Concrete Calculator
Enter the size of your slab, footing, post hole or steps to see how many cubic yards of concrete to order, or how many 40, 60 or 80 lb bags to buy, with extra for waste built in.
Volume of concrete.
How to use Concrete Calculator
- Pick a shape: slab or footing, round hole or column, or steps
- Enter each measurement and pick its unit (you can mix feet and inches)
- Set a quantity if you are pouring several identical holes or footings
- Choose your bag size and how much extra to allow (10% is typical)
- Read the cubic yards to order or the number of bags to buy
About Concrete Calculator
The Concrete Calculator works out how much concrete a pour needs and how to buy it. Pick a shape (a slab, pad, patio, driveway, footing or wall; a round post hole, column or Sonotube form; or a set of solid steps), enter the measurements, and it shows the amount to order in cubic yards, cubic feet and cubic meters, the approximate weight of the pour, and how many 40, 50, 60 or 80 lb bags (or 20 or 25 kg bags) it takes. A waste allowance, 10% by default, is added to the order amount, and the exact volume is shown next to it. A quantity field covers several identical holes or footings at once.
The math is simple. Multiply length × width × thickness, all in feet, to get cubic feet, then divide by 27 to get cubic yards, because a cubic yard is 3 × 3 × 3 = 27 cubic feet. Thickness is usually measured in inches, so divide it by 12 first. For example, a 10 × 12 ft patio 4 inches thick is 10 × 12 × (4 ÷ 12) = 40 cubic feet, or 1.48 cubic yards. A handy shortcut: one cubic yard covers 81 square feet at 4 inches thick. Round holes and columns use π × radius² × depth, and the calculator subtracts the post if you set one in the hole.
Bag counts come from the yield printed on the bag, not from its weight. The QUIKRETE Concrete Mix data sheet lists about 0.60 cubic feet for an 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb, which works out to about 45, 60, 72 and 90 bags per cubic yard. The 20 kg and 25 kg sizes are estimated at the same yield per kilogram (about 11.7 litres for a 25 kg bag), so treat them as approximate. Fast-setting and specialty mixes yield differently; if your bag prints a yield, choose Custom and enter it.
Ground is rarely perfectly flat, forms bow a little and some concrete always gets spilled, so the calculator adds 10% by default; Sakrete, for one, recommends adding 10% to every estimate. You can choose none, 5% or 15% instead. Once a pour reaches about a cubic yard (45 bags of 80 lb), ready-mix delivery is usually easier than mixing by hand. Ask local suppliers about their minimum load and any small-load fees, and use the optional cost fields with your own prices to compare.
The calculator is free, with no sign-up and no limits. Your measurements are calculated in your browser and are not sent anywhere.
Frequently asked questions
- How do I calculate how much concrete I need?
- Multiply length × width × thickness in feet to get cubic feet, then divide by 27 to get cubic yards. Convert inches to feet first by dividing by 12. For example, a 10 × 10 ft slab 4 inches thick is 10 × 10 × (4 ÷ 12) = 33.3 cubic feet, or 1.23 cubic yards. Add about 10% for waste before you order. The calculator does all of this, with any mix of feet, inches, meters and centimeters.
- Why do you divide by 27 to get cubic yards?
- Because a cubic yard is a cube 3 feet on each side, and 3 × 3 × 3 = 27 cubic feet. Dividing a volume in cubic feet by 27 converts it to cubic yards, the unit US ready-mix suppliers sell concrete by.
- How much concrete do I need per square foot?
- At 4 inches thick, each square foot takes 1/3 of a cubic foot of concrete, so one cubic yard (27 cubic feet) covers 81 square feet. At 6 inches thick it is 1/2 cubic foot per square foot, and a cubic yard covers 54 square feet. To work from square footage directly, set Measure by to Total area.
- How many 80 lb bags of concrete are in a yard?
- About 45. An 80 lb bag of standard concrete mix yields about 0.60 cubic feet, and a cubic yard is 27 cubic feet. Using the yields on the QUIKRETE Concrete Mix data sheet, a cubic yard also takes about 60 bags of 60 lb (0.45 cubic feet each), 72 bags of 50 lb (0.375) or 90 bags of 40 lb (0.30). Other mixes vary, so check the yield printed on your bag.
- How many bags of concrete do I need for a 10x10 slab?
