20x20 patio is 400 sq ft ÷ 81 = 4.94 cubic yards.

6 Worked Examples

The formula makes more sense when you see it in action. Here are six real-world examples covering the most common residential projects.

Example 1: 10x10 Patio at 4 Inches

Dimensions: 10 ft long × 10 ft wide × 4 inches thick

Step 1: Convert thickness to feet: 4" ÷ 12 = 0.333 ft

Step 2: Multiply: 10 × 10 × 0.333 = 33.3 cubic feet

Step 3: Divide by 27: 33.3 ÷ 27 = 1.23 cubic yards

Shortcut check: 100 sq ft ÷ 81 = 1.23 yd³, which matches.

Example 2: 20x20 Garage Slab at 6 Inches

Dimensions: 20 ft long × 20 ft wide × 6 inches thick

Step 1: Convert thickness: 6" ÷ 12 = 0.5 ft

Step 2: Multiply: 20 × 20 × 0.5 = 200 cubic feet

Step 3: Divide by 27: 200 ÷ 27 = 7.41 cubic yards

Garage slabs use 6" thickness for vehicle weight. Always use 6" minimum for any slab that will support cars or trucks.

Example 3: 4x50 Sidewalk at 4 Inches

Dimensions: 4 ft wide × 50 ft long × 4 inches thick

Step 1: Convert thickness: 4" ÷ 12 = 0.333 ft

Step 2: Multiply: 4 × 50 × 0.333 = 66.6 cubic feet

Step 3: Divide by 27: 66.6 ÷ 27 = 2.47 cubic yards

Shortcut check: 200 sq ft ÷ 81 = 2.47 yd³, which matches.

Example 4: Circular Fire Pit Pad (8 ft Diameter, 4 Inches)

For circles, use: π × radius² × thickness ÷ 27

Step 1: Radius = diameter ÷ 2 = 8 ÷ 2 = 4 ft

Step 2: Convert thickness: 4" ÷ 12 = 0.333 ft

Step 3: Area = π × 4² = 3.14159 × 16 = 50.27 sq ft

Step 4: Volume = 50.27 × 0.333 = 16.74 cubic feet

Step 5: Divide by 27: 16.74 ÷ 27 = 0.62 cubic yards

Example 5: L-Shaped Patio (Two Rectangles)

Overall L-shape: 20 ft across the top, 12 ft down the right side, with a 10x8 ft cutout in the bottom-left corner. Thickness: 4 inches.

Step 1: Break into two rectangles:

Rectangle A: 20 ft × 4 ft = 80 sq ft (the top section)

Rectangle B: 10 ft × 8 ft = 80 sq ft (the right leg)

Step 2: Total area = 80 + 80 = 160 sq ft

Step 3: Volume = 160 × 0.333 = 53.28 cubic feet

Step 4: Divide by 27: 53.28 ÷ 27 = 1.97 cubic yards

Tip: Sketch the shape on paper and divide it into simple rectangles. Add the areas, then apply the formula once.

Example 6: Sloped Driveway Using Average Thickness

Driveway: 12 ft wide × 40 ft long. Thickness is 4 inches at the top and 8 inches at the bottom due to the slope.

Step 1: Average thickness = (4" + 8") ÷ 2 = 6 inches = 0.5 ft

Step 2: Volume = 12 × 40 × 0.5 = 240 cubic feet

Step 3: Divide by 27: 240 ÷ 27 = 8.89 cubic yards

For slopes, measure the thickness at the thinnest and thickest points, then average them. This gives a reliable estimate without complicated math.

Irregular Shapes and Slopes

Break Complex Shapes into Simple Ones

Any shape can be broken down into rectangles, triangles, and circles. Calculate the area of each piece separately, add them together, then multiply by your thickness and divide by 27.

Rectangles: Length × Width
Triangles: (Base × Height) ÷ 2
Circles: π × Radius² (or 3.14159 × r × r)
Half-circles: (π × Radius²) ÷ 2, common for rounded patio edges
Trapezoids: ((narrow width + wide width) ÷ 2) × Length. A driveway apron that flares from 10 ft to 18 ft over its last 8 ft averages 14 ft wide, so 14 × 8 = 112 sq ft
Cutouts: calculate the full slab, then subtract the area of the drain basin, tree well, or column base

Footings, Piers, and Post Holes

Footings: the same rectangular formula. A 24 × 24 × 12 inch spread footing is 2 × 2 × 1 = 4 cubic feet, or 0.15 cubic yards. Use the footing calculator for pads and the strip footing calculator for continuous footings.
Round piers and form tubes: π × radius² × height. The size printed on the tube is the diameter, so halve it first. A 12-inch tube 4 ft tall is 3.14 × 0.5² × 4 = 3.14 cubic feet, or 0.12 cubic yards. The column calculator handles round and square columns.
Post holes: the hole's cylinder volume minus the post. See bags of concrete for post holes for counts by post and hole size.

