Concrete calculator
How much concrete do you actually need?
Optional — your local prices
We do not publish a price per yard, because concrete pricing is local and moves. Put your own supplier's number in and this will do the arithmetic.
Enter your dimensions above and the volume, bag count and order quantity appear here.
The arithmetic
What the calculator is doing.
None of this is complicated. It is worth seeing once so you can sanity-check any number a supplier gives you.
Volume, in one line
Length × width × thickness, with everything in feet, then divide by 27 to get cubic yards. Thickness is the one that trips people up because it is quoted in inches: four inches is 0.333 feet, six inches is 0.5.
A 10 × 10 patio at 4 inches is 33.3 cubic feet, or 1.23 cubic yards. Add 10% for waste and you are ordering 1.5 — because ready-mix comes in quarter-yard steps and nobody delivers 1.35.
Bags per yard: 45, 60, 90
Those three numbers cover almost every bag question. A cubic yard is 27 cubic feet, and the yield printed on the bag is what does the rest:
| Bag | Yield | Per cubic yard | Per 10×10 at 4″ |
|---|---|---|---|
| 80 lb | 0.60 ft³ | 45 bags | 62 bags |
| 60 lb | 0.45 ft³ | 60 bags | 82 bags |
| 40 lb | 0.30 ft³ | 90 bags | 123 bags |
The last column carries the same 10% waste allowance the calculator uses by default, which is why it is more than 45 × 1.23. Run the numbers above and you will get these.
Why 10% waste, and when to use more
Concrete is lost to the sub-base soaking it up, to spillage, to formwork that is not perfectly true, and to the fact that a slab poured to exactly four inches somewhere is four and a quarter somewhere else. Ten per cent is the normal allowance.
Go higher — 15% or so — if the ground is uneven or unprepared, if you are pouring over gravel that will take some of it, or if the shape is complicated enough that your measurements are estimates. Running out mid-pour is much worse than having a barrow left over: the joint between two batches placed an hour apart is a cold joint, and it is permanent.
How thick?
Four inches for patios, walkways and shed bases. Five to six where a car drives or parks. More than that, and you are into structural territory where the answer comes from an engineer or the building department rather than from a web page.
Thickness gets the attention and the sub-base decides the outcome. A four-inch slab on properly compacted base outlasts a six-inch slab on soft fill, every time. If you are choosing between spending on depth and spending on preparation, prepare.
Cement is not concrete
Worth saying because half the people looking for this tool search for a cement calculator. Cement is the grey powder that binds. Concrete is cement plus sand, aggregate and water. You pour concrete; you do not pour cement, any more than you bake flour.
A general-purpose mix is roughly 1 part cement to 2 parts sand to 3 parts aggregate by volume, with enough water to make it workable and no more — the single most common way to weaken a mix is to add water to make it easier to place.
Weight, if you need it
About 150 lb per cubic foot, so a cubic yard is roughly 4,050 lb — just over two tons. That 10 × 10 patio is about 5,000 lb of material sitting on whatever you put under it, which is the real argument for compacting the base.
Concrete blocks are a different calculation
If you are building a wall rather than pouring a slab, this is not the tool. A standard 8 × 8 × 16 block covers 0.888 square feet of wall face, so wall area in square feet × 1.125 gives you the block count, plus about 5% for cuts and breakage.
Mortar is separate again: roughly three 80 lb bags of mortar mix per 100 blocks, more if the joints are wide or the blocks are dry when laid. And you will still need concrete for the footing under it, which the footing mode above will size.
What actually drives the cost
We have not put a price per yard on this page, and it is a deliberate omission. Concrete is priced locally, it moves with cement and fuel, and a national average published today is wrong in your zip code and wrong again next quarter. Ring two suppliers; it takes five minutes and beats any figure on the internet.
What changes the number when you do ring them:
- Short-load fees. Under a full truck, most suppliers add a charge that can rival the concrete itself on a small pour.
- Access. If the truck cannot reach the pour, you are paying for a pump or for labour barrowing it.
- Mix design. Higher strength, fibre, accelerators and colour all cost more per yard.
- Wait time. Trucks bill by the minute past an allowance. Being ready when it arrives is free money.
- The finish. On a decorative surface this is most of the labour, and it is the part a volume calculator cannot price at all.
Common questions
How do I calculate concrete in cubic yards?
Multiply length by width by thickness with every measurement in feet, then divide by 27. Thickness is usually given in inches, so divide it by 12 first. A 10 by 10 slab at 4 inches is 10 x 10 x 0.333 = 33.3 cubic feet, which is 1.23 cubic yards. Add a waste allowance on top of that — 10% is normal.
How many bags of concrete are in a cubic yard?
45 bags at 80 lb, 60 bags at 60 lb, or 90 bags at 40 lb. A cubic yard is 27 cubic feet and an 80 lb bag yields about 0.6 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.3.
How thick should a concrete slab be?
Four inches is the usual answer for a patio, walkway or shed base. Five to six inches is normal where vehicles drive or park, and heavier still for anything carrying a structure. Thickness is only half of it — what the slab sits on matters as much, and anything structural is a question for your local building department rather than a calculator.
Should I use bags or order ready-mix?
Under about half a cubic yard, bags win. Over a yard, order ready-mix. In between it depends on your supplier's short-load fee. The deciding factor is usually not cost but consistency: every bag is its own batch, and the more batches you mix by hand the more the finished surface shows the difference between them.
Is a cement calculator the same as a concrete calculator?
People search for both and mean the same thing, but cement and concrete are not the same material. Cement is the powder that binds; concrete is cement plus sand, aggregate and water. You pour concrete. You do not pour cement. Any calculator returning cubic yards for a slab is a concrete calculator whatever it is called.
How much does concrete weigh?
Normal-weight concrete is about 150 lb per cubic foot, which is roughly 4,050 lb per cubic yard — a little over two tons. That matters for what a truck can carry, what a suspended slab can take, and whether the ground you are pouring on has been compacted properly.
Pouring something you want to look at?
A volume calculator prices the material. On a pool deck, patio or driveway the material is the small half — what it costs and how it lasts is decided by the finish and by whoever is standing on it at the right moment. That is what we do, on Florida's Gulf Coast, in crushed shell and white cement.