How to Calculate Yards of Concrete (Formulas, Shortcuts & Calculator)

The One Formula You Need

Every concrete calculation comes down to a single formula. Whether you're pouring a patio, a garage slab, or a sidewalk, this is it:

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

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

That's it. Multiply your three dimensions in feet, then divide by 27 to convert cubic feet into cubic yards, which is how concrete is sold. The tricky part isn't the math itself -- it's getting your measurements right before you plug them in.

The #1 Mistake: Forgetting to Convert Inches to Feet

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 "Magic Number 81" Shortcut

For the most common residential thickness (4 inches), just divide your total square footage by 81 to get cubic yards. Pouring a 20x20 patio? That's 400 sq ft ÷ 81 = 4.94 cubic yards. This shortcut works because one cubic yard covers exactly 81 square feet at 4 inches thick.

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 = 1.24 cubic yards

Shortcut check: 100 sq ft ÷ 81 = 1.23 yd³ -- 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 -- 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

Not every project is a simple rectangle. Here's how to handle the shapes that don't fit neatly into the basic formula.

Break Complex Shapes into Simple Ones

Any shape, no matter how complex, 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

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.

If you're working with a particularly tricky shape or slope, our slab calculator handles the math automatically.

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.

Bags vs. Ready-Mix: When to Switch

Bagged concrete from the hardware store and ready-mix delivered by truck use the same material. The difference is cost, labor, and scale. Here's when each option makes sense:

Under 0.5 Cubic Yards -- Use Bags

Small projects like a single post hole, a small pad, or a set of stepping stones. You'll need about 22-23 bags of 80-lb concrete mix. Cost: roughly $135 at $6 per bag. Manageable for one person in a few hours.

0.5 to 1.0 Cubic Yards -- Bags Are Viable, But Consider a Mixer

At this range, you're looking at 23-45 bags of 80-lb mix. It's doable, but physically demanding and time-consuming. A portable mixer rental ($50-75/day) makes the job faster. Total bag cost: $135-$270. Still cheaper than ready-mix for small quantities, but the labor is significant.

Over 1.0 Cubic Yards -- Go Ready-Mix

Ready-mix concrete costs about $150 per cubic yard delivered, while the equivalent in bags runs roughly $270 per cubic yard. That's nearly double the cost for bags, plus hours of backbreaking mixing. A ready-mix truck delivers concrete already mixed and pours it directly into your forms.

Watch for Short-Load Surcharges

Most ready-mix companies charge a short-load fee for orders under their minimum (usually 3-5 cubic yards). This fee can be $50-$150 and is added on top of the per-yard price. Even with the surcharge, ready-mix is usually cheaper than bags once you pass the 1-yard mark. Call your local batch plant and ask about their minimum and short-load pricing before deciding. For more on bag quantities and costs, see our concrete bags guide.

Pre-Calculated Yardage Table

For the most common slab sizes, here's how much concrete you'll need at 4-inch and 6-inch thickness (before waste factor). 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	1.24	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). Want to know how much that concrete will cost? Check our concrete slab cost guide.

Putting It All Together

Calculating yards of concrete is three steps: measure your dimensions in feet (converting inches to feet for thickness), multiply length × width × thickness to get cubic feet, then divide by 27 to get cubic yards. Add a waste factor, and you have your order quantity.

For quick estimates on 4-inch slabs, just divide total square feet by 81. For anything more complex -- circles, L-shapes, slopes, or multi-section pours -- break the shape into simple pieces, calculate each one, and add them up.

Don't want to do the math by hand? Our calculator handles all of these scenarios automatically, including waste factors and cost estimates.