Concrete Calculator

Concrete is not cheap, and it is not light. Knowing exactly how much to purchase for home improvement projects is critical. Returning 80-pound bags of concrete is no fun. And for bigger, more professional jobs, ordering too much from a concrete delivery service can be shockingly expensive.

Figuring out precisely how much concrete a project requires is a tricky question. While standard bags may have square footage coverage areas written on them, that doesn’t tell you how big your site is. Especially when it comes to odd-shaped projects, knowing this is critical.

Luckily, we have concrete volume calculators to do all the number crunching.

What to Look for in a Calculator

Step one, find a powerful, detailed calculator.

Make sure the tool you’re using to calculate has options for more than simple concrete slabs or walls, where a basic length × width × height calculation is performed.

Round footings (piers), circular slabs, tubes, curbs, gutter barriers, and stairs can get tricky, really quick. Even with basic slabs and walls, you’ll want to know more than how many cubic feet are needed. A good concrete estimation tool will output:

• Cubic feet
• Cubic yards
• Cubic meters
• Weight needed
• The number of 60-lb. bags
• The number of 80-lb. bags

Why Do You Need All Those Numbers?

Cubic Feet

Small home improvement projects are often measured in cubic feet of cement mix. When getting assistance from workers at the supply store, they will often speak in terms of cubic feet, as will most friends coming to help.

Cubic Yards

This is an important number to know when dealing with concrete delivery services. Large mixing truck loads are often measured in cubic yards. When dealing with a contractor, it can get confusing if they go back and forth between cubic feet and cubic yards. If you measured in feet and delivered in yards, it’s good to have both numbers handy to avoid expensive mistakes.

And while most concrete delivery services are professional and reputable, it’s always good to know for yourself how much concrete is required for your project. Being armed with a close estimate from the calculator will help ensure you’re being charged for the correct amount. It pays to be informed.

Cubic Meters

You may come across a contractor who uses meters rather than yards.

Weight Needed

This is a crucial number, especially for the DIYer. Your vehicle is only rated to haul a certain weight. Add more, and you risk severely damaging the suspension and tires. Even the biggest pickups reach their limit with bags of concrete.

Also, when you see how much concrete you will be lugging back and forth, you might consider a delivery instead.

Number of 60-lb. bags/ Number of 80-lb. bags

This is where a good concrete calculator will shine. If you’re installing a fence and need footings, you probably won’t be calling a large delivery service to bring a mixing truck. You’ll be going to a building supply store and buying individual bags of concrete mix.

A ballpark guess can leave you with too many or too few bags. Too many means hauling them back to the store and waiting in line. The last thing you want to do at the end of a tiring project is return 80-lb. bags. Too few could send you running to the store at the last minute, or worse, leaving in the middle of a concrete pour to top off with a few more bags.

A concrete calculator can save your back and save you time.

Also, having both 60- and 80-lb. amounts handy can be helpful if the store is out of one or the other size.

A Practical Example

To get a better idea of how valuable a concrete calculator can be, let’s try a real-world example and build a deck.

This deck will require 12 footings. Each footing will be 12 inches across and two feet deep to get below the frost line. A rough estimate might say 24, 60-lb. bags of concrete. Two bags per hole seems about right. We head off to the building store and load up 24 bags to bring home in our minivan.

Run the numbers through the “Hole, Column, or Round Footings” feature of the calculator and find out more precise numbers.

Diameter (d) is 12 inches (make sure the correct parameter is used; not feet or yards). Depth or Height (h) is 2 feet. Quantity is 12 for the number of footings.

Results:

• 18.85 cubic feet
• 2,507 lbs
• 42 60-lb bags
• 32 80-lb bags

Here’s what we learned by taking a minute to use the calculator.

1. We underestimated the number of bags needed. Without the calculator, we would’ve been 2/3 of the way through pouring before figuring out we needed more.
2. These are heavy loads. Even by dividing the trip into two, we’re reaching the limit of our van’s carrying capacity. It’s probably best to plan on three trips and save the van’s suspension and our backs.
3. 60-lb bags will require 20 more trips to the van and back when we load and unload. If we have the strength or a helping hand, 80-lb bags might be a better idea.

Recommended Use-Cases for a Concrete Calculator

The more you get away from a basic, rectangular concrete slab shape, the more a good estimating tool will pay off. In short, the more complicated the shape, the more necessary a calculator becomes.

Footings. Good deck-building practices usually call for a footing at the base. These are almost always poured into circular holes. Simply measuring across the hole at its widest point will give the diameter (d). Drop the tape measure to the deepest point and get the depth or height (h). Assuming your holes are the same size, enter the quantity. Plug those three numbers in, and you will get everything you need to estimate.

Circular Slabs or Tubes. The same measuring protocol for footings is used here, with one more step needed. You’ll still measure at the widest part of the circle for the outer diameter (d1). Now measure across the inner hole to get the second diameter (d2) for inputting. Enter all the numbers into your volume calculator and let it do its magic.

Curb and Gutter Barriers. While this is a slightly more complicated shape, knowing where to measure will make short work of the process. Measure the curb depth, gutter width, curb height, flag thickness, and length.

Stairs. It’s hard to imagine estimating concrete volumes for stairs without help. The combination of the landing area, pitch of the risers, and width makes a calculation complicated. Luckily, the right concrete estimation tool will make short work of this problem. Measure the number of steps, platform depth, width, rise, and run.

Helpful Hints for a Worry-Free Concrete Pour

Now that you know how much concrete to purchase, it’s worth remembering a few valuable facts about concrete to keep your project running smoothly.

The Correct Amount of Water

When adding water, do so in stages. Add some, stir, test, and add some more. Repeat this process until your mix has the consistency of thick oatmeal. A good test is to pick up a bit and see if you can form it into a shape that stays solid. If it leaks through your fingers, it’s too wet. The mix should not be runny. Runny concrete will produce weak, cracked, and porous surfaces. These surfaces don’t last long. If your mix is too wet, add dry concrete and remix.

Concrete Takes Time to Dry

Temperature and humidity can affect curing times, but we can still make some rough estimates. You can remove the forms in two days, and people can walk on the surface. The concrete mix is considered "dry" at this point.

In seven days, it is "partially cured." This means it is suitable for cars, trucks, and heavy machinery.

After a month, the mix is considered "cured."