Total area
cu yd
lb/cu ft
Bag size
Bags needed
cu yd
Total cost

Concrete calculator lets you estimate how much concrete you need to produce and how many bags of cement it requires for a specified purpose accounting for the size of elements, concrete density, weight, and spillage. Cement is usually sold in bags. Knowing the size of the bag (it says how much concrete we'll get out of it) our calculator will help you decide how many bags you need to buy. This way you can also estimate the concrete slab cost. If you would like to estimate the amount of concrete you need for columns, check a sibling calculators for concrete columns and tubes.

Concrete definition

Nowadays, concrete is one of the most common construction materials used in building engineering all over the world. It is composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens over the time. The aggregates are usually sand and gravel (or crushed stone), while the paste is water and portland cement. Portland cement is not a brand name - that is the generic term for the type of cement used basically in all concrete (like e.g. stainless is a type of steel). Through a process called hydration, the cement and water harden and bind the aggregates into a mass similar to a rock. After 28 days concrete achieves around 99% of maximal compressive strength. However, the hardening process continues for years, what results in stronger concrete with passing time.

Why use the concrete calculator?

A lot of us have probably been in this situation: you decide to build your own house, a terrace or a fence. You go to a shop, buy X bags of concrete, and pleased with yourself you get to work right away. Somewhere in the middle you realize you have only one bag left. Irritated you go back to the shop buy another batch of concrete, go back, start working again and then... it's not enough, again. So the day's already over, the shop is closed, work's not done and tomorrow you need to go back to the shop again. Only this time you buy much more, just in case and by the end of work you're left with half of all these extra bags of concrete with absolutely no use for them. You wasted time, you wasted money and it most certainly got on your nerves. All of this can be easily avoided. If you ever wonder "how much concrete do I need", instead of guessing use our concrete calculator.

How much concrete do I need?

  1. First, you need to estimate a volume of concrete that you need. Provide the dimensions of your concrete slabs (length, width and height) and quantity or an area that you want to pave.
  2. Then determine the weight of all those elements - this is how the bagged cement is sold - they tell you how much will the produced concrete weight (for example, you'll get 60 pounds of concrete out of one bag). Given the volume we just calculated and concrete's density (again, you'll find this info on the bag, we provided a commonly spotted value, but it varies), you can calculate the weight of all your elements.
  3. Provide the size of the bag. We'll know how much concrete will go into producing all those slabs.
  4. Finally, you may provide the waste ratio - how much you're planning to lose on spillage, defective elements and other unfortunate events. Now you know how many bags you truly need.

All dimensions are given in feet by default. You can change them to any other unit by simply unlocking the field (the padlock button).

An example calculation

So let's say we have 4 concrete elements 3 feet long, 3 feet wide and 3 feet high each. Now we multiply 4 times (3*3*3). Pre-mixed concrete is usually sold by the cubic yard and this is the volume unit we use by default, but construction elements are usually measured in feet. Therefore our calculator shows the volume in cubic yards - in our example 4 cu yd. 4 cubic yards equals 108 cubic feet.

Sometimes you'll know how many slabs you need. Other times you'll just know what's the area that needs to be paved - feel free to provide any of these values. In our example, we'll have 36 square feet.

Once we have the above measurements we can get to the essence of the matter.

  • Check if the density of concrete from a particular producer is the same as the default value in our calculator (150 lb/cu ft). Feel free to change it if you need.
  • Now you already know the total weight of the needed concrete.
  • Check if the weight of a single bag of cement is the same as the default already given (60 lb). If not, again, change it to appropriate number.
  • To be safe, account for spillage and waste. Experienced contractors usually assume 5-10%.

In our example let's leave the default measures. We need 4 cubic yards of cement, our producer sells cement in bags weighing 60 lb with a 150 lb/cu ft density. Total weight of concrete that we need is 16 201,03 lb. We assume 5% of waste and spillage and voila, we need 284 bags of concrete.

