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How Much Concrete Do I Need? [Solved!]

How Much Concrete Do I Need? [Solved!]

Concrete consists of coarse and fine aggregates bound together by a cement paste which hardens over time, and it is one of the most commonly used construction materials.

Gravel and sand are the most commonly used aggregates, while the cement paste comprises Portland cement and water. It follows that concrete usually achieves up to 99% of its peak compressive strength in about four weeks. But this rocky mass continues to harden over years to come, ultimately becoming stronger.

How Much Concrete Do You Need?

To help answer How Much Concrete Do I Need, a guy pours water into a bucket

Schankz/Shutterstock

It’s possible to order a given amount of concrete only to end up short before completing your construction project. You may frown a little and order more, only to end up with excess by the time you’re done. Before you know it, you’ve wasted money plus your single most invaluable asset – time.

To avoid wasting time and money, we’ll walk you through the fundamental calculations you need to determine how much concrete you need. While you may need a concrete calculator, the good news is that some of these are also available online.

Determining the Amount of Concrete You Need

To figure out how much concrete you need, follow these four simple steps:

  1. First start with an approximation. You’ll need to determine the rough dimensions of your working space in terms of length, breadth, and depth. Next, you’ll need to figure out an appropriate number of concrete slabs. 
  2. If you’re making concrete from scratch, determine all the mass of components you need. A simpler and more convenient way is to go with bagged concrete mix. The weight is indicated on the bag. Some DIYers are skeptical about the concrete mix from hardware stores. But it’s pretty much just what you’d have delivered from a ready-mix concrete plant. 
  3. After determining the mass and density of the concrete (from the bag), and the volume of structure you’ve calculated in step 1, you’ll determine how many concrete bags you’ll need.
  4. Approximate your waste ratio. This is simply an indication of the amount of concrete you’ll likely lose via unfortunate events like spillages and malfunctioning elements. When in doubt, ad 10% to your total needs.

Sample Calculation

Assume your structure comprises three concrete elements, each of which is 4 feet long, 3 feet broad, and 2 feet high (deep). 

The volume (V) of each element is given as:

V=4*3*2=24 cubic feet

We’ll have to multiply this volume by 3 to get the total volume for all elements, which becomes 72 cubic feet. Hardware stores sell ready-mix concrete by the cubic yard.

Pro tip: Most construction elements are measured in cubic yards. So you’ll convert the measured volume into the same units (1 cubic yard is approximately 27 cubic feet). As such, our original volume of 72 cubic feet is equivalent to 2.667 cubic yards.

With these measurements, you can now determine the number of bags of concrete you’ll need. But first confirm the density of the concrete from a producer, say, at 150 pounds per cubic feet (150lb/cu ft.) If the producer sells concrete bags at 150lb/cubic feet and one bag weighs 60 lbs, we proceed as follows:

Mass of Concrete required= 150 lb1 cu feet*72 cfeet=10,800 lbs

Assuming a 5% waste ratio, the total mass required now becomes 10,800 + 5100 * 10,800 = 11,340 lbs.

Since one bag weighs 60 lbs., the total number of bags you need is given as 11,340 lbs. 60 lbs*1 bag = 189 bags. So you need 189 bags of ready-made concrete.

With this formula, you can easily establish the total cost of concrete by multiplying the price per bag by the total number of bags required.

Which Concrete Will You Need?

Guy pouring ready mixed concrete into a rebar-strengthened foundation

Nopphinan/Shutterstock

As a rule of thumb, you should always factor in an extra ¼ inch to your slab thickness to develop a reasonable budget. This also considers the assumption that your concrete is well compacted and graded to the proper depth.

To quickly determine the volume and area of odd shapes, it’s better to divide them into convenient portions of regular shapes like rectangles and triangles. 

Types of Concrete You May Need

There are a few types of concrete, each with its own performance characteristics, compositions and finishes. These include:

  • Ordinary concrete
  • High-Density Concrete
  • Low-Density Concrete
  • Precast concrete
  • Glass Concrete
  • Lime Concrete
  • Rapidly hardening concrete
  • Ready-mix concrete 
  • Self-compacted concrete
  • Reinforced concrete

Ordinary concrete is the most commonly used concrete that comprises sand, coarse aggregates, and cement paste.

Bagged vs. Ready-Mix Concrete

If you’ve decided to use bagged concrete, you should have it delivered to your home. Otherwise, you’ll likely have to transport the bags yourself. And at 60 pounds per bag, that weight adds up quickly. Additionally, you might need mixing equipment, which you can rent at most home improvement or equipment rental stores.

If you’re only dealing with a few bags, a bucket and shovel will do. However, you should avoid bagged concrete if you need to:

  • Fill footings for walls and decks
  • Make small steps and ramps
  • Pour small slabs for sidewalks and patios
  • Repair steps, wall foundations, and terraces
  • Set fencing posts

Ready-mix concrete comes in a typical concrete truck, with an average load of 10 cubic yards. If you order from a supplier, you’ll need to give a total number of yards required. You’ll also incur shortage fees if your order in batches.

Ready-mix concrete is best suited for:

  • Sizeable parking lots
  • Large patios, driveways, and pool decks
  • Slab foundations for a house

Frequently Asked Questions

Cement bags stacked in a house for a piece on how much cement do I need

Sasiko Kaan/Shutterstock

Here are some of the most commonly asked questions about concrete.

How Much Concrete Do I Need for a 1000 sq. ft slab?

It depends on the thickness of the slab, say 6”. If you consider other factors like the waste ratio, you’ll likely end up with between 6,100 and 6,300 cubic feet.

How Many Bags of Concrete Do I Need for 1 Cubic Meter?

One cubic meter is close to 1.308 cubic yards of concrete. This is almost 27 cubic feet. You can interchange these units and determine the number of bags for your project.

So, How Much Concrete Do I Need?

The amount of concrete you need for your project depends on the volume of the structure you intend to make. You can determine the volume of your project by simple calculations for various shapes or use an online calculator.

If you decide to go with bagged concrete, you need to first determine the volume of your structure. You can then obtain the mass and density of concrete as indicated on the bags and calculate the volume of each bag. Doing this will show you exactly how many bags you’ll need.

Depending on the size of your project, ready-mix concrete from a supplier can be a good option. By precisely determining the amount of concrete needed for your project, you’ll likely avoid wasting time and money.

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