How Much Concrete Do You Need? A Room-by-Room Guide
September 18, 2026
Concrete volume = Length × Width × Depth, in feet, giving cubic feet — divide by 27 to convert to cubic yards, the unit ready-mix trucks are ordered in. Always add 5-10% extra to account for spillage and an uneven subgrade.
Skip the math and get your answer instantly:
Open the Concrete Calculator →Worked Example: A Standard Patio Slab
A 10×10 ft patio slab poured 4 inches (0.33 ft) deep: 10 × 10 × 0.33 = 33 cubic feet. Dividing by 27: 33 / 27 ≈ 1.22 cubic yards. Adding a 10% buffer: about 1.34 cubic yards to order.
Bagged Concrete for Small Jobs
For smaller pours (fence posts, small footings), bagged concrete is more practical than a ready-mix truck. An 80 lb bag yields about 0.6 cubic feet of mixed concrete, so roughly 45 bags equal one cubic yard (27 ÷ 0.6).
Typical Slab Depths by Application
| Application | Typical Depth |
|---|---|
| Patio or walkway | 4 inches |
| Driveway | 5-6 inches |
| Shed foundation | 4-6 inches |
| Footings | Per local building code — varies by frost depth and load |
Frequently Asked Questions
How do I calculate concrete volume for a slab?
Multiply length × width × depth, all in feet, to get cubic feet, then divide by 27 to convert to cubic yards for ordering.
How many 80 lb bags of concrete equal a cubic yard?
Roughly 45 bags — an 80 lb bag yields about 0.6 cubic feet of mixed concrete, and a cubic yard is 27 cubic feet.
How much extra concrete should I order?
Add 5-10% beyond your calculated volume to account for spillage, form leakage, and subgrade that isn't perfectly level — running short mid-pour is far more costly than minor leftover material.
What's the difference between cubic feet and cubic yards?
1 cubic yard equals 27 cubic feet. Ready-mix trucks are ordered by the cubic yard; small bagged-concrete jobs are usually calculated in cubic feet first.
How deep should a concrete driveway be?
Typically 5-6 inches, deeper than a standard patio slab (4 inches), since driveways need to support vehicle loads — check local building codes for exact requirements in your area.