How Much Concrete Do I Need? A Simple Guide for DIYers

Ordering the right amount of concrete is one of the most important calculations in any DIY project. Order too little and you are left with an incomplete pour that creates a weak cold joint. Order too much and you waste money on material you cannot return. Here is the straightforward math for getting it right every time.

The Basic Concrete Formula

The formula for calculating concrete volume is length x width x depth, all in the same unit of measurement. For most residential projects, measure in feet and then convert to cubic yards, which is how ready-mix concrete is sold.

To convert cubic feet to cubic yards, divide by 27 (since one cubic yard equals 27 cubic feet). Always add 10% extra to your calculated volume to account for uneven ground, spillage, slight over-excavation, and waste. Running short mid-pour is far worse than having a small amount left over.

Worked example: A patio measuring 12 feet long by 10 feet wide by 4 inches thick (0.33 feet). Volume = 12 x 10 x 0.33 = 39.6 cubic feet. Convert to cubic yards: 39.6 / 27 = 1.47 cubic yards. Add 10% waste: 1.47 x 1.10 = 1.62 cubic yards. Round up to order 1.75 cubic yards.

Common Project Calculations

Patio or slab (4 inches thick): The standard thickness for a residential patio, shed floor, or basketball court is 4 inches. A typical 12x12 foot patio requires approximately 1.8 cubic yards. A larger 20x20 foot patio needs about 5 cubic yards.

Sidewalk (4 inches thick, 3 feet wide): A 30-foot sidewalk that is 3 feet wide and 4 inches thick requires about 1.1 cubic yards. Most residential sidewalks use control joints every 3-4 feet to manage cracking.

Fence post footings (10-inch diameter, 3 feet deep): Each round footing requires about 0.08 cubic yards (2.2 cubic feet). For a fence with posts every 8 feet around a 150-foot perimeter, you need about 19 footings, totaling roughly 1.5 cubic yards.

Driveway (4-6 inches thick): Driveways should be at least 4 inches thick for passenger vehicles and 5-6 inches where heavy vehicles will park. A standard two-car driveway (20x20 feet at 5 inches thick) requires approximately 6.2 cubic yards.

Bags vs Ready-Mix

For small projects under one cubic yard, bagged concrete mix is practical and convenient. For anything larger, ordering a ready-mix truck is more economical and produces a more consistent result.

An 80-pound bag of concrete yields approximately 0.6 cubic feet of mixed concrete. That means you need 45 bags per cubic yard. At roughly $5-6 per bag, one cubic yard of bagged concrete costs $225-$270 in materials alone, not counting your time mixing each bag.

Ready-mix concrete delivered by truck costs $125-$150 per cubic yard in most areas, with a typical minimum order of 1 cubic yard. Some companies charge a short-load fee for orders under 3-5 yards. Even with the delivery fee, ready-mix is almost always cheaper and faster for anything above one cubic yard.

How Many Bags Do I Need?

Here is a quick reference table for bagged concrete:

60-pound bags (0.45 cubic feet per bag): 60 bags per cubic yard
80-pound bags (0.60 cubic feet per bag): 45 bags per cubic yard
90-pound bags (0.68 cubic feet per bag): 40 bags per cubic yard

For a project requiring 0.5 cubic yards, you would need approximately 23 bags of 80-pound mix. That is about 1,840 pounds of dry concrete to mix by hand, which is a significant physical effort. This is another reason to consider ready-mix for anything beyond a small project.

Tips for a Successful Pour

Weather matters. Do not pour concrete when temperatures are below 40°F (4°C) or above 90°F (32°C). Cold weather slows curing dangerously, and hot weather causes the surface to dry and crack before the interior sets. Ideal pouring temperature is 50-70°F. If you must pour in warm weather, dampen the subbase and work in sections.

Get the mix right. Properly mixed concrete should be thick enough to hold its shape when formed into a ball but wet enough to flow into corners and around reinforcement. Too much water weakens the final product. A common mistake is adding extra water to make the mix easier to pour. Instead, use the amount specified on the bag and work the mix into place with a shovel and screed board.

Understand curing time. Concrete reaches approximately 75% of its final strength in 7 days and 100% in 28 days. You can walk on a new slab after 24-48 hours, drive on a driveway after 7 days, and place heavy loads after 28 days. Keep the surface moist for the first 7 days by misting with water or covering with plastic sheeting to prevent premature drying and cracking.

Prepare the subbase. Concrete should be poured on compacted gravel (4 inches minimum) over undisturbed or compacted soil. Never pour directly onto loose fill, topsoil, or organic material. A proper subbase prevents settling and cracking over time.

Use Our Free Concrete Calculator

Our concrete calculator handles slabs, footings, columns, stairs, and custom shapes. Enter your dimensions, and it instantly shows the cubic yards needed, the number of bags for each bag size, and the estimated cost. It includes the 10% waste factor automatically, so the number you see is what you should actually order. No signup, no app download, runs entirely in your browser.