October 27, 2026 · Construction
A simple guide to estimating concrete volume and bags.
Whether you are pouring a patio slab, building a footing, or setting a column, getting the right amount of concrete is critical. Too little, and you will be making an emergency trip to the store. Too much, and you are wasting money. In this guide, I will walk you through the exact formulas you need, with real examples.
Concrete volume is calculated using the standard volume formula:
Volume = Length × Width × Depth
Make sure all measurements are in the same unit. If you measure length and width in feet, measure depth in feet too. A 4-inch slab is 0.33 feet deep (4 ÷ 12 = 0.33).
Let's say you want to pour a 10 ft × 10 ft patio slab that is 4 inches thick.
To convert cubic feet to cubic yards (the unit concrete is usually ordered in), divide by 27: 33 ÷ 27 = 1.22 cubic yards.
Now imagine a footing that is 20 ft long, 1 ft wide, and 1 ft deep.
For a circular column, the formula is different:
Volume = π × (Diameter/2)² × Height
Let's say you have a column with a 1 ft diameter and 8 ft height.
If you are using pre-mixed bags instead of ordering ready-mix, you need to know how many bags to buy. An 80lb bag of concrete yields about 0.6 cubic feet. To find the number of bags, divide your total volume by 0.6.
For the 33 cubic foot slab example: 33 ÷ 0.6 = 55 bags. Always round up and add a waste allowance of 5-10%.
Concrete is not perfectly efficient. Some is lost to spillage, uneven subgrades, and over-excavation. A 5-10% waste allowance is standard. For the slab example, add 10%: 33 × 1.10 = 36.3 cubic feet, or about 61 bags.
Use our free Concrete Calculator to estimate volume and bags instantly. It runs entirely in your browser — no data is uploaded.