# Concrete Calculator - Slab

**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.

**Table of contents:**

## 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?

- 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.
- 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.
- Provide the size of the bag. We'll know how much concrete will go into producing all those slabs.
- 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.

## 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.