The Average Thickness Method for Slopes

When the ground slopes and your slab thickness varies, measure the thickness at the thinnest point and the thickest point. Add them together and divide by two to get the average. Use that average in your formula.

For severe slopes with a wide variation (more than double the thickness difference), measure at three or four points across the slab and average all measurements. This gives a more accurate result than a simple two-point average.

The slab calculator handles the conversions for slabs, and the stairs calculator works from step count, rise, run, and width.

Waste Factor Guide

Always order more concrete than the formula says you need. Concrete gets wasted during the pour: it sticks to the chute, spills off the edges, fills low spots in the ground, and overflows forms that aren't perfectly level. Running short mid-pour is far worse (and more expensive) than having a little left over.

Scenario	Waste Factor	When to Use
Experienced contractor, simple slab	5%	Flat ground, solid forms, professional crew
Standard residential project	10%	Typical patio, sidewalk, or driveway
Irregular shapes or first-timer	15%	L-shapes, curves, or your first concrete pour
Stairs, curves, or complex forms	20%	Stairs, retaining walls, decorative work

To apply the waste factor, multiply your calculated yardage by 1.05, 1.10, 1.15, or 1.20 depending on the scenario. For example, if you calculated 7.41 cubic yards for a garage slab with a 10% waste factor: 7.41 × 1.10 = 8.15 cubic yards. Order 8.25 or 8.5 to be safe, since ready-mix trucks typically deliver in quarter-yard increments. Round up, never down: running short on a continuous slab leaves a cold joint where a second load meets concrete that has already started to set.

Bags vs. Ready-Mix: When to Switch

Per cubic yard, ready-mix costs less than bags even with a short-load fee. Under about a yard, the plant's minimum order and any flat small-load charge usually make bags cheaper, so get a quote.

Under 0.5 Cubic Yards: Use Bags

Small projects like a single post hole, a small pad, or a set of stepping stones. That is up to 23 bags of 80-lb concrete mix (13.5 cu ft ÷ 0.60 = 22.5), about $115 to $184 at $5 to $8 per bag. Manageable for one person in a few hours.

0.5 to 1.0 Cubic Yards: Bags Work, a Mixer Helps

At this range, you're looking at 23-45 bags of 80-lb mix. It's doable, but physically demanding and time-consuming. A rented portable mixer makes the job faster. Bags cost $115 to $184 for 23 up to $225 to $360 for 45. At this size the plant's minimum order and any flat small-load charge usually make a truck cost more than the bags, so get a quote before you buy.

Over 1.0 Cubic Yards: Go Ready-Mix

Ready-mix concrete costs about $110 to $150 per cubic yard, while the same yard in 80-lb bags costs $225 to $360. Per yard, ready-mix stays cheaper even with a short-load fee, and the truck delivers concrete already mixed and pours it directly into your forms.

Watch for Short-Load Surcharges

On an order smaller than a full truck, a short-load fee is added to the per-yard price, and some plants also bill a minimum order or a flat small-load charge. Those flat charges are what make bags cheaper under about a yard. Ask your local plant for its minimum and short-load pricing before deciding (see ordering ready-mix below). For bag quantities and costs, see the concrete bags guide.

Ordering Ready-Mix: What the Plant Will Ask

A ready-mix order is your yardage plus six specs: strength (PSI), slump, aggregate size, air content, admixtures, and site access. Have all of them written down before you call.

Minimums and Short-Load Fees

Minimum delivery is typically 1 cubic yard. Some plants deliver less but bill the minimum.
Short-load fees apply to orders smaller than a full truck (about 10 yards). Cost guides put the fee at $40 to $60 per cubic yard; some plants charge a flat fee instead.
Truck size: a standard mixer carries about 10 cubic yards. Some plants run smaller trucks of 3 to 5 yards.
Volumetric (mobile-mix) trucks batch the concrete at your site and bill for what they pour, often with no short-load fee but a higher per-yard rate. For a 1 to 2 yard job, ask whether one serves your area.
Pump trucks are billed separately from the concrete, usually with a minimum number of hours. You need one when the mixer cannot reach the forms.
Standby time: the ticket includes a set unloading time, and the plant bills by the minute beyond it.
Combine jobs. A driveway, walkway, and shed pad poured on the same day can fill a full truck and avoid the short-load fee.

