How to Calculate Concrete Volume for Slabs, Footings & Posts
Concrete is one of the most fundamental structural materials in residential and commercial construction. Whether you are pouring a patio slab, installing fence post footings, or setting up a monolithic foundation, ordering the correct volume of concrete is essential to prevent costly job delays or leftover waste fees.
The Standard Concrete Formula
Concrete volume is measured in cubic yards. To calculate how much concrete you need for a rectangular slab:
Cubic Yards = Cubic Feet ÷ 27
Because slab depth is typically specified in inches (such as a 4-inch sidewalk or 6-inch driveway), you must divide the inch thickness by 12 to convert it to feet before multiplying.
Estimating Pre-Mixed Bags vs. Ready-Mix Truck Delivery
For smaller jobs under 1 to 2 cubic yards (such as fence posts, small walkways, or AC pad slabs), pre-mixed concrete bags (Sakrete or Quikrete) are often the most practical choice. Here is a general rule of thumb for bag yields:
- 80 lb bag: yields approximately 0.60 cubic feet (requires ~45 bags per cubic yard).
- 60 lb bag: yields approximately 0.45 cubic feet (requires ~60 bags per cubic yard).
- 50 lb bag: yields approximately 0.375 cubic feet (requires ~72 bags per cubic yard).
For larger pours exceeding 2 cubic yards, ordering a ready-mix truck delivery is significantly faster and physically easier. Ready-mix trucks typically carry 9 to 11 cubic yards per load.
Accounting for Excavation Waste & Spillage
In real-world jobsite conditions, soil subgrades are rarely perfectly level. Excavation flex, sub-base settling, and form board bowing can easily consume 5% to 10% more concrete than theoretical dimensions suggest. Professional general contractors always add a 10% safety margin to ready-mix orders.