How Much Concrete Do I Need? Calculator, Formula & Tips

Ordering too little concrete means a costly second delivery. Ordering too much means paying for excess you’ll waste. Getting the calculation right saves money and frustration on any project from a small patio to a full foundation. Here’s how to calculate exactly what you need.

The Basic Concrete Volume Formula

For rectangular slabs (patios, sidewalks, driveways, garage floors):

Volume (cubic feet) = Length (ft) × Width (ft) × Thickness (ft)
Volume (cubic yards) = Volume (cubic feet) / 27

Example: A 12 ft × 20 ft patio, 4 inches thick:

• Convert thickness: 4 inches = 0.333 feet
• Volume: 12 × 20 × 0.333 = 79.92 cubic feet
• Cubic yards: 79.92 / 27 = 2.96 cubic yards

For cylindrical columns/posts:

Volume = π × r² × h

Example: A 12-inch diameter sonotube, 4 feet deep:

• Radius: 6 inches = 0.5 feet
• Volume: 3.14159 × 0.5² × 4 = 3.14 cubic feet
• Per tube: 0.116 cubic yards

Common Project Calculations

Project	Typical Dimensions	Concrete Needed
Sidewalk (4 ft × 50 ft × 4”)	200 sq ft	2.47 cu yd
Single car driveway (10 × 20 × 5”)	200 sq ft	3.09 cu yd
Double car driveway (16 × 40 × 5”)	640 sq ft	9.88 cu yd
Patio (12 × 16 × 4”)	192 sq ft	2.37 cu yd
Garage floor (20 × 22 × 5”)	440 sq ft	6.79 cu yd
Footing (12” × 12” × 50 linear ft)	—	1.85 cu yd
Deck pier (12” dia × 48” deep) each	—	0.116 cu yd

Bag Calculations vs Ready-Mix Truck

For small projects, pre-mixed bags are practical. For anything over 1 cubic yard, a ready-mix truck delivery is more economical.

Bags needed per cubic yard:

Bag Size	Bags per Cubic Yard
40 lb	85 bags
60 lb	57 bags
80 lb	43 bags

Our 2.96 cubic yard patio would need about 127 bags of 80-lb concrete — clearly a job for a truck delivery, not bag mixing.

Cost comparison:

• 80-lb bags: ~$6 each → 127 bags = $762 + your labor mixing
• Ready-mix delivery: ~$150–$180/cubic yard → 2.96 yards = $444–$533 delivered

The break-even point is roughly 1 cubic yard or 40-45 bags of 80-lb mix.

Ordering Tips

Always add 5–10% extra to your calculated volume for waste, spillage, uneven subgrade, and slight over-excavation. For our 2.96 yard patio, order 3.25 cubic yards.

Truck minimums: Most concrete suppliers have a minimum order (often 1 cubic yard) and charge short-load fees for orders under their minimum full-truck delivery (typically 8–10 yards).

Timing matters: Concrete begins setting in 60–90 minutes. For large pours, ensure you have enough help to spread, screed, and finish before it sets. In hot weather (above 85°F), the window shrinks significantly.

Subgrade prep: Compact your base material and check for low spots before the truck arrives. An uneven base wastes concrete filling unexpected dips.

Calculate instantly with our Concrete Calculator.