The Specs to Have Ready

Volume: your total with waste factor, rounded up to the next quarter yard.
PSI: 2,500 to 3,000 for footings; 3,000 to 3,500 for sidewalks, patios, basement floors, and shed pads; 3,500 to 4,000 for driveways and garage floors; 4,000 and up for shop floors and exposed slabs in freeze-thaw climates. The plant's standard residential mix is usually the right starting point.
Slump: 3 to 4 inches for footings, 4 to 5 inches for slabs and most flatwork, 5 to 6 inches for walls, columns, and pumped pours. Order the low end of the range: water raises slump on site, but nothing lowers it.
Aggregate: 3/4 inch is the default for residential work. 3/8 inch (pea gravel) suits pours thinner than 3 inches or tightly spaced rebar.
Air entrainment: needed for exterior concrete where it freezes. The IRC (Table R402.2) calls for 5 to 7 percent total air in porches, steps, carport slabs, and garage floors in moderate and severe weathering areas. Give air-entrained flatwork a broom finish; a hard steel-trowel finish can blister or delaminate the surface.
Admixtures: a retarder in hot weather, an accelerator in cold weather. Choose a non-chloride accelerator if the pour has rebar or wire mesh, because calcium chloride corrodes steel.
Access: driveway width, overhead clearance (a loaded truck is about 13 ft tall and weighs 60,000 to 80,000 lb), and whether the truck can back up to the pour or you need wheelbarrows or a pump.
Washout: where the driver can rinse the chute. Washout water must not go into a storm drain; a lined wheelbarrow or plastic tub works for one truck.

A Complete Order, in One Call

"I'd like a delivery Tuesday morning: 5.5 yards of 3,500 PSI, 4-inch slump, 3/4-inch aggregate, 5 percent air. The truck can back straight to the pour from a paved driveway, there are no overhead wires, and I have a washout area set up."

When the Truck Arrives

Check the slump. ASTM C94 allows ±1/2 inch when the specified slump is 2 inches or less, ±1 inch from over 2 through 4 inches, and ±1-1/2 inches above 4 inches. A load ordered at 4 inches is in spec from 3 to 5 inches; one that arrives at 7 inches is not, and you can reject it before it goes in the forms. At 4 inches the concrete holds its shape like thick oatmeal; at 7 inches it flows flat.
Limit water on site. Each extra gallon of water per cubic yard adds about 1 inch of slump and costs roughly 150 to 200 PSI of compressive strength, so two extra gallons per yard can turn a 4,000 PSI mix into a 3,600 to 3,700 PSI one. Water you ask for beyond what the mix design allows moves the strength risk from the plant to you.
Hot weather shortens working time. Pre-wet the subgrade, have the whole crew on site before the truck arrives, and book the earliest slot you can get.
Cold weather: ACI 306 applies when the air temperature has fallen to, or is expected to fall below, 40°F during the protection period. Never place concrete on frozen ground. Protect the pour with insulated blankets or a heated enclosure, or use an accelerator, and reschedule if you cannot protect it. Protection times by temperature are in the concrete cure time chart.

Pre-Calculated Yardage Table

Cubic yards for common slab sizes at 4 and 6 inches thick, before waste. These are the two most common residential thicknesses: 4 inches for patios and sidewalks, 6 inches for driveways and garages.

Slab Size	Square Feet	At 4" Thick (yd³)	At 6" Thick (yd³)
10 x 10	100	1.23	1.85
10 x 12	120	1.48	2.22
12 x 12	144	1.78	2.67
12 x 20	240	2.96	4.44
16 x 20	320	3.95	5.93
20 x 20	400	4.94	7.41
20 x 24	480	5.93	8.89
24 x 24	576	7.11	10.67
30 x 30	900	11.11	16.67

Remember to add your waste factor on top of these numbers. For a standard residential project, multiply by 1.10 (10% waste). Prices per yard and per square foot are in the concrete slab cost guide.

Price sources

Prices checked October 31, 2026. These are national ranges from the cost guides below; prices vary by region, so get a local quote before you order.

HomeGuide, Concrete Delivery Cost (updated May 1, 2026). Retrieved October 31, 2026.
HomeGuide, How Much Does Concrete Cost Per Yard? (updated January 6, 2026). Retrieved October 31, 2026.
Angi, Concrete Cost Per Yard (updated September 10, 2026). Retrieved October 31, 2026.
LawnStarter, How Much Does Concrete Cost Per Yard? (updated September 24, 2026). Retrieved October 31, 2026.
This Old House, How Much Does a Concrete Slab Cost? (updated March 17, 2026). Retrieved October 31, 2026.