Concrete Calculator – Slabs, Footings, Post Holes, and Bags
Our free concrete calculator tells you how much concrete you need for slabs, footings, columns, post holes, and stairs. Enter your sizes in feet and inches (or switch to metric), add as many pours as your project has, and you get the volume in cubic yards, cubic feet, and cubic meters, the number of 40, 60, and 80 lb bags, the ready-mix order, and the cost. Copy the result or print it for the store.
How to Use the Concrete Calculator
You only need a tape measure and a rough idea of the shape you want to fill:
- Pick the shape. Each pour has five options: slab, footing, column, post holes, and stairs. The fields change with the shape.
- Enter the sizes. Lengths go in feet, thicknesses and diameters in inches. You can type 12.5 or 12 1/2, and feet with inches work too, using the foot and inch marks.
- Set how many. Ten identical post holes or two equal pads need only one entry with “How many” set to 10 or 2.
- Add more pours. A patio with a footing around it, or a deck with twelve posts and a set of steps, is one project. Tap “Add another pour” for each part, and the calculator adds them up.
- Choose the extra. The default is 10% for spills, uneven ground, and forms that bulge a little. Use 5% for a perfect form on a level base and 15% for rough ground.
- Add prices if you like. Enter the price of one bag or the price per cubic yard of ready-mix, and the calculator shows the cost of both options.
- Copy or print. The result list goes to your clipboard or prints on US Letter or A4, with all pours, sizes, and settings.
The Formulas Behind the Calculator
Concrete is ordered and mixed by volume. Every shape comes down to “area × depth.” The calculator converts all sizes to the same unit first, because the most common mistake is multiplying feet by inches.
| Shape | Formula | Example |
|---|---|---|
| Slab or pad | length × width × thickness | 10 × 10 ft, 4 in thick = 33.3 cu ft |
| Footing | total length × width × depth | 20 ft long, 16 × 8 in = 17.8 cu ft |
| Column or tube form | π × radius² × height | 12 in round, 4 ft high = 3.1 cu ft |
| Post hole | (hole area − post area) × (depth − gravel) | 10 in hole, 36 in deep, 6 in gravel, 4×4 post = 1.15 cu ft |
| Stairs | width × rise × run × n × (n + 1) ÷ 2 | 4 steps, 7 × 11 in, 4 ft wide = 21.4 cu ft |
To turn cubic feet into cubic yards, divide by 27, because a cubic yard is 3 × 3 × 3 feet. One cubic meter is about 1.31 cubic yards, or 35.3 cubic feet.
Example: a 10 × 10 Slab
A 10 × 10 ft patio slab, 4 inches thick, is 10 × 10 × 0.333 = 33.3 cubic feet. With 10% extra, that is 36.7 cubic feet, or 1.36 cubic yards. In bags, 36.7 ÷ 0.60 = 61.1, so you need 62 bags of 80 lb mix. With 60 lb bags, it is 82, and with 40 lb bags, 123. That is a lot of mixing, which is why the calculator also shows the ready-mix order: 1.5 cubic yards, rounded up to the next quarter yard.
Example: Ten Fence Post Holes
Ten holes, 10 inches wide and 36 inches deep, with 6 inches of gravel at the bottom and a 4×4 post (3 1/2 × 3 1/2 inches actual size) in each. One hole holds 78.5 square inches minus 12.25 for the post, which is 66.3 square inches, over 30 inches of depth. That makes 1,989 cubic inches, or 1.15 cubic feet per hole. Ten holes need 11.5 cubic feet, or 12.7 with 10% extra, and that is 22 bags of 80 lb mix.
How Many Bags of Concrete Do I Need?
Bagged concrete mix does not turn into the same volume as the dry bag. What counts is the yield printed on the bag. The calculator uses the yields from the Quikrete Concrete Mix data sheet, which are typical for standard concrete mix:
| Bag size | Yield per bag | Bags per cubic yard | Bags per cubic foot |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 3.33 |
| 60 lb | 0.45 cu ft | 60 | 2.22 |
| 80 lb | 0.60 cu ft | 45 | 1.67 |
These are guide values. Fast-setting mixes, high-strength mixes, and mortar mixes can yield a little more or less, so check the bag. You can find the yields in the Quikrete Concrete Mix data sheet and on the bag itself.
The 80 lb bag is usually the cheapest per cubic foot, but it is heavy. For fence posts, a few small repairs, or work where you carry the bags far, 40 or 60 lb bags are easier on your back.
Bags or Ready-Mix?
Above about one cubic yard, which is 45 bags of 80 lb mix and roughly 3,600 pounds of dry material, mixing by hand gets hard. That is also when many builders call a ready-mix supplier. Ready-mix is usually sold in quarter-yard or half-yard steps, and many plants charge a short-load fee for small orders, so ask before you order. The calculator rounds the order up to the next quarter yard.
A few rules of thumb help with the choice:
- Bags work well for post holes, small pads, steps, and repairs. You can mix one bag at a time and pour at your own pace.
- A mixer rental makes sense from about 20 bags upward. You still carry every bag, but the mixing is faster and more even.
- Ready-mix is the better choice for driveways, larger patios, and slabs that have to be poured in one go, because a slab poured in parts can show cold joints.
