Concrete Calculator

Worked example

A 10 ft by 10 ft slab, 4 inches thick, needs 1.23 cubic yards of concrete.

The math takes three steps:

1. Turn the thickness into feet: 4 inches ÷ 12 = 1/3 ft (about 0.33 ft).
2. Multiply length × width × thickness: 10 × 10 × 1/3 = 33.33 cubic feet.
3. Divide by 27 to get cubic yards: 33.33 ÷ 27 = 1.23 cubic yards.

That job takes 56 bags of 80 lb mix, because the QUIKRETE Concrete Mix data sheet lists 0.60 cubic feet per 80 lb bag. With 10% extra for waste, plan on 1.36 cubic yards or 62 bags.

How to calculate concrete

Work out the volume in cubic feet, then divide by 27 to get cubic yards. Ready-mix suppliers sell concrete by the cubic yard, and one cubic yard holds 27 cubic feet.

1. Measure each side in feet. To turn inches into feet, divide by 12.
2. Multiply the sides together, using the formula for your shape below.
3. Divide the cubic feet by 27 to get cubic yards.
4. Add extra for waste, then round up when you order.

For a slab 4 inches thick, a shortcut skips the first three steps: divide the area in square feet by 81. A 10 ft × 10 ft slab covers 100 square feet, and 100 ÷ 81 = 1.23 cubic yards.

Each cubic yard covers 81 square feet at that thickness, because 27 ÷ 1/3 = 81.

The formula for each shape

Multiply the measurements your shape needs, then divide by 27. Each example below uses the values the calculator opens with.

• Slab: length × width × thickness. A 10 ft × 10 ft slab at 4 inches is 33.33 cubic feet, or 1.23 cubic yards.
• Circle: 3.1416 × radius × radius × thickness. The radius is half the distance across. A circle 10 ft across at 4 inches is 26.18 cubic feet, or 0.97 cubic yards.
• Footing: length × width × depth. A footing 20 ft long, 2 ft wide and 12 inches deep is 40 cubic feet, or 1.48 cubic yards.
• Round column: 3.1416 × radius × radius × height. A column 12 inches across and 4 ft high is 3.14 cubic feet, or 0.12 cubic yards.
• Square column: width × depth × height. A 12 by 12 inch column, 4 ft high, is 4 cubic feet, or 0.15 cubic yards.
• Wall: length × height × thickness. Take door and window areas off length × height first. A wall 20 ft long, 4 ft high and 8 inches thick is 53.33 cubic feet, or 1.98 cubic yards.
• Curb and gutter: add the curb and the gutter, then multiply by the length. The curb is its depth × its full height, which is the height above the slab plus the slab thickness. The gutter is its width × the slab thickness. A 20 ft run (6 by 6 inch curb, 12 inch gutter, 6 inch flag) is 20 cubic feet, or 0.74 cubic yards.
• Steps: treat each step as a solid block from the ground, add them up, then multiply by the width. Step 1 is one rise tall, step 2 is two rises tall, and so on. The top step uses the platform depth in place of the run. Three steps 4 ft wide (7 inch rise, 11 inch run, 36 inch platform) make 27.42 cubic feet, or 1.02 cubic yards.
• Monolithic slab: work out the flat slab, then add the thicker edge. The edge is a ring: take the whole area, subtract the area inside the edge, then multiply by the edge depth. This counts each corner once. A 20 ft × 20 ft slab at 4 inches (edge 12 inches wide, 12 inches deeper) is 209.33 cubic feet, or 7.75 cubic yards.

If your job has more than one part, work out each part and add the results. The quantity box does this for parts that are the same size.

How much concrete do I need?

A 4 inch slab needs 1.23 cubic yards for every 100 square feet. The table shows common slab sizes at 4 inches thick, before any extra for waste.

Slab size	Cubic yards	80 lb bags
10 × 10 ft	1.23	56
10 × 12 ft	1.48	67
12 × 12 ft	1.78	80
12 × 20 ft	2.96	134
20 × 20 ft	4.94	223
24 × 24 ft	7.11	320

For a 6 inch slab, multiply the cubic yards by 1.5. The bag counts use the QUIKRETE yield of 0.60 cubic feet per 80 lb bag.

How many bags of concrete do I need?

Divide the cubic feet you need by the cubic feet one bag makes, then round up to a whole bag.

The QUIKRETE Concrete Mix data sheet lists these yields per bag:

Bag size	Concrete per bag	Bags for 1 cubic yard
80 lb	0.60 cubic feet	45
60 lb	0.45 cubic feet	60
50 lb	0.375 cubic feet	72
40 lb	0.30 cubic feet	90

For 30 kg bags, the SAKRETE Concrete Mix data sheet from Sika Canada lists 0.014 cubic meters per bag. That works out to about 72 bags for 1 cubic meter.

Yields are approximate. They change with how much water you add and how well you pack the mix, so check the bag you buy.

How many yards of concrete should I order?

Order 4% to 10% more than your measured amount. The National Ready Mixed Concrete Association (NRMCA) gives that range in its CIP 31 sheet on ordering ready-mix.

