Concrete Calculator for Slabs, Footings and Posts
Our concrete calculator works out how much concrete you need in cubic yards, cubic feet and cubic meters. Use it as a concrete slab calculator for patios and driveways, or for footings, walls, columns and post holes. It also counts premixed bags and adds a waste allowance.
How Concrete Is Measured
Concrete is sold by volume. In the US, ready mixed concrete is ordered in cubic yards, while bags list how many cubic feet they fill. Most other countries use cubic meters for both.
One cubic yard is a cube 3 feet on each side, which holds 27 cubic feet. That is enough for a 10 by 8 foot slab about 4 inches thick. A concrete calculator turns your measurements into these units, so you order the right amount the first time.
How to Use the Concrete Calculator
- Choose US units (feet and inches) or metric units (meters and centimeters).
- Pick a shape for the first pour and enter its measurements.
- Add more pours for other parts of the job, and set a quantity for repeats.
- Set a waste allowance and, if you like, a price per cubic yard or meter.
The results show the total in cubic yards, cubic meters and cubic feet, the number of 40, 60 and 80 lb bags and the approximate weight. A line for each pour appears under the results, so you can check every part. It helps most on a bigger project.
Concrete Formulas
Every shape comes down to area times thickness. The concrete calculator converts inches to feet, or centimeters to meters, before it multiplies. Then it divides cubic feet by 27 to get cubic yards.
Slabs, Footings and Walls
Multiply length × width × thickness. A slab of 10 by 10 feet and 4 inches thick is 10 × 10 × 4 ÷ 12 = 33.33 cubic feet, or 1.23 cubic yards. Footings and walls use the same formula with their own width and height.
Round Columns, Posts and Round Slabs
Use π × (diameter ÷ 2)² × height. A post hole 10 inches across and 3 feet deep holds about 1.64 cubic feet. Ten post holes need 16.36 cubic feet, or 0.61 cubic yards.
How Much Concrete Do I Need?
Start by measuring the area in feet and the thickness in inches. Enter them in the concrete slab calculator, add 5% to 10% for waste and round up your order. For uneven ground, measure the depth in several places and enter the average in the concrete slab calculator.
The table below shows common slab sizes at 4 inches thick, with 10% extra included. Each figure comes from the concrete slab calculator. Use it as a quick check before you enter your own numbers.
| Slab size | Cubic yards with 10% extra |
| 10 × 10 ft | 1.36 |
| 12 × 12 ft | 1.96 |
| 20 × 20 ft | 5.43 |
| 24 × 24 ft | 7.82 |
For a 6 inch slab, multiply these figures by 1.5. Ready mix suppliers usually sell in steps of a quarter or half yard. Order slightly more rather than run short during a pour.
Concrete Calculator Examples
A Patio Slab
A 10 by 10 foot patio, 4 inches thick, needs 1.23 cubic yards. With 10% extra, the concrete slab calculator gives 1.36 cubic yards, or 1.04 cubic meters. That is 62 bags of 80 lb concrete and about 5,500 pounds in total.
A Driveway
A 20 by 24 foot driveway poured 5 inches thick needs 7.41 cubic yards. With 10% waste the concrete slab calculator shows 8.15 cubic yards. This size calls for a truck, not bags.
A Strip Footing
A footing 40 feet long, 16 inches wide and 8 inches deep holds 35.56 cubic feet, or 1.32 cubic yards. Add 10% and you need about 1.45 cubic yards. Measure the trench before you order, since footings are often dug wider.
Fence Post Holes
Ten post holes, each 10 inches across and 3 feet deep, need 16.36 cubic feet of concrete. That is 28 bags of 80 lb mix, or 30 bags with 10% extra. Set the quantity to 10 in the concrete calculator instead of adding ten rows.
Recommended Slab Thickness
| Project | Typical thickness |
| Patio or walkway | 4 inches |
| Driveway for cars | 4 to 5 inches |
| Driveway for heavy trucks or RVs | 6 inches or more |
| Garage floor | 4 to 6 inches |
| Shed base | 4 inches |
These are common starting points for the concrete slab calculator, not a design for your site. Soil, climate and loads all change what is needed. For structural work such as foundations, follow local building codes and an engineer's advice.
