The formula, step-by-step examples for every project type, a quick reference table, and answers to the most common concrete calculation questions — all in one place.
To calculate concrete, multiply Length × Width × (Thickness in inches ÷ 12) to get cubic feet, then divide by 27 to convert to cubic yards. That's the complete formula. Everything below shows you how to use it for real projects.
Enter your dimensions and get your cubic yard total, bag count, and material cost in seconds.
Use the Concrete Calculator →Concrete is ordered and priced by the cubic yard (CY). One cubic yard equals 27 cubic feet — a block 3 feet wide, 3 feet long, and 3 feet tall. Here is the two-step formula every contractor uses:
Why divide by 27? One yard = 3 feet, so 1 cubic yard = 3 × 3 × 3 = 27 cubic feet. This is the same unit ready-mix suppliers use when they quote a job. Always confirm your contractor is quoting in cubic yards, not linear yards or square yards.
A backyard patio, 5% waste, ready-mix order
Order: 1.6 cubic yards of concrete
Plan your gravel sub-base before pouring — typically 4 inches of compacted crushed stone under a patio slab.
Single-car driveway section, 5% waste
Order: 2.6 cubic yards of concrete
Heavy-vehicle driveways should use 6-inch depth. For a full driveway cost breakdown, see our Driveway Cost Calculator.
Deck footings — cylinder formula applies here
Order: ~39 bags of 60 lb concrete (bagged is best under 1 CY)
One cubic yard of concrete is a block 3 feet × 3 feet × 3 feet — roughly the size of a washing machine. When a contractor says "order 3.5 yards," they mean 3.5 cubic yards — not linear yards or square yards. Here is what 1 CY looks like in practice:
| What You're Asking | The Answer |
|---|---|
| Coverage at 4" thick | ~81 square feet per CY |
| Coverage at 6" thick | ~54 square feet per CY |
| Weight of 1 CY | ~4,000 lbs (2 tons) wet |
| 80 lb bags per CY | ~45 bags (0.60 cu ft per bag) |
| 60 lb bags per CY | ~60 bags (0.45 cu ft per bag) |
| Standard ready-mix truck | 8–10 CY per load |
For anything over 1 cubic yard, a ready-mix truck is almost always faster and cheaper per yard than buying and mixing bags. Under 1 CY — especially for sonotube footings — bagged concrete mixed on-site is the practical choice for most homeowners and contractors.
Find your square footage on the left, then read across to your pour thickness. Numbers shown are net volume — add 5–10% when placing your order.
| Area (sq ft) | 3" Thick | 4" Thick | 5" Thick | 6" Thick |
|---|---|---|---|---|
| 50 sq ft | 0.46 CY | 0.62 CY | 0.77 CY | 0.93 CY |
| 100 sq ft | 0.93 CY | 1.23 CY | 1.54 CY | 1.85 CY |
| 150 sq ft | 1.39 CY | 1.85 CY | 2.31 CY | 2.78 CY |
| 200 sq ft | 1.85 CY | 2.47 CY | 3.09 CY | 3.70 CY |
| 300 sq ft | 2.78 CY | 3.70 CY | 4.63 CY | 5.56 CY |
| 400 sq ft | 3.70 CY | 4.94 CY | 6.17 CY | 7.41 CY |
| 500 sq ft | 4.63 CY | 6.17 CY | 7.72 CY | 9.26 CY |
| 750 sq ft | 6.94 CY | 9.26 CY | 11.57 CY | 13.89 CY |
| 1,000 sq ft | 9.26 CY | 12.35 CY | 15.43 CY | 18.52 CY |
Our free calculator handles any dimension — including irregular shapes, sonotube footings, and multiple sections.
Calculate Concrete Yards →Residential driveways are poured at 4 inches thick for standard passenger vehicles. If RVs, trucks, or heavy loads will use the driveway, go to 5 or 6 inches. Measure the full length and width including the garage apron and any street flare. For an irregular driveway, break it into rectangles, calculate each section, and add the totals. Always plan your compacted gravel sub-base — typically 4 to 6 inches — before setting forms.
Considering asphalt instead? Use our Asphalt Calculator to compare tonnage and cost for the same area.
A backyard patio is typically 4 inches of concrete — enough for foot traffic, outdoor furniture, and a gas grill. If you're adding a hot tub or heavy planter, pour at 5 or 6 inches. For an L-shaped patio, divide it into two rectangles, calculate each one, and add both totals together.