- At 4 inches thick, a 10 × 10 ft slab is 1.23 cubic yards: about 56 bags of 80 lb, 75 bags of 60 lb or 112 bags of 40 lb. With 10% extra for waste, buy 62, 82 or 123. At 6 inches thick it is 1.85 cubic yards, or 84 bags of 80 lb before waste.
- How much concrete do I need for a 12x12 slab?
- At 4 inches thick, a 12 × 12 ft slab is 48 cubic feet, or 1.78 cubic yards. That is 80 bags of 80 lb (88 with 10% extra) or 107 bags of 60 lb. For the material cost, multiply the bags or cubic yards by your local price. An installed slab also pays for labor, a gravel base, forms and finishing, which vary a lot from area to area.
- How much concrete do I need for a concrete pad?
- Treat a pad as a small slab: enter its length, width and thickness. A 4 × 4 ft pad 4 inches thick takes 0.20 cubic yards, about 9 bags of 80 lb (10 with 10% extra). An 8 × 10 ft shed base at 4 inches is 0.99 cubic yards, about 45 bags of 80 lb (49 with 10% extra), which is around the point where ready-mix starts to make sense.
- How do I estimate concrete for a footing?
- A strip footing is a long, narrow slab: enter its length, its width, and its depth as the thickness. For several identical pad footings, enter one and set Quantity to the number you are pouring. Round pier footings and Sonotube forms go under Hole / column.
- How many bags of concrete do I need for a fence post?
- It depends on the hole. A 10 inch wide hole 3 feet deep holds about 1.6 cubic feet; with a 4×4 post in it (3.5 × 3.5 inches actual), the concrete is about 1.4 cubic feet. That is about 2.3 bags of 80 lb or 3.1 bags of 60 lb, so with 10% extra, buy 3 bags of 80 lb or 4 bags of 60 lb per post. Use the Hole / column shape, pick your post size and set Quantity to the number of posts.
- How many 20 kg bags of concrete make 1 m³?
- About 107, if each 20 kg bag yields about 9.4 litres, which is the same yield per kilogram as a standard 80 lb bag. With 25 kg bags (about 11.7 litres each) it is about 86. These are estimates, because yields vary by product: check the bag, and if it lists a yield, enter it with the Custom bag size.
- Is it cheaper to buy bags or ready-mix concrete?
- Bags suit small jobs such as fence posts, repairs and pads under about a cubic yard. Past about a cubic yard (45 bags of 80 lb), ready-mix is usually easier and often cheaper per yard, but many suppliers charge extra for small loads. Enter your local bag and ready-mix prices in the optional cost fields to compare them for your pour.
- How thick should a concrete slab be?
- 4 inches is the usual thickness for patios, walkways, sidewalks and most residential slabs. Driveways and garage floors that carry heavier vehicles are often poured 5 to 6 inches thick. Your plans or local building code have the final say. The 4, 5 and 6 in buttons under Thickness fill these in.
- How much does a cubic yard of concrete weigh?
- Roughly 3,800 to 4,050 lb. Normal-weight concrete is roughly 140 to 150 lb per cubic foot (about 2,250 to 2,400 kg per cubic meter), and a cubic yard is 27 cubic feet. The calculator shows the approximate weight of your pour at 150 lb per cubic foot, the top of that range, so the real pour may weigh a little less.
- Should I order extra concrete?
- Yes. Ground is rarely perfectly level, forms flex and some concrete always gets spilled, and running short mid-pour is a real problem. The calculator adds 10% by default, and Sakrete recommends adding 10% to every estimate. You can change it to none, 5% or 15%, and the exact volume before the allowance is shown under the result.
- Can I mix units like feet and inches?
- Yes. Each measurement has its own unit selector, so you can enter length and width in feet and thickness in inches, or use meters and centimeters. The calculator converts everything to the same unit before it works out the volume.
- How do I convert cubic meters to cubic yards?
- Multiply cubic meters by about 1.308 to get cubic yards, or multiply cubic yards by about 0.765 to get cubic meters. The calculator shows cubic yards, cubic feet and cubic meters side by side, so you can order in whichever unit your supplier uses.
- Is the Concrete Calculator free to use?
- Yes, it is 100% free with no account, subscription, or usage limits. You can calculate as many pours as you need.
- Is my data uploaded or stored anywhere?
- No. Your measurements are calculated in your browser and are not sent anywhere, and nothing is saved after you close the page.