Concrete Slab Calculator

Calculation Result

You Should Order

2.72 yd³

Includes 10% recommended waste allowance

Net Volume (yd³)2.47 yd³

Net Volume (ft³)66.7 ft³

Surface Area400 ft²

Waste Factor+10%

Estimated Concrete Cost:$366.67

Premixed Bag Estimates

(Includes 10% waste)

80 lb bags123

60 lb bags163

40 lb bags245

Bag estimates are approximate based on standard yield (e.g. 0.60 cu. ft. per 80 lb bag).

How Much Concrete Do I Need?

Calculating concrete volume for a slab is a fundamental step in home improvement and construction planning. Whether pouring a backyard patio, a shed foundation, a walkway, or a garage floor, ordering the correct volume prevents costly delivery delays or overpaying for unused material.

Concrete is sold and measured by volume in cubic yards (yd³) or packaged in premixed bags (typically 80 lb, 60 lb, or 40 lb). To determine your requirement, measure the length, width, and thickness of your formwork, convert all dimensions to feet, multiply them to get total cubic feet, and divide by 27.

How the Concrete Calculation Works

The mathematical formula for calculating rectangular slab concrete volume uses three simple steps:

1. Volume (cubic feet) = Length (ft) × Width (ft) × Thickness (ft)

2. Volume (cubic yards) = Volume (cubic feet) ÷ 27

3. Order Volume = Cubic Yards × (1 + Waste Allowance % / 100)

Tip: To convert thickness from inches to feet, divide the inch measurement by 12 (e.g., 4 inches = 4 ÷ 12 = 0.3333 feet).

Worked Example: 20 ft × 20 ft Slab

Step 1: Convert Thickness to Feet

For a 2-inch slab depth: 2 inches ÷ 12 = 0.1667 feet.
(For a standard 4-inch depth: 4 inches ÷ 12 = 0.3333 feet).

Step 2: Calculate Surface Area

20 feet × 20 feet = 400 square feet.

Step 3: Calculate Net Volume

400 sq. ft. × 0.1667 ft = 66.67 cubic feet.
66.67 cu. ft. ÷ 27 = 2.47 cubic yards.

Step 4: Apply 10% Waste Factor

2.47 cu. yd. × 1.10 = 2.72 cubic yards to order.

Concrete Waste Allowance

It is standard construction practice to add 5% to 10% waste allowance to your calculated volume. Actual excavations are rarely perfectly level, sub-base gravel settles, and formwork flexes slightly under wet concrete pressure.

10% Waste (Standard): Best for standard backyard patios, shed bases, and flat grade pours.
15% Waste (Uneven Grade): Recommended for rocky soil, sloped ground, or deep footings.

Premixed Bags vs. Ready-Mix Delivery

Premixed Bagged Concrete

Ideal for small slabs under 1 cubic yard (e.g., small walkways or post footings). One 80 lb bag yields approximately 0.60 cu. ft. You need ~45 bags per cubic yard.

Ready-Mix Truck Delivery

Best for jobs larger than 1 to 2 cubic yards. Truck delivery ensures uniform mixing, eliminates manual labor, and finishes significantly faster.

Practical Considerations & Local Building Codes

Always check local municipal building codes before pouring concrete. Heavy vehicle driveways or frost-line footings may mandate minimum 5" to 6" thickness, rebar (#4 / 1/2-inch grid), wire mesh, or vapor barriers.

Frequently Asked Questions

Why should I add 10% extra for waste allowance?

Excavated ground is rarely completely level, wooden forms flex under the weight of wet concrete, and material sticks to mixing tools. Ordering a 10% waste buffer prevents running out of concrete mid-pour.

How many 80 lb bags of concrete equal 1 cubic yard?

It takes approximately 45 bags of 80 lb premixed concrete (or 60 bags of 60 lb concrete) to yield 1 cubic yard (27 cubic feet).

What is the standard thickness for a concrete slab?

Standard residential patios, walkways, and shed slabs are typically 4 inches deep. Driveways or heavy vehicle pads should be at least 5 to 6 inches thick with steel reinforcement.

How long should a concrete slab cure before walking on it?

Concrete sets enough for light foot traffic within 24 to 48 hours. However, full structural curing takes 28 days. Avoid parking heavy vehicles for at least 7 days.

Slab Dimensions