Concrete Calculator

Accurately determining ready-mix concrete quantities is essential for pouring building slabs, patios, footings, and sidewalks without running out mid-

Construction

Length

Length of the pour in decimal feet (e.g. 12.5 for 12 ft 6 in)

Width

Width of the pour in decimal feet

Thickness / Depth

Thickness in inches (converted to feet as in ÷ 12)

Waste & Overage Allowance

Covers uneven subgrade, spillage, and form deflection (5-10% standard)

Price per Cubic Yard (Optional)

Enter delivered ready-mix price per cu yd to estimate material cost

Formula & Methodology

Depth (ft) = Thickness (in) ÷ 12 Net Volume (cu ft) = Length (ft) × Width (ft) × Depth (ft) Net Volume (cu yd) = Net Volume (cu ft) ÷ 27 Gross Volume to Order (cu yd) = ⌈Net Volume (cu yd) × (1 + Waste % ÷ 100) ÷ 0.25⌉ × 0.25 Metric Volume (m³) = Gross Volume (cu yd) × 0.76455486

Because concrete is ordered in cubic yards while slab depth is measured in inches, thickness must be converted to decimal feet by dividing by 12 before multiplying planar area. The resulting cubic feet are converted to cubic yards by dividing by 27 (since 1 cu yd = 3 ft × 3 ft × 3 ft = 27 cu ft). A waste margin (typically 10%) is applied, and the ordered quantity is rounded strictly up to the nearest 0.25 cubic yard supplier increment to prevent expensive cold joints.

Concrete to Order (Rounded Up)

1.5 cu yd

Overview

Accurately determining ready-mix concrete quantities is essential for pouring building slabs, patios, footings, and sidewalks without running out mid-pour or overpaying for surplus mix. This Concrete Calculator determines the exact cubic yardage required from length, width, and thickness measurements, factoring in subgrade absorption and form deflection with a custom waste reserve.

Estimates are for material planning and budgeting purposes only. This calculator does not provide structural engineering, architectural design, or official building code compliance advice. Always verify measurements, local building codes, and permit requirements with a licensed contractor or local building authority before purchasing materials or commencing construction.

Worked Examples

Example 1: Standard 10 ft × 10 ft Slab at 4 in Thick with 10% Waste (Verification Case)

Inputs: Length = 10 ft, Width = 10 ft, Thickness = 4 in, Waste = 10%, Price = $0.00

Convert thickness from inches to feet: 4 in ÷ 12 = 0.3333 ft.
Calculate net cubic feet: 10 ft × 10 ft × 0.3333 ft = 33.33 cu ft.
Convert cubic feet to cubic yards (divide by 27): 33.33 cu ft ÷ 27 = 1.23 cu yd.
Add 10% waste reserve: 1.2346 cu yd × 1.10 = 1.36 cu yd.
Round UP to the next ready-mix supplier batch increment (0.25 cu yd): 1.50 cu yd.

Final Answer: 1.50 cu yd to order (1.23 net cu yd + 0.12 waste reserve; exact unrounded: 1.36 cu yd)

Example 2: Thicker 10 ft × 10 ft Slab at 6 in Depth with 10% Waste (Verification Case)

Inputs: Length = 10 ft, Width = 10 ft, Thickness = 6 in, Waste = 10%, Price = $150.00/cu yd

Convert depth to feet: 6 in ÷ 12 = 0.50 ft.
Calculate net cubic feet: 10 ft × 10 ft × 0.50 ft = 50.00 cu ft.
Convert to cubic yards: 50.00 cu ft ÷ 27 = 1.85 cu yd.
Add 10% waste buffer: 1.8519 cu yd × 1.10 = 2.04 cu yd.
Round UP to supplier 0.25 yd increment: 2.25 cu yd.
Calculate estimated material cost: 2.25 cu yd × $150.00 = $337.50.

Final Answer: 2.25 cu yd to order (1.85 net cu yd; $337.50 estimated material cost)

Example 3: Tape Measure Dimensions (12.5 ft [12 ft 6 in] × 10 ft at 4 in with 10% Waste)

Inputs: Length = 12.5 ft (12 ft 6 in), Width = 10 ft, Thickness = 4 in, Waste = 10%

Confirm decimal length: 12 ft 6 in = 12 + (6 ÷ 12) = 12.5 ft.
Convert 4 in thickness: 4 in ÷ 12 = 0.3333 ft.
Calculate net cubic volume: 12.5 ft × 10 ft × 0.3333 ft = 41.67 cu ft.
Convert to cubic yards: 41.667 cu ft ÷ 27 = 1.54 cu yd.
Add 10% waste allowance: 1.5432 cu yd × 1.10 = 1.70 cu yd.
Round UP to next 0.25 yd truck increment: 1.75 cu yd.

Final Answer: 1.75 cu yd to order (1.54 net cu yd + 0.15 waste reserve; exact: 1.70 cu yd)

How to Use This Calculator

Measure the length and width of your pour area in feet and enter them into the dimensions fields.

Enter the slab thickness in inches (common residential flatwork uses 4 inches).

Select an appropriate waste factor (10% is standard for flat slabs; use 15% for rough ground excavations).

Optionally enter your local supplier concrete price per cubic yard to estimate cost.

Click Calculate to view net theoretical volume, exact cubic yards, rounded truck quantity, and 80-lb bag equivalents.

Reference Table

Slab Dimensions (ft)	Thickness (in)	Net cu ft	Net cu yd	Total to Order (10% Waste)
10 × 10	4 in	33.33	1.23	1.50 cu yd
10 × 10	6 in	50.00	1.85	2.25 cu yd
12 × 10	4 in	40.00	1.48	1.75 cu yd
15 × 10	4 in	50.00	1.85	2.25 cu yd
20 × 10	4 in	66.67	2.47	2.75 cu yd
20 × 20	4 in	133.33	4.94	5.50 cu yd
24 × 24	4 in	192.00	7.11	8.00 cu yd

Frequently Asked Questions

Why do I divide cubic feet by 27 instead of 3 or 9 to get cubic yards?

A cubic yard measures 3 feet long by 3 feet wide by 3 feet high, which equals 3 × 3 × 3 = 27 cubic feet. Dividing by 9 gives square yards (an area measurement), not cubic yards (volume). Always divide cubic feet by 27.

Why must depth in inches be converted before multiplying length and width?

Length and width are measured in feet. Multiplying 10 ft by 10 ft by 4 inches directly would incorrectly treat the depth as 4 feet (resulting in 400 cubic feet instead of the true 33.33 cubic feet). Four inches equals 4 ÷ 12 = 0.3333 feet.

Why should I always round my concrete order up to the nearest quarter or half yard?

Ready-mix batch plants dispense concrete in increments (commonly 0.25 or 0.5 cubic yards). Running short by even a fraction of a yard halts the pour, causes cold joints where batches fail to bond, and incurs expensive short-load delivery fees for a second truck.

What waste percentage should I add to my concrete calculations?

Add 5% to 8% for pours placed inside smooth wooden forms on level ground. Add 10% to 15% for ground excavations, uneven dirt subgrades, and footings where trench walls bulge or lose material.

How many 80-pound bags of concrete equal one cubic yard?

One standard 80-pound bag yields approximately 0.60 cubic feet of mixed concrete. Since 1 cubic yard equals 27 cubic feet, you need 27 ÷ 0.60 = 45 bags of 80-lb concrete to equal 1 cubic yard (or 60 bags of 60-lb concrete).