How Much Concrete Do I Need?

Whether you're pouring a patio, setting fence posts, or building a driveway, getting the right amount of concrete is critical. Order too little and you'll end up with weak cold joints. Order too much and you'll waste money. This guide walks you through exactly how to calculate concrete for any project.

The Basic Concrete Formula

All concrete calculations start with one fundamental formula:

Volume (cubic feet) = Length (ft) x Width (ft) x Depth (ft)
Cubic Yards = Volume (cubic feet) / 27

Concrete in the United States is sold by the cubic yard. One cubic yard equals 27 cubic feet (3 ft x 3 ft x 3 ft). If you're working in metric units, the formula uses meters and the result is in cubic meters.

The most common mistake people make is forgetting to convert thickness from inches to feet. If your slab is 4 inches thick, you must use 4/12 = 0.333 feet in your calculation, not 4.

Step-by-Step Calculation Process

Step 1: Measure Your Project Area

Use a tape measure to determine the length, width, and thickness (or depth) of the area you plan to fill with concrete. For irregular shapes, break the area into simple rectangles or circles and calculate each section separately.

Pro tip: Measure in feet and inches, then convert inches to decimal feet. For example, 10 feet 6 inches = 10.5 feet.

Step 2: Calculate Volume

Multiply your three measurements together. Remember to convert thickness from inches to feet first by dividing by 12.

Example: A patio that is 12 feet long, 10 feet wide, and 4 inches thick:

• Convert thickness: 4 inches / 12 = 0.333 feet
• Volume: 12 x 10 x 0.333 = 40 cubic feet
• Cubic yards: 40 / 27 = 1.48 cubic yards

Step 3: Add a Safety Margin

Always add 5-10% to your calculated volume. This accounts for spillage, uneven subgrade, slight variations in form dimensions, and rounding errors. For most projects, 10% extra is a safe bet.

Using the patio example above: 1.48 x 1.10 = 1.63 cubic yards. You would order 1.75 cubic yards to be safe (most ready-mix companies sell in quarter-yard increments).

Step 4: Choose Bags or Ready-Mix

For small projects (under 1 cubic yard), bags are practical. For larger projects, ready-mix truck delivery is more efficient and cost-effective.

Bag Size	Yield per Bag	Bags per Cubic Yard
40 lb	0.30 cu ft	~90 bags
60 lb	0.45 cu ft	~60 bags
80 lb	0.60 cu ft	~45 bags

Common Project Examples

Patio (12 x 14 ft, 4 inches thick)

Volume: 12 x 14 x 0.333 = 56 cu ft = 2.07 cubic yards. With 10% extra: 2.28 cubic yards. This project is best served by ready-mix delivery. Estimated cost at $150/yard: $342.

Driveway (10 x 40 ft, 5 inches thick)

Volume: 10 x 40 x 0.417 = 167 cu ft = 6.17 cubic yards. With 10% extra: 6.79 cubic yards. You'll want 4000 PSI concrete for vehicle traffic. Estimated cost at $160/yard: $1,086.

Sidewalk (3 x 30 ft, 4 inches thick)

Volume: 3 x 30 x 0.333 = 30 cu ft = 1.11 cubic yards. With 10% extra: 1.22 cubic yards. This could be done with bags (about 55 bags of 80-lb mix) or a small ready-mix order. Estimated cost at $150/yard: $183.

Fence Post Holes (20 posts, 10 in diameter, 36 in deep)

Each hole: pi x (5/12)^2 x 3 = 1.64 cu ft. Total for 20 posts: 32.7 cu ft = 1.21 cubic yards. With 10% extra: 1.33 cubic yards. About 60 bags of 80-lb mix, or you could order a ready-mix truck. Estimated cost at $150/yard: $200.

Garage Floor (20 x 22 ft, 5 inches thick)

Volume: 20 x 22 x 0.417 = 183 cu ft = 6.80 cubic yards. With 10% extra: 7.48 cubic yards. Definitely order ready-mix. Use 4000 PSI with fiber reinforcement for best results. Estimated cost at $160/yard: $1,197.

Shed Foundation (8 x 10 ft, 4 inches thick)

Volume: 8 x 10 x 0.333 = 26.7 cu ft = 0.99 cubic yards. With 10% extra: 1.09 cubic yards. Right on the borderline between bags and ready-mix. About 45 bags of 80-lb mix. Estimated cost at $150/yard: $164.

Understanding Concrete Mix Types

Not all concrete is the same. The PSI rating tells you how much compressive force the concrete can withstand after 28 days of curing.

• 2500 PSI: General use, non-structural applications like fence posts and small pads.
• 3000 PSI: The standard residential mix. Suitable for sidewalks, patios, basement floors, and most slabs.
• 3500 PSI: A step up from standard. Good for driveways in moderate climates.
• 4000 PSI: High-strength mix for driveways, garage floors, and areas with heavy vehicle traffic.
• 4500+ PSI: Commercial and industrial applications, structural columns, and load-bearing walls.

Cost Factors to Consider

The total cost of a concrete project depends on more than just the concrete itself:

• Concrete material: $120-$200 per cubic yard for ready-mix, or $5-$8 per bag.
• Delivery fee: Most plants charge $50-$100 for delivery, sometimes included for large orders.
• Short load fee: Orders under 5-10 cubic yards may incur a surcharge of $25-$75 per yard.
• Saturday/overtime delivery: Expect a 10-20% premium for non-standard delivery times.
• Reinforcement: Rebar and wire mesh add $0.50-$1.50 per square foot.
• Labor: If hiring a contractor, expect $5-$12 per square foot for flatwork (pour, finish, cure).
• Base preparation: Gravel, compaction, and excavation add $1-$3 per square foot.

Try the Calculator

Skip the manual math and use our free concrete calculator below. Select your shape, enter your dimensions, and get instant results including volume, bag count, and cost estimate.