Sidewalks pour at 4 inches for residential use. A standard walkway is 3 to 4 feet wide. Example: a 40-foot long, 4-foot wide walkway at 4 inches thick — 40 × 4 × (4 ÷ 12) = 53.33 cu ft ÷ 27 = 1.97 CY. Order 2.1 CY after adding the 5% waste factor.
Round footings use the cylinder formula (π × r² × height). Spread footings use the standard rectangle formula. The critical variable is depth — always check your local building code for the required frost depth, which ranges from 18 inches in warm southern climates to 48 inches in northern states. Use the Sonotube tab in our Concrete Calculator — it handles round tubes automatically.
Steps are the trickiest to estimate by hand because each tread and riser is a different volume. Treat each step as a solid rectangle: length × tread depth × riser height. Calculate each step separately, then add them all together. Include the landing pad at the bottom if you're pouring one.
Garage floors are poured at 4 to 6 inches thick. Use 4 inches for a standard two-car garage on stable soil. If you plan to park heavy trucks or use the floor as a workshop with heavy equipment, pour at 6 inches. A properly compacted sub-base is especially important for garage floors — soft or wet soil under a garage slab is the most common cause of cracking.
Ready-mix concrete suppliers and contractors always quote in cubic yards. Bagged concrete from a home improvement store lists yield in cubic feet. Here is a quick guide:
| Situation | Unit | Why |
|---|---|---|
| Ready-mix truck order | Cubic yards (CY) | Suppliers price by the yard |
| Bagged concrete (store) | Cubic feet | Bags list yield in cubic feet |
| Building permit drawings | Cubic yards | Standard unit in construction |
| Contractor quote | Cubic yards | Confirm it's CY — not tons |
To convert: cubic feet ÷ 27 = cubic yards. Going the other way: cubic yards × 27 = cubic feet.
Always order more than your exact calculation. The standard is 5% for regular rectangular slabs and 10% for irregular shapes or uneven ground. Here is why:
Running out of concrete mid-pour is one of the most expensive mistakes on a job. The extra concrete you don't use costs far less than a cold joint repair or tearing out a partially cured slab and starting over.
10 × 10 = 100 sq ft × (4 ÷ 12) = 33.33 cu ft ÷ 27 = 1.23 cubic yards. Add 5% waste and order 1.30 CY. At 6 inches thick, that becomes 1.85 CY before waste — order 1.95 CY.
20 × 20 = 400 sq ft × (4 ÷ 12) = 133.3 cu ft ÷ 27 = 4.94 cubic yards. With 5% waste, order 5.19 CY — your supplier will likely round to 5.25 or 5.5 CY. Use our Concrete Calculator for the exact figure.
30 × 30 = 900 sq ft × (4 ÷ 12) = 300 cu ft ÷ 27 = 11.11 CY. Add 5% waste and order 11.67 CY — round to 12 CY. At 6 inches thick, that jumps to 16.67 CY net, order 17.5 CY.
"Cement yards" and "concrete yards" mean the same thing in everyday usage — both refer to cubic yards of mixed concrete. The formula: CY = (L × W × Thickness in inches ÷ 12) ÷ 27. Technically, cement is just one ingredient in concrete (mixed with sand, gravel, and water), but the terms are used interchangeably on job sites and by suppliers.
Break the irregular area into rectangles. Calculate each rectangle separately using the standard formula (L × W × thickness ÷ 12 ÷ 27), then add all the totals together. For this type of project, use a 10% waste factor instead of the standard 5% — irregular perimeters always waste more concrete than a clean rectangle.
Always estimate high, then add the waste factor on top. Running short mid-pour forces you to stop — the concrete already placed begins to cure, and when the second load arrives you get a cold joint: a visible seam that is a structural weak point. Leftover concrete can go into a post hole, a small footing, or a garden path. Cold joints cannot be fixed without removing and reporing the slab.
For rectangular spread footings, use the standard formula: L × W × (depth ÷ 12) ÷ 27. For round sonotube footings, use the cylinder formula: π × (diameter ÷ 2 ÷ 12)² × depth in feet ÷ 27. Multiply by the number of tubes, then add 5% waste. The Sonotube tab in our free calculator handles all of this automatically.
Instant cubic yards, bag count, and ready-mix cost for slabs, driveways, patios, and sonotube footings.
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