Concrete slab cost

It's easy with the calculator - once you know how much concrete you need (in bags) and how many slabs you'll produce out of it, simply provide the price of the bag and you'll receive the concrete slab cost, a cost to pave your desired area, cost per unit of volume and the total cost of the material you need.

How to make concrete?

The production of concrete is quite time-consuming. In this process, various ingredients are mixed together - water, aggregate, cement, and any additives. Once the ingredients are mixed, workers must cast the concrete in place before it hardens. In modern usage, most concrete production takes place in large industrial facilities (concrete plants).

Careful proportioning and mixing of the ingredients is the key factor to achieve a strong and durable concrete. Not enough paste in the concrete mixture will result in rough, honeycomb surfaces and porous concrete. A mixture overloaded with cement paste will be easy to place and will create smooth surface but on the other hand, it will result in uneconomical concrete, susceptible to cracking.

Concrete vs cement

Here is one of the main reasons why cement and concrete are so often confused: there is cement in concrete. When cement is blended with water, it creates a paste. Then, that paste is combined with aggregates like gravel and sand what creates the concrete.

Cement is made of calcium and silica-rich materials (such as limestone and clay). It has the unique adhesive properties that make it an excellent binding agent. On its own, cement is prone to cracking. Compared with concrete, which can last hundreds of years, cement is much less durable.

Types of concrete

Concrete is produced in a variety of compositions, finishes and performance characteristics to meet a wide range of needs. You can find some of the types below:

  1. Ordinary or plain concrete - one of the most commonly used. Made of cement, sand and coarse aggregates.

  2. Lightweight concrete - density lower than 1920 kg/m³. It has very low thermal conductivity.

  3. High-density concrete - also called heavy weight. The density varies between 3000-4000 kg/m³. It is mostly used in atomic power plants (good radiation protection).

  4. Reinforced concrete - most important type. In this concrete type, steel in various forms is used as reinforcement to give very high tensile strength.

  5. Precast concrete - the term refers to prefabricated, useful concrete blocks. They have various shapes.

  6. Prestressed Concrete - a special type of reinforced concrete in which the reinforcement bars are tensioned before being embedded in the concrete.

  7. Air-entrained concrete - special prepared plain concrete in which air is entrained in the form of thousands uniformly distributed particles.

  8. Glass concrete - recycled glass used as an aggregate.

  9. Rapid hardening concrete - mostly used in underwater construction and road repairing.

  10. Asphalt - the combination of aggregates and asphalt. Used to build highways, airports, and embankments.

  11. Lime concrete - lime is used as a binding material with aggregates. Used on floors, domes, etc.

  12. Roller compacted concrete - very small amount of cement in mixture. Mostly used as filling material.

  13. Stamped concrete - ordinary concrete with some little differences. Mostly used for architectural purposes.

  14. Pumped concrete - used for high rise buildings.

  15. Vacuum concrete - a higher quantity of water is added to the concrete mix, and then the mixture is poured into the formwork.

  16. Permeable concrete - water is able to pass through it. Used in areas where storm water issues occur.

  17. Sprayed concrete - it is fabricated in the same way as ordinary concrete, but the way of placing it differs. It is pneumatically projected at high velocity onto a surface. Used e.g. in tunnels construction.

  18. Ready-mix - prepared in concrete plant and transported with concrete mixer trucks what allows for delivery prior to setting time.

  19. Self-consolidated - compacted by its own weight. There is no need of performing manual compaction.

Other considerations

Now, this was relatively easy; determining the volume of elements other than rectangular might be a little bit more tricky, though. To make out the volume other common three dimensional object, go to our volume calculator.

While we're around these areas, check out the density calculator, where you can calculate the density of any given object based on its weight and volume.

Mateusz Mucha, Joanna Andrzejewska and Filip Derma

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Concrete Calculator - Slab | How To Calculate Concrete