The extra covers the small losses every pour has. For a 20 ft × 20 ft slab at 4 inches, 4.94 cubic yards becomes 5.14 to 5.43 cubic yards. The waste box above starts at 10%, the top of that range.

Measure again before you call the supplier. The same NRMCA sheet warns against ordering too much concrete or small clean-up loads.

Ready-mix or bags: which should I order?

Bags suit small jobs and ready-mix suits large ones. The switch point is how many bags you are willing to mix by hand.

One cubic yard takes 45 bags of 80 lb mix at the yield QUIKRETE lists, and you carry, open and mix each one. A truck delivers the same cubic yard already mixed.

Before you decide, ask a local ready-mix supplier two things: the smallest load they deliver, and whether a small load costs extra. Then enter both prices in the cost boxes above to compare.

How thick should a concrete slab be?

A concrete floor slab on the ground must be at least 3½ inches thick under section R506.1 of the 2021 International Residential Code.

For driveways, the QUIKRETE guide Concrete Driveways and Curbs says 4 inches is enough for passenger cars. It recommends 5 to 6 inches if trucks and other heavy vehicles use the driveway.

Cities can change these rules, and a slab under a building follows the plans for that building. Check with your local building department, then type your own thickness. The calculator starts at 4 inches as a sample value only.

How much does concrete weigh?

Concrete mix weighs about 140 lb per cubic foot, according to the QUIKRETE Concrete Mix data sheet. That works out to 3,780 lb per cubic yard, because 140 × 27 = 3,780.

The result card shows the weight of your job under the cubic yards. Use it to check what your truck or trailer can carry before you buy bags.

A metric example

A slab 3 m long, 3 m wide and 10 cm thick needs 0.90 cubic meters of concrete (3 × 3 × 0.10 = 0.90).

The SAKRETE data sheet from Sika Canada lists 0.014 cubic meters per 30 kg bag, so that is 65 bags. With 10% extra for waste, plan on 0.99 cubic meters or 71 bags. Tap Metric above the calculator to work in meters and centimeters.

FAQs

How many bags of concrete are in a yard?

One cubic yard takes 45 bags of 80 lb mix or 60 bags of 60 lb mix. With smaller bags, it takes 72 bags of 50 lb mix or 90 bags of 40 lb mix.

These counts divide 27 cubic feet by the per-bag yields in the QUIKRETE Concrete Mix data sheet: 0.60, 0.45, 0.375 and 0.30 cubic feet.

How many 80 lb bags make a yard?

It takes 45 bags of 80 lb mix to make one cubic yard. The QUIKRETE Concrete Mix data sheet lists 0.60 cubic feet per 80 lb bag, and 27 ÷ 0.60 = 45. Add your waste allowance on top, because the yield changes with the water you add.

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

A 10 ft × 10 ft slab needs 1.23 cubic yards at 4 inches thick, or 1.85 cubic yards at 6 inches thick. In 80 lb bags, that is 56 bags at 4 inches or 84 bags at 6 inches.

With 10% extra for waste, order 1.36 cubic yards for the 4 inch slab.

How many yards of concrete do I need for a 24 × 24 slab?

A 24 ft × 24 ft slab at 4 inches thick needs 7.11 cubic yards, or 320 bags of 80 lb mix. With 10% extra for waste, order 7.82 cubic yards.

A slab that size suits ready-mix better than bags. At 6 inches thick, the same slab needs 10.67 cubic yards.

Why do you divide by 27 for cubic yards?

You divide by 27 because one cubic yard holds 27 cubic feet. A cubic yard is a cube 3 feet on each side, and 3 × 3 × 3 = 27.

You measure a job in feet, so the math gives cubic feet, and dividing by 27 turns them into the cubic yards a supplier sells.

How many cubic feet does a 50 lb bag of concrete fill?

A 50 lb bag of concrete mix makes 0.375 cubic feet, according to the QUIKRETE Concrete Mix data sheet. That fills a post hole about 6 inches square and 18 inches deep. It takes 72 of these bags to make one cubic yard.

How much concrete is in a truck?

A ready-mix truck holds 8 to 12 cubic yards, according to the NRMCA sheet CIP 31 on ordering ready mixed concrete. So one truck can carry a 24 ft × 24 ft slab at 4 inches, even with 10% extra.

Ask your supplier how much their trucks carry before you book.

How much does a yard of concrete cost?

The price of a cubic yard depends on where you live, the mix you order, the size of the load and the delivery distance. Ask two or three local ready-mix suppliers for a quote. Then type the price into the cost box above to see the total for your job.

Do I need rebar or mesh in a slab?

It depends on what the slab carries, the ground under it and your local building code. This calculator works out concrete volume only.

For a driveway, a slab under a building or any heavy load, follow the plans for the job. If there are no plans, ask your local building department or an engineer.

How accurate is this calculator?

The volume is exact for the measurements you enter. We test every shape against answers worked out by hand. The slab, circle, footing, column, wall, curb and steps also match calculator.net.

Bag counts are close but not exact, because each bag’s yield changes with the water you add.