Concrete Calculator

Calculate concrete volume for slabs, footings, and columns

About Concrete Calculator

A concrete calculator estimates the volume of concrete needed for a slab, footing, or column based on its dimensions, and converts that volume into the number of pre-mixed bags or cubic yards to order. It helps avoid ordering too much (wasted material) or too little (a delayed pour).

Formula

Volume = Length × Width × Depth (converted to cubic yards or cubic feet)

How It Works

Select the shape you're pouring (slab, footing, column, or sonotube)
Enter the dimensions — length, width, and thickness/depth for a slab, or diameter and height for a column
The calculator finds the total volume needed
Result is converted into cubic yards (for ready-mix trucks) or the number of bags of a given size (e.g., 60 lb or 80 lb bags)

Tips

Always add 5-10% extra to your calculated volume to account for spillage, uneven subgrade, and formwork variance
Concrete is typically ordered in cubic yards for large pours (ready-mix truck) and in bags for small pours (patios, footings, fence posts)
Depth is one of the most commonly underestimated dimensions — measure at multiple points if the subgrade isn't perfectly level

Frequently Asked Questions

How do I calculate how much concrete I need for a slab?

Multiply length × width × depth (in feet) to get cubic feet, then divide by 27 to convert to cubic yards. For example, a 10×10 ft slab at 4 inches (0.33 ft) deep: 10 × 10 × 0.33 = 33 cubic feet ≈ 1.22 cubic yards.

How many 80 lb bags of concrete do I need per cubic yard?

An 80 lb bag yields about 0.6 cubic feet of concrete, so you need roughly 45 bags per cubic yard (27 cubic feet ÷ 0.6).

Should I order extra concrete beyond my calculated amount?

Yes — most contractors add 5-10% extra to account for spillage, form leakage, and subgrade that isn't perfectly level, since running short mid-pour is far more costly than minor leftover material.

What's the difference between cubic feet and cubic yards for ordering concrete?

1 cubic yard = 27 cubic feet. Ready-mix trucks are ordered by the cubic yard; bagged concrete for small jobs is usually calculated in cubic feet.

How deep should a typical concrete slab be?

A standard patio or walkway slab is usually 4 inches thick; slabs supporting vehicles (driveways) are typically 5-6 inches; footings and structural slabs may require greater depth per local building code.