Concrete Calculator
Home & EnergyPick a slab (also for footings and walls) or a round column, enter the size in feet and inches, and add a little extra for waste. You get the cubic yards to order, cubic feet, cubic meters, the number of bags and the weight.
Step by step
10 × 10 × (4 / 12) = 33.333
- Volume of one piece: 10 × 10 × (4 / 12) = 33.333 cubic feet.
- Multiply by the number of pieces (1) and add the extra (5%): 33.333 × 1 = 33.333, then 33.333 × (1 + 5 / 100) = 35 cubic feet.
- A cubic yard is 27 cubic feet: 35 / 27 = 1.3 cubic yards, which is 0.99 cubic meters.
- One bag of 80 lb yields 0.6 cubic feet, so 35 / 0.6, rounded up, is 59 bags.
Once every entry is valid, you’ll see each step of the calculation here.
Formula
- 12
- inches in a foot
- 24
- 2 × 12: halves the diameter to get the radius and turns inches into feet
- 27
- cubic feet in a cubic yard (3 × 3 × 3)
- pieces
- the number of identical slabs, columns or holes, 1 for a single pour
- extra
- the percent added for waste, 5 by default
- 0.0075
- cubic feet of concrete one pound of premix makes: 0.60 cubic feet from an 80 lb bag
How it works
Concrete is sold by volume, so the calculator multiplies the three dimensions. For a slab, footing or wall, volume = length × width × thickness, with the thickness turned from inches into feet by dividing by 12. A 10 ft by 10 ft patio that is 4 inches thick holds 10 × 10 × (4 / 12) = 33.3 cubic feet. Ready-mix is ordered in cubic yards, and one cubic yard is 27 cubic feet (3 ft × 3 ft × 3 ft), so that patio is 33.3 / 27 = 1.23 cubic yards.
For a round column, a tube form or a fence post hole the volume is a cylinder: π × radius² × depth, with the radius in feet (half the diameter, divided by 12). A 12 inch column that is 4 feet deep takes 3.14 × 0.5² × 4 = 3.14 cubic feet. Enter the number of identical pieces to total several holes at once. Then comes the extra for waste, which the calculator sets to 5 percent: an exact order leaves you short when concrete is spilled or the ground is lower than you measured, so raise it for hand-dug holes or uneven bases. To turn dimensions given in inches into feet, use the inches to feet converter.
A bag of premixed concrete yields about 0.0075 cubic feet per pound of bag: 0.30 cubic feet for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb and 0.60 for 80 lb. The calculator divides the volume by the yield and rounds up to whole bags, which gives 56 bags of 80 lb for a 10 by 10 by 4 inch slab with no extra. Yields differ a little between products, so check the label. As a rule of thumb, bags suit small jobs and ready-mix suits anything much over a cubic yard, which is 45 bags of 80 lb; suppliers often charge a short-load fee for small deliveries. The weight uses 150 pounds per cubic foot, about 4,050 pounds or 2 tons per cubic yard.
The formula
One yard is exactly 0.9144 meters, so one cubic yard is 0.764554857984 cubic meters, which is how the page converts to cubic meters. The weight of normal-weight concrete is about 145 to 150 pounds per cubic foot; the calculator uses 150.
The result assumes square forms, a level base and an even thickness. A slab with thickened edges, a gravel base that settles, or forms that bow out will take more, which is what the extra percentage is for. Check local code and your plans for thickness, reinforcement and mix strength before you order.
Example
How much concrete does a 10 by 10 foot patio need?
- A 4 inch slab holds 10 × 10 × (4 / 12) = 33.3 cubic feet. With 5 percent extra that is 35 cubic feet.
- Divide by 27: 1.3 cubic yards, or 0.99 cubic meters. In 80 lb bags that is 59 bags, and the pour weighs about 5,250 lb.
Your pour needs 1.3 cubic yards (35 cubic feet), extra 5% included. In bags of 80 lb that is about 59 bags.
Load this example into the calculatorFrequently asked questions
How much concrete do I need?
Measure the length, width and thickness, multiply them in feet to get cubic feet, divide by 27 to get cubic yards and add 5 to 10 percent for waste. A 10 by 10 foot slab that is 4 inches thick needs 1.23 cubic yards, or about 1.30 with 5 percent extra.
How do I calculate how much concrete I need?
