Concrete Calculator

What the concrete calculator does

You have a slab to pour, and you need to know how much concrete to buy. It is a number worth getting right. Order too little and the truck pulls away before the job is done. Order too much and you have paid for concrete that ends up as a lump in the corner of the yard.

This calculator does the measuring math for you. You give it the shape and the dimensions, and it hands back the volume in cubic yards and cubic metres, plus how many pre-mixed bags that works out to. Below is how it gets there, a worked example, and the few things the bag label and the ready-mix yard will not tell you.

How to use it

1. Pick what you are pouring. Choose a slab, wall, footer, stairs, columns, or a curb from the dropdown. The fields change to match the shape.
2. Enter each measurement. Type the dimensions and choose the unit sitting next to each one. You can mix units, so a slab can be in feet for length and inches for thickness, straight off the tape.
3. Set the quantity. Use this when you are pouring more than one of the same shape, like four identical footings.
4. Press Calculate for the breakdown, or Reset to clear it.

How the volume is worked out

Concrete is sold by volume, so volume is what we are after. For anything box-shaped, a slab, a wall, a footer, that is just the three sides multiplied together:

Volume = length × width × thickness

A column is round, so it uses the formula for a cylinder instead, where the radius is half the diameter you typed in:

Volume = π × radius² × height

Before any of that, the calculator quietly converts every measurement to one common unit so the multiplication lines up, then multiplies by your quantity. Stairs and curbs are just a handful of these box shapes added together, one stacked block per step.

There is one shortcut worth knowing if you ever want to check the math by hand. Concrete is ordered in cubic yards, and there are 27 cubic feet in a cubic yard. So if you measure everything in feet, divide the cubic feet by 27 to get cubic yards. A 10 by 10 slab at 4 inches thick is 33.3 cubic feet, and 33.3 divided by 27 is about 1.23 cubic yards.

Units, rounding, and how precise this is

You can enter measurements in inches, feet, or yards, in any mix you like. The answer comes back two ways: cubic yards, which is how a ready-mix yard takes your order in the US, and cubic metres for everywhere that works in metric. One cubic yard is 0.7646 cubic metres, so the calculator simply multiplies by that to give you both.

Volume is rounded to two decimal places, which is plenty for ordering. Bag counts are always rounded up to a whole bag, never down, because you cannot buy three-quarters of a bag, and rounding down would leave you short on the day. So "needs 55.6 bags" shows up as 56.

The number you get is the exact volume of the shape you typed in, worked out cleanly and rounded only at the very end. On a real pour you will want to buy a little on top of that, and the next two sections cover how much and why.

A worked example: a 10 ft by 10 ft slab

Say you are pouring a 10 ft by 10 ft slab, 4 inches thick. Here is the whole thing, start to finish.

First the thickness in feet: 4 divided by 12 is 0.333 ft. So the volume is 10 × 10 × 0.333 = 33.3 cubic feet, which is 33.3 ÷ 27 = 1.23 cubic yards, or about 0.94 cubic metres.

Turning that into bags depends on the bag size, since each one fills a different amount of space. For 1.23 cubic yards you are looking at roughly:

• 56 bags of 80 lb mix
• 75 bags of 60 lb mix
• 89 bags of 50 lb mix

That is a lot of bags to mix by hand for a slab this size, which is the sort of thing the result is quietly telling you. There is more on when to stop mixing bags and call a truck further down.

How many bags of concrete are in a yard?

This is the question almost everyone ends up asking, so here it is on its own. A cubic yard is 27 cubic feet, and each bag size fills a known slice of that:

The 80 lb bag is the cheapest way to buy a cubic yard in bags, which is why most people reach for it. The lighter bags cost a bit more per yard of concrete, but they are kinder on your back, and that counts when you are the one carrying every single one of them.

Ordering the right amount

The calculator gives you the exact volume of the shape. On a real pour, a little always gets away from you. Forms leak a touch, the ground underneath is never perfectly level, and there is always some left clinging to the wheelbarrow. So crews order a bit more than the math says, on purpose.

A good habit is to add about 5 to 10 percent on top. Use the lower end for tidy forms on well-compacted ground, and the higher end when the ground is uneven or the shape is fiddly. The reasoning is simple. Going a little over costs you one spare bag or a slightly larger order. Coming up short can stop the pour, and concrete will not wait for you. A fresh batch poured against one that has already started to set leaves a weak seam where the two meet.

So take the number, add your margin, and round up to something easy to order.

Bags or a ready-mix truck?

Bags make sense for small jobs. You buy them when you need them, mix at your own pace, and there is no truck to book. Once a pour gets near a full cubic yard, though, the math turns on you. A cubic yard is about 45 bags of 80 lb mix, and mixing 45 bags by hand is a long, heavy afternoon.

That is roughly the crossover. Under half a yard, bags almost always win. Between half a yard and a yard, it comes down to how much help you have and how much mixing you can stomach. Past a yard, a ready-mix truck is usually faster, and often cheaper per yard, though most yards set a minimum order of around a yard and add a small fee if you go under it.

Questions people ask

How much concrete do I need for a 10x10 slab?

For a 10 ft by 10 ft slab at 4 inches thick, about 1.23 cubic yards, or 0.94 cubic metres. At 6 inches thick it climbs to roughly 1.85 cubic yards, since the thickness scales the whole thing.

How many 80 lb bags make a cubic yard?

About 45, since an 80 lb bag fills roughly 0.6 cubic feet and a cubic yard is 27 cubic feet.

Does the result include extra for waste?

No. The number is the exact volume of the shape you entered. Add about 5 to 10 percent yourself when you order, to cover spillage and uneven ground.

Why does it show both cubic yards and cubic metres?

Cubic yards is how ready-mix is ordered in the US. Cubic metres is the metric equivalent used most other places. One cubic yard is 0.7646 cubic metres.

How much does a yard of concrete weigh?

A cubic yard of normal-weight concrete weighs roughly 4,000 pounds, a little under two tons. Worth knowing before you load bags into a car or a small trailer.

References

A quick note on where these figures come from. The unit conversions, that one yard is exactly 0.9144 metres and one pound is 0.45359237 kilograms, follow the definitions in the US National Institute of Standards and Technology guide. The weight of concrete, about 2,400 kg per cubic metre for a normal-weight mix, is the standard value used across engineering references and tabulated in the Portland Cement Association's Design and Control of Concrete Mixtures. And how much each bag fills comes from the manufacturers' own product data, such as Quikrete's, which prints the yield right on the bag.

1. National Institute of Standards and Technology (NIST), Special Publication 811, Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811
2. Portland Cement Association (PCA), Design and Control of Concrete Mixtures. https://www.cement.org
3. Quikrete, concrete calculator and product data (bag yield by weight). https://www.quikrete.com/calculator/main.asp