Wet concrete weighs about 150 pounds per cubic foot, so the calculator also shows the weight. A 1.36 cubic yard slab is roughly 5,500 pounds of concrete, which is good to know before you load a trailer.
Thickness and Size Guide
The right thickness depends on what the concrete has to carry. These are common starting points for DIY projects. Local building codes and soil conditions come first, especially for footings.
- Walkways and patios: 4 inches on a compacted gravel base.
- Shed floors and hot tub pads: 4 to 6 inches, depending on the load.
- Driveways: 4 inches for cars, 5 to 6 inches for heavier vehicles.
- Fence posts: a hole about three times the post width, and about one third of the post length in the ground, below the frost line where it freezes.
- Tube forms for deck footings: 8, 10, and 12 inch round forms are the common sizes. The quick buttons in the calculator fill them in.
Before you seal or color the finished surface, give the concrete time to cure. Our guides to the best concrete sealer and the best concrete stain explain when and how to finish it.
What Each Shape Means in the Calculator
Slab
A flat, rectangular pour such as a patio, a pad for an air conditioner, or a shed floor. For an L-shaped patio, split it into two rectangles and add a second pour.
Footing
A continuous footing under a wall or along the edge of a slab. Enter the total length of all sides, plus the width and depth in inches. For a 10 × 12 ft shed with a footing on all four sides, the total length is 44 ft.
Column
A round column or a footing poured in a cardboard tube form. Enter the diameter in inches and the height in feet.
Post Holes
The calculator takes the hole volume and subtracts the post and the gravel at the bottom. Choose a square post (a 4×4 is 3 1/2 inches, a 6×6 is 5 1/2 inches), a round post, or “No post (footing only)” for a footing that holds a post anchor later.
Stairs
Solid steps poured on the ground. Enter the number of steps, the rise (height) and run (tread depth) of each step, and the stair width. A landing at the top extends the top step. Stairs that span over open space need a structural design, and the calculator does not cover that.
Tips for Ordering and Pouring
- Measure the real depth. Ground is never perfectly flat. Measure the depth in several spots and use the average, or a little more.
- Do not skimp on the extra. Running out halfway through a slab is worse than having two bags left. Unopened bags keep if you store them dry.
- Check the weather. Do not pour on frozen ground or when frost is expected, and keep fresh concrete damp in hot, dry weather.
- Plan the finish. If you want to paint the surface later, our guide on how to paint concrete patios walks you through cleaning, etching, and the right paint, and our article on concrete etching covers the acid wash.
- Fix cracks early. Small cracks in older concrete can often be filled. See our guide to the best concrete crack filler.
- Protect the surface. Garages and driveways last longer with a coating or sealer. Compare the costs in how much an epoxy garage floor costs and our guide to the best driveway sealer.
Privacy and Honest Limits
The concrete calculator runs entirely in your browser. There is no sign-up, and nothing you enter is sent anywhere. All results are estimates. Real yields depend on the mix, the amount of water, and how much the base settles. The bag label, your supplier, and local building codes always come first.
Frequently Asked Questions
How Do I Calculate How Much Concrete I Need?
Multiply length × width × thickness, with all three in feet. A 4 inch slab is 0.333 ft thick. Divide the cubic feet by 27 to get cubic yards, and add 5 to 10% extra.
How Many 80 lb Bags of Concrete Make a Yard?
An 80 lb bag yields about 0.60 cubic feet, so one cubic yard (27 cubic feet) needs 45 bags. With 60 lb bags, it is 60 bags, and with 40 lb bags, 90 bags.
How Many Bags of Concrete for a 10 × 10 Slab?
A 10 × 10 ft slab, 4 inches thick, is 33.3 cubic feet. That is 56 bags of 80 lb mix without extra, or 62 bags with 10% extra.
How Much Concrete Do I Need for a Fence Post?
A 10 inch hole, 36 inches deep with 6 inches of gravel and a 4×4 post, takes about 1.15 cubic feet. That is two 80 lb bags per post. Wider or deeper holes need more.
How Many Cubic Yards Is a 12 × 12 Slab?
A 12 × 12 ft slab, 4 inches thick, is 48 cubic feet, or 1.78 cubic yards. With 10% extra, order about 2 cubic yards.
How Much Does a Yard of Concrete Weigh?
Normal concrete weighs about 150 pounds per cubic foot, so a cubic yard weighs roughly 4,000 pounds. Lightweight concrete weighs less.
Should I Add Extra Concrete for Waste?
Yes. Most pros add 5 to 10%, and more on uneven ground. The calculator uses 10% as the default, and you can change it.
Is the Concrete Calculator Free?
Yes. It is free, needs no account, works in imperial or metric units, and lets you copy or print the results.
Robert Thompson has worked as a self-employed carpenter for over 15 years. He lives with his family in Billings, Montana, where he also successfully operates his business as a woodworker. His main business is the complete interior construction of buildings. He is also passionate about creating high-quality wood furniture with epoxy and other items made from burl wood. Besides the woodwork, Robert is also a skilled DIY professional for all kinds of materials. He always has a solution ready for every craft and construction challenge. Originally, Robert studied civil engineering and has advanced degrees in carpentry and business management. Robert has been working for Craft Art since 2020 and enriches the blog with his immense knowledge and skills in the field of woodworking and home improvement. He writes the articles about DIY and Repair.
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