Concrete Calculator

Concrete calculator: Slabs, Pads, Blocks and Ready-Mix Yardage

First, choose the form you want to pour into. A flat slab, a thick footing or pad, a block wall, and a round column or post footing are the four projects that most people actually build with a concrete calculator. Since the math for each shape is different, the tool’s input boxes change when you switch between them. 

Choose between feet and inches or meters and centimeters as your unit of measurement. This concrete calculator will give you cubic yards, a bag count, and a rough price all at once. There’s no need to sign up, and it’s free. This page goes over all four forms with a real-life example for each one. It then goes over the steps that are the same no matter what you are pouring: how many bags you need, when to order ready-mix instead, how much it costs, and how the math changes in the UK.

Concrete Slab Calculator (Patios, Floors, Driveways)

For most do-it-yourself projects, a slab is the best shape to use because it’s easy to measure. Patios, shed floors, paths, and driveways are all made of slabs.

Length times width times thickness equals volume in cubic feet.

There is one catch: all three numbers must be in the same amount. Most of the time, thickness is given in inches, so divide it by 12 to get it in feet.

For example, a patio that is 12 feet by 10 feet and 4 inches thick

• Step 1. 4 times 12 is 0.33 feet, which is the thickness.
• Step 2: Add them all up: 12 x 10 x 0.33 = 39.6 cubic feet
• Step 3: Figure out how many cubic yards it is: 39.6 ÷ 27 = 1.47

People get lost at that last step, so it needs its own line. When you buy concrete by the cubic yard, you get 27 cubic feet of space. This is because a yard is 3 feet long and 3 times 3 times 3 is 27. If you need cubic yards instead of cubic feet, divide by 27.

Common slab thicknesses

PSI stands for pounds per square inch, and it’s used to rate how strong concrete is. A bigger number means the mix is stronger. A backyard patio should have 3,000 PSI. For anything that a car stops on, you should go higher.

One more way to measure. Don’t believe the plan drawing; measure the ground yourself. The ground in a dugout isn’t always level, and a patio that’s 4 inches deep in one end and 5 inches deep in the other will use up more concrete than your math shows. The part near the end of this page called “Waste Allowance” tells you how much extra to add.

Before you pour, check to see if you need a base of packed gravel. A lot of slabs do. We use our gravel and base material calculator to figure out that layer on its own.

Concrete Pad / Footing Calculator (Sheds, Posts, Foundations)

The formula for a slab is the same for a footing or concrete pad calculator. What changes is the shape of the numbers. For slabs, they are wide and thin, while footings are small on the outside and deep inside.

Length times width times depth equals volume in cubic feet.

An example: a base that is 3 feet by 3 feet and 1 foot deep

• 1/3 x 1/3 x 1 = 9 cu ft
• 9 times 27 is 0.33 cubic yards.

You could stand on that base with room to spare, which is a third of a yard away. What makes it work is depth. On the other hand, the patio above only took up 1.47 yards and 120 square feet of space. When it comes to sound, deep beats wide.

You need to figure out one and multiply it by the number of footings you are filling. Six 3x3x1 footings that are all the same need 6×9 = 54 cubic feet, which is 54×27 = 2 cubic yards.

For footings, you need a stronger mix than for a patio. Something that sits on a base will hold its own weight, and it can go in wet ground while a decorative slab can’t. A lot of residential footings need 3,000 to 4,000 PSI, and some city codes need even more. Frost level is also important. In cold places, footings have to go below the frost line, which in some parts of the northern US and Canada can be 42 inches or deeper. This depth changes your volume a lot.

Before you dig, check with the building department in your area. The code sets the size, depth, and strengthening of the footings; these are not choices.

Concrete Block Calculator (Block Walls)

Block walls are not the same. There is no solid amount being poured; you are counting units. This block concrete calculator counts the blocks first, then the mortar and any fill.

Wall area (sq ft) ÷ Block face area (sq ft) = Number of blocks

A normal CMU block is 8 inches by 8 inches by 16 inches. From the outside, the face is 8 inches tall and 16 inches wide.

8 times 16 times 144, or 0.89 square feet, is the block face area.

You can find 1.125 blocks per square foot by dividing 1 by 0.89. It’s easy to get to that number. To find out how many blocks you have, multiply the size of your wall by 1.125.

Shown: a 10-foot-long, 4-foot-tall wall

• Step 1: Figure out how big the wall is: 10 times 4 is 40 square feet.
• Step 2: Find the number of blocks: 40 x 1.125 = 45
• Step 3: Add 5 to 10 percent for breaks and cuts: 45 blocks times 1.10 percent = 50 blocks

When you place your order, round up. To get five extra blocks, you have to go to the yard twice, which costs more than five spare blocks.

