Concrete Calculator: Yards and Bags

How the volume is calculated

Concrete volume is length times width times depth, with every dimension converted to feet before the multiplication, then divided by 27 to turn cubic feet into cubic yards. A cubic yard is 27 cubic feet, which is why ready-mix is ordered in yards and bag math starts in feet. The page offers three shapes, and the units on the labels are part of the formula.

A slab uses length in feet, width in feet, and thickness in inches. Cubic feet = length × width × (thickness ÷ 12). A footing uses length in feet, width in inches, and depth in inches. Cubic feet = length × (width ÷ 12) × (depth ÷ 12). A round column uses diameter in inches and height in feet. The radius in feet is diameter ÷ 12 ÷ 2, and cubic feet = π × radius² × height. "Number of pours" multiplies the result, so four identical piers are one column calculated with a count of 4, not four separate runs you have to add by hand.

The headline yardage is the exact volume, before waste. The line labeled with a 10 percent cushion is that yardage times 1.10. Bag counts are different: they are based on the exact cubic feet, not on the cushioned yardage. A 60-pound bag is treated as 0.45 cubic feet, and an 80-pound bag as 0.60 cubic feet. Bags are rounded up. If you want bags to include the same 10 percent, add 10 percent to the bag count yourself or multiply your thickness slightly before you calculate. Ordering the printed bag count and also assuming the waste is already inside it will leave you short.

Worked examples

Example 1: A 4-inch slab

A slab 20 feet by 10 feet at 4 inches thick is 20 × 10 × (4 ÷ 12) = 66.67 cubic feet. Divide by 27 and the exact order is 2.47 cubic yards. With the 10 percent cushion the panel shows 2.72 cubic yards. Bag counts from the 66.67 cubic feet, rounded up: 149 bags at 60 pounds (66.67 ÷ 0.45) and 112 bags at 80 pounds (66.67 ÷ 0.60). Those bag counts do not include the 10 percent. A truck order that uses the cushioned 2.72 yards and a bag order that uses 112 bags are not the same quantity of concrete.

Example 2: A footing

A footing 30 feet long, 16 inches wide, and 12 inches deep is 30 × (16 ÷ 12) × (12 ÷ 12) = 40 cubic feet, or 1.48 cubic yards. The cushioned figure is 10 percent higher, about 1.63 cubic yards. Bags from the exact 40 cubic feet: 89 of the 60-pound bags, or 67 of the 80-pound bags. Width and depth are both in inches on this shape. Typing 16 as if it were feet makes the footing enormous.

Example 3: Four round piers

Each pier is 12 inches in diameter and 8 feet tall. One pier is π × (0.5 feet)² × 8, about 6.28 cubic feet. Set the count to 4 and the total is about 25.13 cubic feet, or 0.93 cubic yards, 1.02 cubic yards with the cushion. At 0.60 cubic feet per 80-pound bag, the exact volume rounds up to 42 bags. The diameter field is the full width of the pier, not the radius. A 12-inch pier entered as radius 12 is four times the concrete.

When to use this

Use it before you call a ready-mix plant or before you load bags into a cart. The plant will ask for yards. The store shelf is bags. This page speaks both, from the same dimensions, so you can see the size of the job before you pick a method. It is the right tool for a patio, a small slab, a footing trench you have formed, or a set of identical sonotube piers.

It is the wrong tool for the surface area of a wall you are painting, or for the square feet of a lawn. Those are the Paint Coverage Calculator and the Sod Calculator. If you only need to convert a volume you already know, such as liters to gallons, use the Volume Converter. This page builds the volume from dimensions.

Yards, bags, and the waste line

Ready-mix is sold by the cubic yard. The cushioned number is there because subgrade is rarely a perfect plane, forms flex, and some concrete stays in the chute. Shorting a pour in the middle is expensive. The 10 percent is a fixed multiplier on the yardage line. The page does not offer 15 percent. If the excavation is rough or the forms are complicated, increase a dimension slightly, or take the exact yards and add a larger allowance by hand, and tell the dispatcher which number you are ordering.

The bag yields are the ones this calculator uses: 0.45 cubic feet for a 60-pound bag and 0.60 cubic feet for an 80-pound bag. Bag labels vary by brand and by mix. Read the yield printed on the bag you are actually buying and, if it differs, divide your cubic feet by that yield and round up. Do not assume every 80-pound bag on the shelf is 0.60. The page also ignores the weight you will carry. An 80-pound bag count is a smaller number and a heavier lift.

Choosing truck or bags is a local price question this page does not answer. A short load from a truck often carries a minimum charge that can make bags competitive on a small pad. A multi-yard slab is a long day of mixing by hand. Get a delivered price per yard, including any short-load fee, and a shelf price per bag, then compare those quotes against the two quantities this page gives you. The Unit Price Calculator helps when the bag sizes differ and you want cost per cubic foot. Thickness itself should follow the use of the slab and the local code, not a guess from a web page. A sidewalk, a driveway that will see trucks, and a footing are different sections. Confirm the inches with the plan or the building department, then type those inches here.

Common mistakes

• Inches and feet in the wrong box. Slab thickness is inches. Slab length and width are feet. Footing width and depth are inches. Column diameter is inches and column height is feet. One swapped unit multiplies or divides the order by 12.
• Ordering bags from the cushioned yards without noticing. The bag numbers use exact cubic feet. The "with 10% waste" number is yards only. Pick one cushion and apply it once.
• Entering a diameter as a radius, or a radius as a diameter. The column field is diameter, the distance across.
• Using the count field for shapes that are not the same. Identical piers belong in the count. Different pads belong in separate calculations that you add in cubic feet.
• Forgetting that a 4-inch slab is not half of an 8-inch slab in bags if you round each one up. Compute the thickness you are actually pouring. Rounding up happens once at the end, which is what you want.
• Measuring the outside of the forms when the concrete fills the inside. Use the interior dimensions the concrete will occupy.
• Trusting a bag yield that does not match the sack in your hand. 0.45 and 0.60 are this tool's assumptions. The printed yield on the product wins.

Limitations

Rectangular slabs, straight footings, and round columns only. No steps, no thickened edges, no slope on the top of the slab, and no deduction for rebar or for pipes that displace a little volume. Those details are small on a patio and can matter on a footing with a lot of steel. The tool does not price the order, does not know the plant's minimum load, and does not add fiber, color, or a particular mix design. Waste is a flat 10 percent on the yard line and zero percent on the bag line. Subgrade that is low by an inch over a large slab can eat that cushion. Check the grade with a straightedge before you lock the number with the dispatcher.

FAQ

Why are there two yard numbers?

The first is the geometric volume. The second is that volume times 1.10, labeled as a waste cushion. Order the one you mean, and say which one it is when you call. Ordering the larger number and then adding another 10 percent at the plant stacks the cushion.

Do the bag counts include waste?

No. They are the exact cubic feet divided by 0.45 or 0.60, rounded up to a whole bag. Add your own allowance if the subgrade is uneven, or raise the thickness slightly and recalculate so the bags and the yards tell the same story.

How do I convert the result to cubic meters?

Take the cubic feet from the panel and convert with the Volume Converter. Do not convert each side to meters and also leave the inputs in feet. Pick one system before you multiply.

Related calculators

• Volume Converter
• Square Footage Calculator for the surface, once the volume is known
• Unit Price Calculator to compare bag prices per unit
• Board Feet Calculator for form lumber
• Sod Calculator for the lawn around the new slab
• How to read a paint label if the same project includes coating the finished surface