Ordering too little concrete means stopping mid-pour to wait for another truck — a nightmare that can ruin your slab with cold joints. Ordering too much wastes money on excess material you'll have to dispose of. Getting the math right before you call the ready-mix plant saves headaches and money.
This guide covers how to calculate concrete for the most common DIY projects: slabs, footings, columns, and steps. All measurements use the standard formula that converts to cubic yards — the unit ready-mix concrete is sold in.
The Basic Formula
Concrete volume is simply length × width × depth (thickness). Convert all measurements to feet before multiplying, then divide by 27 to convert cubic feet to cubic yards (since 1 cubic yard = 27 cubic feet).
Cubic yards = (Length × Width × Depth) / 27. Example: a 10-foot × 20-foot driveway slab, 4 inches thick. Convert 4 inches to feet: 4/12 = 0.333 feet. Volume = 10 × 20 × 0.333 = 66.7 cubic feet / 27 = 2.47 cubic yards. Order at least 2.5 cubic yards, adding a 5-10% waste factor.
Always add a 5-10% waste factor to account for uneven subgrade, spillage, and the fact that it's better to have a little extra than run short. Most contractors add 10% for slabs and 5% for footings.
Slabs: Driveways, Patios, Floors
Residential slabs are typically 4 inches thick for patios and light-use areas, and 6 inches thick for driveways and garage floors. Thickening to 6" at the edges (a thickened edge slab) adds structural support without significantly increasing concrete volume.
4-inch slab at 100 square feet = (100 × 0.333) / 27 = 1.23 cubic yards. 4-inch slab at 400 square feet (about a 20×20 garage) = (400 × 0.333) / 27 = 4.93 cubic yards → order 5.5 cubic yards with waste factor.
For large slabs, consider whether to pour all at once or in sections. A single continuous pour produces a stronger, crack-resistant slab, but requires enough crew to finish it before it sets. In hot, dry weather, concrete sets faster — have enough people ready.
Footings
Footings support structural loads and must be sized to the soil bearing capacity and load above. A typical residential perimeter footing might be 16" wide × 8" deep. Calculate the volume per linear foot and multiply by total footing length.
Linear footing volume per foot = (width in feet × depth in feet × 1 linear foot). A 16"×8" footing: (1.33 × 0.67 × 1) = 0.89 cubic feet per linear foot. For 100 linear feet of footing: 100 × 0.89 = 89 cubic feet / 27 = 3.3 cubic yards.
Spread footings (pad footings) for posts or columns are rectangular volumes. A 24"×24"×12" spread footing: (2 × 2 × 1) = 4 cubic feet / 27 = 0.148 cubic yards each. Ten of these footings = 1.48 cubic yards — add to your slab order.
Round Columns and Cylindrical Forms
Round concrete forms (Sonotubes) are used for deck footings, fence posts, and column bases. The formula for a cylinder: Volume = π × r² × height, where r is the radius (half the diameter) in feet.
A 12" diameter (6" radius = 0.5 feet) Sonotube, 48" (4 feet) deep: V = 3.14159 × (0.5)² × 4 = 3.14159 × 0.25 × 4 = 3.14 cubic feet. In cubic yards: 3.14 / 27 = 0.116 cubic yards.
For a deck with 12 post footings of this size: 12 × 0.116 = 1.39 cubic yards. If you're also pouring a small patio nearby, it often makes sense to coordinate both pours for the same concrete truck delivery to minimize delivery charges.
Practical Tips for Ordering Concrete
Ready-mix concrete is typically ordered in full cubic yard increments, with a short-load fee charged for less than a minimum quantity (often 3-4 cubic yards). If your project calculates to 2.7 yards, you'll often pay the same for 3 yards — and the extra is insurance against running short.
Specify the right mix: residential slabs typically use 3,000 PSI mix (standard) or 4,000 PSI (driveways, where freeze-thaw cycling is a concern). Specify air-entrainment if you're in a climate that freezes. Order fiber reinforcement for crack resistance in slabs.
Use our free Concrete Calculator to handle the math for any shape — slabs, footings, columns, steps, and curbs. Enter your dimensions in feet or inches and it calculates cubic yards, adds your specified waste factor, and tells you how many bags of premix to buy if you're doing a smaller project by hand.