There are two things that are left out of the block count:

Bricks. For every 12 to 15 standard blocks, you will need about one 80-pound bag of mortar mix. Four bags would be enough for our 50-block wall.

Fill the core. If your wall has to hold weight or the building code says it needs filled cores, you also need to pour concrete or grout into the hollow centers, usually with rebar. That is a different order and estimate of volume with the help of a concrete volume calculator.

Before you do the math, take away the holes. If the gate on that 10×4 wall is 3 feet by 2 feet, then your real area is 40 – 6 = 34 square feet, which is 34 x 1.125 = 39 blocks.

Round Slab or Column Calculator

Round shapes are used for patios, deck post footings, fence posts, and Sonotube columns. For circles to change, so does the method.

Size (in cubic feet) = π x radius x height

For π, use 3.14. Radius is half of diameter, and this is the step that people almost always forget.

Example: a footing with a circle of 3 feet and a depth of 1 foot

• Step 1: Find the radius: 3d x 2 = 1.5 ft
• Step 2: Square it: 1.5 times 1.5 equals 2.25
• Step 3. Multiply by 1: 3.14 x 2.25 x 1 = 7.07 cubic feet
• Step 4. Cubic yards: 7.07 x 27 = 0.26 cubic yards

Do your math in inches first for narrow tubes. There are 0.5 feet of space around a 12-inch sonotube that is 4 feet deep. This means that 3.14 × 0.25 × 4 = 3.14 cubic feet, which is about 0.12 cubic yards. That’s about 25 cubic feet, which is a little less than a cubic yard.

Bags or Ready-Mix? Concrete Bag Calculator vs Ordering by the Yard

You’ve got your sound. Now comes the choice that will cost you money.

How much does one bag cost?

• Size of the bag
• About the yield
• 60-pound bag
• 0.45 ft³
• 80-pound bag
• That’s 60 cubic feet

Total cubic feet times bag yield equals bags.

Look at the patio from before: it’s 39.6 cubic feet.

• 39.6 x 0.45 = 88 60-pound bags
• 39.6 x 0.60 = 66 80-pound bags

97 bags of 60-pound mix or 73 bags of 80-pound mix after adding 10% for waste.

Think about that. There are 97 60-pound bags that weigh 5,820 pounds. That’s almost three tons that you need to buy, load, drive home, unload, and mix. You also need to work quickly so that the first batch doesn’t set before the last one goes in. A 120-square-foot patio that was filled by one person from bags is a really bad day.

The rule of thumb: if your project needs more than about 1 cubic yard, order ready-mix.

About 60 bags of 80-pound mix make up a cubic yard. After that, a delivery truck is usually less expensive, always faster, and gives you one continuous pour with no cold gaps between batches.

Bags are still better for small jobs like a few post footings, a mower pad, a fix, or any place a truck can’t get to. If it’s less than half a yard, bags are the best choice.

Just a quick note on brand tools. There are brand-specific concrete calculators run by companies that make bags and ready-mix, and people will sometimes look for them by a company name that they can’t remember or spell correctly, like “Quick Creek concrete calculator.” What’s helpful about these tools is that they are set up to work with a certain company’s bag sizes and results. The math on this page for volume is the same both ways, so figure out your cubic feet here first. Then, look at the yield printed on the bag you buy, because it can be a little different.

Concrete Cost Calculator: Estimating the Price

Prices change all the time and vary a lot from place to place, so use this information as a guide only, not as a price. Before you decide, call two local suppliers.

Ready-mix delivery, general US range

Bagged mix, general US range

The 12×10 patio (1.47 cubic yards) was worked on in both ways.

Ready-mix: Use 1.62 yards and add 10% waste to get there. Then, round up to the next number the supplier will sell you, which is usually 1.75 or 2 yards. It costs $340 for 2 yards, which is $170. You’ll end up with about $440 after adding a short-load fee of about $100.

At $6 each, 97 bags of 60-pound mix add up to about $582. That’s before you add in the cost of renting a mixer, gas, and your whole weekend.

Ready-mix costs less and saves you three tons of work for this size job. The picture changes to show small drops. You would need 20 bags of 60-pound mix, which is about $120, for that single 3x3x1 base, which is 9 cubic feet. A ready-mix truck, on the other hand, would charge you a minimum load plus a short-load fee.

Neither range comes with forms, a dirt base, rebar, finishing tools, or labor if you hire someone to do the work. The concrete calculator itself is often less than half of the bill for a finished patio that was put in by a professional.

Concrete Calculator UK: Metric Measurements

Users in the UK, Australia, and Canada use metric units, and this isn’t just a matter of changing the names. It comes in different sizes, and you have to order it in a different unit.

