Concrete Calculator
Find out how much concrete you need for a slab, footing or column — in cubic yards, cubic feet and 40 / 60 / 80 lb bags. Instant results, imperial or metric.
How to use this concrete calculator
- Pick a shape. Use Slab for rectangular pours (patios, floors, walkways) or Footing / Column for round piers and posts.
- Enter your dimensions. Length and width in feet, thickness in inches. For footings, enter the diameter in inches, the height in feet, and how many you are pouring.
- Choose a bag size. 80 lb bags are the usual pick; switch to 60 or 40 lb to match what your store stocks.
- Set a waste factor. Leave it on 10 percent unless your forms are dead-level and tight.
- Read the results. Bag count and volume update together. Use Add another area to total several pours at once.
How the concrete math works
Every estimate is volume = area × thickness. For a slab that is length × width × thickness. For a round footing it is π × radius² × height. The tool converts your inches to feet, multiplies, then divides by 27 because a cubic yard holds 27 cubic feet.
Bag counts come from each bag’s yield — the volume of wet concrete one bag makes:
| Bag | Approx. yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Bags or ready-mix?
Bagged mix is ideal for small jobs — a mailbox post, a few footings, a 4 × 4 pad. Once a project needs more than about a cubic yard, mixing 45-plus bags by hand becomes a real slog and the first batches can set before the last are poured. A ready-mix truck is then cheaper per yard and gives a more consistent slab. Most suppliers have a minimum load (often one cubic yard) and a short-load fee below it.
Why the 10 percent waste factor
Subgrade is never perfectly flat, hand-dug footing holes bell out at the bottom, and some concrete always clings to the mixer and wheelbarrow. Running short mid-pour means a cold joint or a second trip, so 10 percent extra is cheap insurance. On rough ground or deep footings, many pros go to 15.
Metric mode
Switch to metric to enter metres and centimetres and read the answer in cubic metres. Bag options change to 20, 25 and 30 kg pre-mix, with yields near 0.009, 0.011 and 0.014 m³ per bag.
Concrete for common slabs (4″ thick, before waste)
| Slab | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 4 × 4 ft | 0.20 | 9 | 12 |
| 8 × 8 ft | 0.79 | 36 | 48 |
| 10 × 10 ft | 1.24 | 56 | 75 |
| 12 × 12 ft | 1.78 | 80 | 107 |
| 10 × 10 ft @ 6″ | 1.85 | 84 | 112 |
Add roughly 10 percent to every figure for waste. Planning numbers only; your slab may vary with subgrade and finish.
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10 ft by 10 ft slab poured 4 inches thick is about 1.24 cubic yards of concrete. That is roughly 56 bags of 80 lb mix, 75 bags of 60 lb, or 112 bags of 40 lb before waste. Add about 10 percent for spillage and uneven ground and plan on around 62 bags of 80 lb. Past roughly one cubic yard, ordering ready-mix by the truck is usually cheaper and far less work than mixing bags.
When should I order ready-mix instead of bags?
A good rule of thumb is one cubic yard, or about 40 to 60 eighty-pound bags. Below that, bagged mix is convenient. Above it, hand-mixing that many bags is slow and the concrete can begin curing before you finish, so a ready-mix truck gives a stronger, more uniform pour.
How thick should a concrete slab be?
Four inches is standard for patios, walkways and shed floors. Driveways and anything that carries vehicles should be 5 to 6 inches, usually over a compacted gravel base. Always confirm thickness before you calculate, because it scales the volume directly.
How much does a cubic yard of concrete weigh?
Cured concrete weighs roughly 3,700 to 4,050 pounds per cubic yard, or about 150 pounds per cubic foot. That matters for delivery planning and for making sure the subgrade can support the finished slab.
What is the difference between 40, 60 and 80 lb bags?
The number is the dry weight. A 40 lb bag yields about 0.30 cubic feet of mixed concrete, a 60 lb bag about 0.45 cubic feet, and an 80 lb bag about 0.60 cubic feet. Fewer, larger bags mean less opening and mixing, which is why 60 and 80 lb bags are the usual choice for slabs.
Should I add a waste factor?
Yes. Uneven subgrade, over-excavation, spillage and slab edges all use extra concrete. Five percent is a minimum; 10 percent is the safer default, especially for footings dug by hand where the hole is rarely a perfect cylinder.
These results are estimates for planning only and are not engineering or professional advice. Concrete yield varies with mix, water content and site conditions. Confirm quantities with your supplier and follow local building codes before you pour.