Garage Floors and Shed Bases
A garage floor is usually 4 to 6 inches thick over a compacted gravel base. A 24 by 24 foot garage at 5 inches needs about 8.89 cubic yards, or 9.78 with 10% extra. Enter the size in the concrete slab calculator and change the thickness to see how fast the volume grows.
A shed base of 8 by 10 feet at 4 inches needs only 0.99 cubic yards, or 1.09 with waste. That is about 49 bags of 80 lb mix, so a small ready mix load may be easier. The concrete calculator shows both options side by side.
Bags or Ready Mix Concrete
Premixed bags are handy for small jobs such as post holes, steps and repairs. An 80 lb bag makes about 0.60 cubic feet, a 60 lb bag 0.45 cubic feet and a 40 lb bag 0.30 cubic feet. A full cubic yard takes 45 bags of 80 lb mix, which is a lot of mixing by hand.
For about one cubic yard or more, a ready mix truck is usually faster and more even. Many suppliers charge a short load fee for small orders, so compare it with the cost of bags. The concrete calculator shows both bag counts and cubic yards, so you can weigh the two options.
Estimating the Cost
Enter a price per cubic yard to see the cost of the concrete itself. Prices vary widely by region and mix, so ask local suppliers for a quote. Delivery, pumping and short load fees can add a lot on top.
Bags are priced per bag, so multiply the bag count by the store price. Remember the extra costs of a bigger job: gravel base, rebar or mesh, forms and tools. Labor is often the biggest cost when you hire a contractor.
Concrete vs Cement
People often use cement and concrete as if they were the same. Cement is the grey powder that acts as glue. Concrete is a mix of cement, sand, gravel and water that hardens into stone.
That means a cement calculator and a concrete slab calculator usually answer the same question. You order or buy concrete by volume, and the cement is already inside the mix. If you mix your own, a common ratio is 1 part cement, 2 parts sand and 3 parts gravel.
Tips for a Successful Pour
- Measure twice and add 5% to 10% for spills, uneven ground and form bulges.
- Compact the gravel base well so the slab does not crack or sink.
- Keep new concrete damp for the first week.
Concrete gains most of its strength in the first week and reaches its design strength after about 28 days. Foot traffic is usually fine after one or two days. Wait about a week before you drive a car onto a new driveway.
Weight of Concrete
Normal concrete weighs about 150 pounds per cubic foot, or around 2,400 kilograms per cubic meter. One cubic yard weighs about 4,050 pounds. The concrete calculator shows the total weight, which helps when you plan delivery, trailers or a structure that will carry the load.
Check your post holes and footings before you build. Our Square Footage Calculator gives the area of odd shapes first. You can then enter it in the concrete slab calculator with the right thickness.
Why Use Our Concrete Calculator
It handles slabs, footings, walls, columns, post holes and round slabs in US or metric units. Several pours can be added together, with waste, bag counts, weight and cost in one view. It runs in your browser with no sign up. Enter your measurements and plan your pour now.
Frequently asked questions
Is this concrete calculator free?
Yes, it is free with no sign up and no limit.
How much concrete do I need for a 10 by 10 slab?
At 4 inches thick, about 1.23 cubic yards, or 1.36 with 10% extra.
How many 80 lb bags make a cubic yard?
About 45 bags, since each one fills about 0.60 cubic feet.
Does the concrete slab calculator include waste?
Yes, set the waste allowance and it adds it to every total.
Is a cement calculator the same thing?
Yes, people usually mean concrete when they say cement.
When should I order ready mix instead of bags?
For about one cubic yard or more, a truck is usually easier.
What thickness should I enter in the concrete slab calculator?
Use 4 inches for patios and walkways and 5 to 6 inches for driveways.
How long before I can walk on new concrete?
Usually after one or two days, but driving should wait about a week.
Related calculators
Other tools people use alongside this one.