Its volume is equal to its length times its width times its thickness.

Millimeters are used to measure thickness, so divide by 1,000 to get meters. A 100 mm rock is 0.1 m thick.

In this case, the patio is 4 m x 3 m and 100 mm thick.

1.2 cubic meters is 4 times 3 times 0.1.

There is no step to split by 27. One great thing about the metric system in this case is that sellers already sell in units of meters cubed.

What changes compared to the US method

For the metric case, 1.2 times 0.0125 equals 96 bags of 25 kg mix, or about 105 bags if you count the waste. That’s 2.6 tons of bags for a small patio. That’s why UK suppliers recommend ready-mix or a volumetric mixer for areas bigger than about 0.5 m³.

In the UK, ready-mix usually costs between £90 and £130 per cubic meter, plus the cost of delivery. Small orders have minimum load charges. There are a lot of volumetric mixers in the UK. They batch on-site and only charge you for what you use. For small pours, they often work out better.

To switch between units, 1 cubic meter is equal to 1.308 cubic yards, and 1 cubic yard is equal to 0.765 cubic meters. If you use the above calculator’s unit switch, it will take care of this for you.

Don’t Forget: Waste Allowance and Reinforcement

Always order more than your math says. Add 5 to 10 percent to any amount you figure out. The ground is never completely level, and some forms always end up on the ground because they are too heavy for the ground. If the ground is smooth and well-prepared, use 5%. If the ground is rough or dug by hand, use 10%.

Not having enough is much worse than having too much. When you add a second batch of concrete an hour after the first one starts to set, it makes a cold joint, which is a constant weak spot in your slab.

Reinforcement is separate. If you want to build a patio with wire mesh, you’ll need steel. For footings and work that has to hold weight, you’ll need rebar. This calculator can only figure out the volume of concrete. It is not a structural planning tool and does not measure, space, or explain reinforcement. Our rebar calculator works with both spacing and quantity.

FAQs

Q1. How much concrete do I need for a 10×10 slab?

A 10×10 slab that is 4 inches thick needs 10×10 times 0.33, which is 33 cubic feet, or 1.22 cubic yards. For every yard you buy, add 10% for waste, and you should get about 1.35 yards, which most suppliers will give you. The same slab grows to 50 cubic feet, or 1.85 cubic yards, when it’s 6 inches thick.

Q2. How many bags of concrete do I need for a small job?

Then divide that number by the bag yield. It takes about 0.45 cubic feet to fill a 60-pound bag and 0.60 cubic feet to fill an 80-pound bag. For example, a 9-cubic-foot base needs 20 bags of 60-pound mix or 15 bags of 80-pound mix. Round up and buy an extra bag or two.

Q3. How many blocks do I need for a wall?

Because an 8-by-16-inch block face takes up 0.89 square feet, multiply the wall size in square feet by 1.125. A wall that is 10 feet by 4 feet is 40 square feet, which means that there are 45 blocks. Add 5 to 10 percent to account for breaks and cuts, and then take away the area of any door or gate entrances.

Q4. Should I buy bags or order ready-mix concrete?

Set your separating line at about 1 cubic yard. After taking into account minimum load fees, bags are more useful and cheaper below that. On top of that, ready-mix usually costs less per yard, comes in one load, and saves a lot of work. That’s 4,800 pounds to move by hand, which is about 60 bags of 80-pound mix.

Q5. How much does concrete cost per cubic yard?

It is normal for ready-mix to cost $140 to $200 per cubic yard in the US. Small orders also have a short-load fee of $50 to $150. You can expect to pay between £90 and £130 per cubic meter in the UK, plus delivery. Because these costs change based on fuel, cement, and how far the truck goes, get two quotes from different businesses in your area before making a budget.

Q6. Is the UK concrete calculator different from the US one?

In three real ways, yes. In the UK, they order by the cubic meter instead of the cubic yard, so there is no step to divide by 27. Instead of 60 lb and 80 lb, bags come in 20 kg and 25 kg sizes. A 25 kg bag produces about 0.0125 m³. The thickness of a slab is given in millimeters, usually 100 mm, which is equal to 4 inches in the US.

Conclusion

Measure your shape, figure out the cubic feet, divide that number by 27 to get the cubic yards, and then add 5 to 10 percent for waste. This was the formula for all the projects on this page. The patio came in at 1.47 yards, the footing at 0.33 yards, the round column at 0.26 yards, and the block wall at 45 blocks, not a volume. 

Once you know that number, the choice between bags and trucks is easy, since it stops making sense to haul bags after about one cubic yard. Use the concrete calculator at the top of this page to get your own measurements. Then, print out the answer and bring it with you to the supplier. Then dig your forms and check three places to see how deep they are before anyone pours.