How to Estimate Concrete for DIY Projects

The Unit You Need: Cubic Yards

Concrete is measured by volume. In the US, the standard unit is the cubic yard(cu yd), which equals 27 cubic feet. Ready-mix trucks deliver in cubic yards. Bags of dry concrete mix are labeled by weight, but their yield (the volume of mixed concrete they produce) is given in cubic feet on the bag.

The key step in any concrete estimate is converting your project dimensions into cubic feet, then dividing by 27 to get cubic yards — or matching the result to bag yields for smaller jobs.

Calculating Volume for Common Pour Shapes

Rectangular Slabs (Patios, Driveways, Shed Floors)

Volume = Length × Width × Thickness (all in feet) ÷ 27 = Cubic yards

Example: A 10 × 12 ft patio at 4-inch thickness:

• Thickness in feet: 4 in ÷ 12 = 0.333 ft
• Volume: 10 × 12 × 0.333 = 40 cu ft
• Cubic yards: 40 ÷ 27 = 1.48 cu yd

Standard slab thicknesses by application:

• Walkways and patios: 4 inches
• Driveways: 4–6 inches (6 for heavy vehicles)
• Garage floors: 4–6 inches (4 is code minimum in most jurisdictions)
• Structural slabs and load-bearing pads: engineer-specified, often 6–8 inches or more

Footings (Strip or Continuous)

Strip footings are rectangular beams under foundation walls. The formula is the same: Length × Width × Depth ÷ 27.

Example: A 20 ft footing, 16 in wide, 12 in deep:

• Width in feet: 16 ÷ 12 = 1.33 ft
• Depth in feet: 12 ÷ 12 = 1.0 ft
• Volume: 20 × 1.33 × 1.0 = 26.6 cu ft ÷ 27 = 0.99 cu yd

Round Columns and Post Holes

Cylindrical volumes use the circle area formula: π × r² × Depth, where r is the radius in feet.

Example: An 8-inch-diameter post hole, 36 inches deep:

• Radius in feet: 4 in ÷ 12 = 0.333 ft
• Depth in feet: 36 ÷ 12 = 3.0 ft
• Volume: π × (0.333)² × 3.0 = 3.14 × 0.111 × 3.0 = 1.046 cu ft

For post holes, you must also subtract the post displacement (the volume the post itself occupies in the hole). A 4×4 post (3.5 × 3.5 in actual dimension) displacing 36 inches of hole: 0.292 × 0.292 × 3.0 = 0.256 cu ft.

Net concrete needed per hole: 1.046 − 0.256 = 0.79 cu ft.

The Post Hole Concrete Calculator handles this automatically for any post size and hole depth.

Bags vs. Ready-Mix: Which Should You Use?

The decision between bags and ready-mix is primarily driven by project volume and logistics.

Bag Yields

Bag yields are based on Portland Cement Association and manufacturer specifications:

• 40 lb bag: 0.30 cu ft of mixed concrete
• 60 lb bag: 0.45 cu ft of mixed concrete
• 80 lb bag: 0.60 cu ft of mixed concrete

Example: For a post-hole project requiring 12 cu ft total: 12 ÷ 0.60 = 20 bags of 80 lb mix.

Always round up. Returning an unopened bag is easy; running short mid-pour is not.

Understanding Concrete Strength (PSI)

Concrete is rated by compressive strength in PSI (pounds per square inch), measured at 28 days after pouring. Higher PSI means a harder, stronger slab — but also a higher water-to-cement ratio that requires more careful mixing.

• 2,500 PSI: Residential walkways, patios, steps. Minimum for most non-structural applications.
• 3,000 PSI: Driveways, garage floors. Most common residential specification.
• 3,500–4,000 PSI: Structural footings, load-bearing slabs, and applications exposed to freeze-thaw cycles.
• 4,000+ PSI: Commercial applications, engineered structures. Requires professional specification.

Pre-mixed bags are formulated to reach a specific PSI when mixed per the bag instructions. Over-adding water (a common DIY shortcut to make the mix more workable) significantly reduces the final strength.

Always Order 10% Extra

Add at least 10% to your calculated volume before ordering. This accounts for:

• Irregular form surfaces that consume more concrete than a geometric calculation predicts
• Spillage and material left in the mixer
• Subgrade settling that creates uneven depth across a slab
• Re-pouring if the first attempt cannot be finished (rare with ready-mix, common with bags)

Running short of ready-mix mid-pour is a serious problem: the first concrete begins curing while you wait for more, creating a cold joint (a weak plane in the slab) at the boundary. For slabs, it is almost always worth over-ordering slightly.

Ready-Mix Ordering Tips

• Know the minimum order. Most ready-mix plants have a minimum delivery of 1 cubic yard, with a short-load surcharge (typically $50–$150) for orders under their standard minimum (often 3–4 yards).
• Confirm the PSI and slump. Slump is a measure of workability (how wet the mix is). A 4-inch slump is standard for residential pours. Higher slump is easier to work but weaker.
• Have your forms and tools ready before the truck arrives. You typically have 60–90 minutes of working time from delivery. The truck driver may charge a waiting fee if pour is delayed.
• Plan access. A ready-mix truck is heavy. Know your driveway and site access before scheduling delivery.

Worked Example: 12 × 20 ft Shed Floor

Project: A detached shed floor, 12 × 20 ft, 4 inches thick. Six 4×4 anchor posts to be set around the perimeter in 10-inch-diameter holes, 30 inches deep.

Slab volume:

• 12 × 20 × 0.333 ft = 79.92 cu ft ÷ 27 = 2.96 cu yd

Post holes (6 holes):

• Hole radius: 5 in ÷ 12 = 0.417 ft
• Per hole volume: π × 0.417² × 2.5 = 1.37 cu ft
• Post displacement: 0.292 × 0.292 × 2.5 = 0.21 cu ft
• Net per hole: 1.37 − 0.21 = 1.16 cu ft
• 6 holes: 6 × 1.16 = 6.96 cu ft ÷ 27 = 0.26 cu yd

Total: 2.96 + 0.26 = 3.22 cu yd + 10% = order 3.6 cu yd → round to 4 cu yd

Use the Concrete Calculator and Post Hole Calculator to verify this instantly.