The formula is straightforward once you understand the units. Concrete is sold in cubic yards in the USA — one cubic yard equals 27 cubic feet. To figure out your volume in cubic feet, multiply the length (in feet) × width (in feet) × depth (in feet). Then divide by 27 to get cubic yards.
For example: a 20×20 foot patio with a 4-inch slab works out to 20 × 20 × 0.333 = 133.3 cubic feet, divided by 27 = 4.94 cubic yards. Most contractors round up 10% for waste, so you'd order about 5.5 yards. Our concrete calculator does all of this automatically the moment you hit calculate.
Concrete Slab Thickness — What's the Right Depth?
Thickness matters more than most homeowners realize. Use the wrong depth and you either waste money on excess concrete or end up with a slab that cracks under load. Here's a quick depth guide:
Sidewalks and garden paths: 3–4 inches is typically enough for pedestrian traffic
Patios and backyard slabs: 4 inches standard, 6 inches if heavy furniture or hot tub
Residential garage floors: 4 inches minimum, 6 inches recommended for vehicles
Driveways: 4 inches for passenger cars, 6 inches if you park trucks or heavy equipment
Structural footings: 8–12 inches or per your engineer's specifications
Always account for your base material too. A proper concrete job usually starts with 4–6 inches of compacted gravel base beneath the slab — use our gravel calculator to figure that out before you pour.
Bags vs. Ready-Mix Concrete — Which Should You Order?
If your project is small — think a fence post, repair patch, or a single step — bagged concrete from the hardware store makes sense. You can pick up 60lb or 80lb bags for around $6–$9 each, mix them yourself, and avoid the minimum order requirements of a ready-mix truck.
Once you cross about 1 cubic yard (roughly 27 cubic feet), calling a ready-mix plant is almost always cheaper per yard and saves a lot of labor. In 2025, ready-mix concrete typically runs $125–$185 per cubic yard depending on your location, mix strength, and current fuel prices. Add about $20–$30 per cubic yard for delivery.
A rough rule of thumb: one 80lb bag of concrete covers about 0.6 cubic feet, or 0.022 cubic yards. So for a 4-inch slab that's 10×10 feet (= 0.33 cubic yards), you'd need about 15 bags of 80lb concrete. This gets expensive fast — for that same slab, a ready-mix truck delivery runs $75–$120 total at current prices, which is often cheaper than 15 bags at $8 each ($120).
Concrete Strength (PSI) — What Mix Do You Need?
Concrete mixes are rated in PSI — pounds per square inch compressive strength. The most common residential options:
2,500 PSI: Sidewalks, small patios, non-structural applications
3,000 PSI: Standard residential slabs, driveways, garage floors — most common choice
3,500–4,000 PSI: Heavy driveways, commercial slabs, areas with freeze-thaw cycles
5,000+ PSI: Structural and industrial applications
In cold climates, specify air-entrained concrete (ASTM C260) — tiny air bubbles help the slab resist cracking from freeze-thaw cycles. This is standard in northern states and worth specifying even if your contractor doesn't mention it.
Real Example: Calculating Concrete for a 20×30 Garage
Let's walk through a real calculation. You're pouring a 20×30 garage floor at 4 inches thick.
Area: 20 × 30 = 600 sq ft
Volume: 600 × (4÷12) = 600 × 0.333 = 200 cubic feet
Cubic yards: 200 ÷ 27 = 7.41 cubic yards
Add 10% waste: 7.41 × 1.1 = 8.15 cubic yards → order 8.5 yards
At $145/yard: material cost = $1,232
Add delivery fee (~$150) + labor (~$5–7/sqft) for a full installed estimate
Running these numbers before you call contractors means you walk into every conversation knowing exactly what to expect — and you can spot if someone's quoting you an unusually high volume.
Common Mistakes People Make When Ordering Concrete
Not accounting for waste: Always add 5–10% over your calculated volume. Leftover concrete at the end of a pour is normal; running short mid-pour is a disaster.
Measuring the wrong depth: 4 inches of actual concrete is not 4 inches of total floor height — your gravel base, vapor barrier, and any rebar take up space too.
Skipping the gravel base: Concrete needs a stable, well-draining base. Without proper compacted gravel underneath, slabs crack faster.
Forgetting control joints: In large slabs, control joints every 8–12 feet let the slab crack predictably rather than randomly.
Estimix Tools That Work Together
Concrete projects rarely need just concrete. Before your pour, calculate your gravel base using our gravel calculator. If you're doing a driveway and haven't decided on material yet, our asphalt cost calculator lets you compare side-by-side. Once the slab is done, plan your interior using the flooring calculator or paint calculator for walls.
Every calculator on Estimix is built with USA pricing in mind — real numbers, not guesses.