How to Calculate Concrete Volume Accurately for Slabs, Footings & Posts

The Concrete Rule: Once It's Poured, It's Set

In construction, there is an adage: *"You can always cancel unused lumber, but concrete waits for no one."* Ordering too little ready-mix concrete leaves you with cold joints, weakened structural integrity, and emergency short-load charges that can easily double delivery fees. Ordering too much wastes hundreds of dollars and leaves you with hard-to-dispose curing material.

This guide walks through the exact formulas professional contractors use to size slabs, round post footings, and calculate bagged concrete requirements.

Understanding the Cubic Yard

In North America, ready-mix concrete and bulk landscape supplies (gravel, mulch, sand, topsoil) are sold by the **Cubic Yard ($yd^3$)**.

$$1 \text{ Cubic Yard} = 27 \text{ Cubic Feet} = 46,656 \text{ Cubic Inches} \approx 0.76455 \text{ m}^3$$

A standard 8-yard or 10-yard concrete truck carries approximately 10 cubic yards of concrete, weighing over 40,000 lbs (20 tons).

Formula 1: Slabs, Walkways, and Patios

For any rectangular prism (length $\times$ width $\times$ thickness):

Convert all dimensions into feet first:

Length in feet ($L$)

Width in feet ($W$)

Thickness in inches divided by 12 ($T_{ft} = \text{thickness in inches} / 12$)

Multiply to find total cubic feet:

$$\text{Cubic Feet} = L \times W \times (\text{Thickness in inches} / 12)$$

Divide by 27 to obtain cubic yards:

$$\text{Cubic Yards} = \frac{L \times W \times (\text{Thickness} / 12)}{27}$$

Practical Example:

A garage driveway slab measuring $24 \text{ ft}$ long by $20 \text{ ft}$ wide, poured to a standard heavy-duty thickness of $5 \text{ inches}$:

Thickness in feet: $5 / 12 = 0.4167 \text{ ft}$

Volume in cubic feet: $24 \times 20 \times 0.4167 = 200 \text{ cu ft}$

Volume in cubic yards: $200 / 27 = 7.41 \text{ cu yd}$

Formula 2: Circular Columns & Sonotube Footings

Deck posts, fence footings, and pole-barn piers require calculating cylinder volumes:

$$\text{Volume} = \pi \times r^2 \times h$$

Where:

$r$ is the radius in feet (diameter in inches divided by 24)

$h$ is the depth in feet

Practical Example:

12 post footings, each $12 \text{ inches}$ in diameter and $4 \text{ feet}$ deep:

Radius: $12 / 24 = 0.5 \text{ ft}$

Volume per footing: $3.14159 \times (0.5)^2 \times 4 = 3.1416 \text{ cu ft}$

Total for 12 footings: $3.1416 \times 12 = 37.7 \text{ cu ft}$

In cubic yards: $37.7 / 27 = 1.40 \text{ cu yd}$

Calculating Ready-Mix vs. Bagged Concrete

For small projects under $1.5 \text{ cu yd}$, pre-mixed dry bags (such as Quikrete or Sakrete) are generally more economical than calling a batch plant truck:

Bag Weight	Yield in Cubic Feet	Bags per 1 Cubic Yard
80 lb (36.3 kg)	$0.60 \text{ cu ft}$	45 bags
60 lb (27.2 kg)	$0.45 \text{ cu ft}$	60 bags
50 lb (22.7 kg)	$0.375 \text{ cu ft}$	72 bags

If your project requires $1.2 \text{ cu yd}$:

At 80-lb bags: $1.2 \times 45 = 54 \text{ bags}$.

At 60-lb bags: $1.2 \times 60 = 72 \text{ bags}$.

The Crucial 10% Waste & Over-Excavation Margin

Never order the exact theoretical volume! On a real job site:

Soil sub-bases are uneven (a half-inch dip across a 400 sq ft patio consumes an extra 0.6 cubic yards).

Wooden forms bow outward under hydrostatic pressure.

Residue adheres to the pump truck chute or wheelbarrow basins.

Spillage occurs during pouring and screeding.

**Rule of Thumb:**

Standard slab on grade: **Add 10%** to your total.

Complex stamped curves or step forms: **Add 12% to 15%**.

Controlled steel forms on laser-graded gravel: **Add 5% minimum**.

Always round your final ready-mix truck order up to the nearest quarter or half yard.