Volume = length × width × thickness, all in feet, so turn inches into feet by dividing by 12. For a round column use π × radius² × height. Divide the cubic feet by 27 to order in cubic yards, or by the bag yield (0.6 cubic feet for an 80 lb bag) to count bags.
What is the standard thickness of concrete?
It depends on the use. Residential patios, walkways and driveways are typically 4 inches thick, garage floors and heavy-duty driveways often 5 to 6 inches, and commercial slabs 6 to 8 inches or more. Your plans or local building code decide the thickness for a structural slab.
How much is 1 yard of concrete?
One cubic yard is 27 cubic feet and weighs about 4,050 pounds, roughly 2 tons. It covers 81 square feet at 4 inches thick. The price of a cubic yard of ready-mix depends on your region, the strength of the mix and delivery, so ask local suppliers for a quote.
How much concrete do I need for a 10x10 slab?
A 10x10 slab that is 4 inches thick needs 33.3 cubic feet, which is 1.23 cubic yards. At 6 inches thick it needs 50 cubic feet, or 1.85 cubic yards. Add 5 to 10 percent on top of either figure to cover waste.
How many 80 lb bags of concrete for a slab?
An 80 lb bag yields about 0.6 cubic feet, so divide the cubic feet of your slab by 0.6 and round up. A 10x10 slab at 4 inches needs 56 bags, a 100 square foot slab at 4 inches also needs 56, and at 6 inches it needs 84.
How many cubic feet is a 50-pound bag of concrete?
A 50 lb bag yields about 0.375 cubic feet, or 0.014 cubic yards. The other sizes follow the same 0.0075 cubic feet per pound: 0.30 cubic feet for a 40 lb bag, 0.45 for 60 lb and 0.60 for 80 lb. The label on your bag has the exact yield.
Should I order concrete by the yard from a ready mix company or just use bags?
Bags are practical for small jobs such as a few post holes or a small pad. Past about a cubic yard, which is 45 bags of 80 lb, the mixing effort and cost usually favor ready-mix. Many suppliers add a short-load fee for small deliveries, so compare both for jobs of 1 to 3 cubic yards.
How much does concrete weigh?
Normal-weight concrete weighs about 145 to 150 pounds per cubic foot. The calculator uses 150, which is about 4,050 pounds per cubic yard, so a 10x10 slab at 4 inches weighs about 5,000 pounds, or 2.5 tons.
How much does concrete cost per square foot?
Multiply your supplier's price per cubic yard by the cubic yards per square foot, which is the thickness in inches divided by 12 and by 27. As an example, not a quote, at $150 per cubic yard a 4 inch slab uses 0.0123 cubic yards per square foot, so the concrete alone costs about $1.85 per square foot before delivery, forms, base, reinforcement and labor.
How do I calculate concrete for a column, a sonotube or a post hole?
Use the round column option: enter the diameter in inches and the depth or height in feet. A 12 inch tube that is 4 feet deep takes 3.14 cubic feet, about 5.2 bags of 80 lb. For several identical holes, set the number of pieces.
How much rebar do I need for my concrete slab?
That depends on the slab's use, the soil and local code, so follow your plans or ask your building department or an engineer. Residential slabs are often reinforced with welded wire mesh or rebar laid out at a set spacing. This calculator estimates the concrete only, not the steel.
How much concrete do I need for 100 square feet?
It depends on the thickness. At 4 inches, 100 square feet needs 33.3 cubic feet, which is 1.23 cubic yards; at 6 inches it needs 50 cubic feet, or 1.85 cubic yards. Per square foot, multiply the thickness in inches by 0.0031 to get cubic yards, then add your extra for waste.
How do I calculate concrete for a triangle or an irregular shape?
Find the area first. A triangle is half the base times the height, and an irregular shape can be split into rectangles and triangles whose areas you add. Then enter any length and width whose product is that area, such as a length of 10 feet and a width of the area divided by 10, together with the thickness.
How much extra concrete should I order?
Many contractors order 5 to 10 percent more than the calculated volume. Spillage, uneven ground, a slightly deeper base and forms that bow out all use concrete, and a second delivery for a small shortfall can cost more than the extra you would have ordered.
- Concrete Calculator: How Much Do I Need? (Bags and Yards) (Concrete Network)
- NIST Guide to the SI, Appendix B: Conversion Factors (National Institute of Standards and Technology)
- Concrete (Wikipedia)