How Much Concrete Do I Need? A Simple Guide for Any Project
Ordering the right amount of concrete prevents costly overages and frustrating shortages. This post explains how to calculate volume for slabs, footings, and columns so you can order with confidence.
Why Getting Concrete Right Matters
Concrete is sold by the cubic yard for ready-mix deliveries and by the bag for small projects. Ordering too little means stopping work mid-pour while you wait for more material, which can create cold joints that weaken the structure. Ordering too much wastes money and requires disposing of unused concrete, which comes with its own logistical and environmental considerations.
Most concrete projects involve simple rectangular, circular, or cylindrical shapes. The math for these shapes is straightforward: length times width times depth for rectangular slabs, and pi times radius squared times depth for circular pads and cylinders. Once you calculate the volume, convert it to cubic yards to match the units used by concrete suppliers.
Adding a waste factor of 5 to 10 percent to your calculated volume is standard practice. This accounts for uneven subgrade, spillage, and the natural variation in pouring irregular forms. Always round up to the nearest quarter yard when ordering from a ready-mix supplier.
Calculating Volume for a Concrete Slab
For a rectangular concrete slab, multiply the length by the width by the thickness. All three measurements must use the same unit. If the slab is 12 feet long, 10 feet wide, and 4 inches thick, convert the thickness to feet first: 4 inches divided by 12 equals 0.333 feet. Multiply 12 times 10 times 0.333 to get 40 cubic feet.
To convert cubic feet to cubic yards, divide by 27 (since 1 cubic yard equals 27 cubic feet). Dividing 40 by 27 gives approximately 1.48 cubic yards. Adding a 10 percent waste factor: 1.48 times 1.10 equals 1.63 cubic yards. You would order 1.75 cubic yards, rounding up to the nearest quarter.
For an L-shaped or other non-rectangular slab, divide the area into separate rectangles and calculate each one individually. Add the volumes together to get the total, then convert to cubic yards and apply the waste factor.
Calculating Volume for Footings and Walls
Concrete footings are the rectangular strips or pads that support a structure's foundation. Calculate their volume just like a slab: length times width times depth. For a footing 20 feet long, 12 inches (1 foot) wide, and 12 inches (1 foot) deep, the volume is 20 times 1 times 1, which equals 20 cubic feet, or about 0.74 cubic yards.
Foundation walls are calculated the same way, treating the wall as a long, thin slab. If the wall is 40 feet long, 8 inches (0.667 feet) thick, and 4 feet tall, the volume is 40 times 0.667 times 4, which equals 106.7 cubic feet or about 3.95 cubic yards.
When a footing and wall share the same form, add their volumes together to get the total concrete needed. For complex foundation systems with multiple footings, walls, and piers, calculate each element separately and sum them, then apply the waste factor to the total.
Calculating Concrete for Cylindrical Columns and Posts
Cylindrical concrete forms, such as post holes and round columns, require a different formula. The volume of a cylinder equals pi (approximately 3.14159) times the radius squared times the height. The radius is half the diameter.
For a round concrete post hole 10 inches in diameter and 3 feet deep, the radius is 5 inches or 0.417 feet. Applying the formula: 3.14159 times 0.417 squared times 3 equals 3.14159 times 0.174 times 3, which equals approximately 1.64 cubic feet. Dividing by 27 gives about 0.06 cubic yards.
For multiple post holes, calculate the volume of one hole and multiply by the number of holes. If you are setting 8 fence posts with this same hole size, the total is 8 times 0.06, or 0.48 cubic yards. For small quantities like this, it is often more practical to buy pre-mixed bags rather than ordering ready-mix concrete.
Choosing Between Ready-Mix and Bagged Concrete
Ready-mix concrete is delivered by truck and is cost-effective for projects requiring more than 1 cubic yard. Most suppliers have a minimum order of 1 yard, and some charge extra for small loads. Ready-mix ensures a consistent mix throughout the pour and is necessary for large projects like driveways and floor slabs.
Bagged concrete is sold in 40-pound, 60-pound, and 80-pound bags. An 80-pound bag typically yields about 0.6 cubic feet of concrete. For a project requiring 3 cubic feet, you would need approximately 5 bags. Bagged concrete is more expensive per cubic foot than ready-mix but is practical for small repairs, post holes, and other projects under a quarter yard.
When deciding between the two, calculate your total volume first. Under 0.5 cubic yards, bagged concrete is usually more convenient despite the higher per-unit cost. Over 1 cubic yard, ready-mix is almost always more economical and produces better results because the entire pour is mixed to the same consistency from a single batch.