Pick what you are pouring and enter its dimensions.
A cubic yard of concrete fills 27 cubic feet, which is a 4 inch slab covering about 81 square feet. Everything else on this page comes from that one conversion. Measure in the units written on your plans, feet for length and width, inches for thickness, and the calculator does the rest, including the bags and the weight.
How Much Concrete a Slab Needs
Thickness is the part people get wrong, because it is the one dimension given in inches while the others are in feet. Divide the inches by 12 first, then multiply:
Cubic yards = cubic feet ÷ 27
A 10 by 12 foot patio at 4 inches is 10 × 12 × (4 ÷ 12) = 40.00 cubic feet, which is 1.481 cubic yards. Add 10 percent for waste and it becomes 44.00 cubic feet, or 1.630 cubic yards.
| Slab thickness | Cubic feet per 100 sq ft | Cubic yards per 100 sq ft | Bags of 80 lb |
|---|---|---|---|
| 3 in | 25.00 | 0.93 | 42 |
| 3.5 in | 29.17 | 1.08 | 49 |
| 4 in | 33.33 | 1.23 | 56 |
| 5 in | 41.67 | 1.54 | 70 |
| 6 in | 50.00 | 1.85 | 84 |
| 8 in | 66.67 | 2.47 | 112 |
| 10 in | 83.33 | 3.09 | 139 |
| 12 in | 100.00 | 3.70 | 167 |
No waste is included in that table, and the bag counts are rounded up to a whole bag. How thick your slab has to be is set by what it carries and by the code your project falls under, so that question belongs with your local building department. The one figure the product itself fixes is the minimum: the bagged mix is designed for pours of 2 inches or more.
Bags or a Ready Mix Truck
Bag yields are printed on the bag, and they are all the same material at the same rate. Every size yields 0.0075 cubic feet per pound of dry mix, which is why the dry weight of a cubic yard comes out identical whichever bag you buy:
| Bag | Yield | Bags per cubic yard | Dry weight you carry |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 3,600 lb |
| 50 lb | 0.375 cu ft | 72 | 3,600 lb |
| 60 lb | 0.45 cu ft | 60 | 3,600 lb |
| 80 lb | 0.60 cu ft | 45 | 3,600 lb |
| 90 lb | 0.675 cu ft | 40 | 3,600 lb |
That 3,600 pound figure is the useful one. It is a pickup load of material, carried twice, once into the mixer and once into the forms, for every cubic yard. Two cubic yards by hand is 90 bags of 80 lb and more than three and a half tons of lifting.
Whether a truck is cheaper, and at what size, depends on your supplier rather than on arithmetic. Plants set their own full load size, their own minimum, and their own charge for a part load, and those three numbers decide it. Ask for the rate sheet before you compare, and give them your figure in cubic yards, because that is what they sell in.
Round Columns and Post Holes
A round pour uses the circle: volume = π × radius² × height. With the diameter in inches and the height in feet, the radius in feet is the diameter divided by 24.
| Tube diameter | Cubic feet per foot of height | Bags of 80 lb per foot | Feet of column per cubic yard |
|---|---|---|---|
| 6 in | 0.196 | 0.33 | 137.5 |
| 8 in | 0.349 | 0.58 | 77.4 |
| 10 in | 0.545 | 0.91 | 49.5 |
| 12 in | 0.785 | 1.31 | 34.4 |
| 14 in | 1.069 | 1.78 | 25.3 |
| 16 in | 1.396 | 2.33 | 19.3 |
| 18 in | 1.767 | 2.95 | 15.3 |
| 24 in | 3.142 | 5.24 | 8.6 |
The post takes up room
Setting a post in concrete is not the same as filling an empty hole, because the post is in the hole. Most calculators ignore that and send you home with bags you do not need. A post is also smaller than its name: under the American Softwood Lumber Standard a dry 4x4 is dressed to 3.5 by 3.5 inches and a dry 6x6 to 5.5 by 5.5 inches.
Six holes 10 inches across and 30 inches deep hold 1.364 cubic feet each. A 4x4 running the full depth displaces 0.213 cubic feet, so the real figure is 1.151 cubic feet per hole and 6.91 for the six, which is 12 bags of 80 lb. Ignore the posts and you buy 14. Pick your post size in the calculator and it comes out of the volume.
Steps, and the Formula Nobody Writes Down
A solid flight of steps is the shape people guess at, usually by treating it as a box and ordering too much. Seen from the side it is a staircase, and the volume of a staircase with n steps is:
The n(n + 1) ÷ 2 is there because the bottom step is one riser tall, the next is two risers tall, and so on, so the heights add up as 1 + 2 + 3 and not as n times the same thing. Three steps 4 feet wide with an 11 inch tread and a 7 inch riser come to 12.833 cubic feet, which is 0.475 cubic yards or 22 bags of 80 lb.
A box of the same outside size is 4 feet wide by 2 feet 9 inches of run by 1 foot 9 inches of rise, which is 19.25 cubic feet: half as much again as the steps actually hold. The overshoot is 2n ÷ (n + 1), so it is 1.5 times at three steps, 1.67 at five, and it only approaches double on a long flight.
Where the Figures Go Wrong
- Thickness left in inches. A 4 in slab entered as 4 feet is twelve times too much concrete. Inches divided by 12, every time.
- Yards and cubic yards. Concrete is sold by the cubic yard, 27 cubic feet. A cubic yard is not three cubic feet.
- Ordering the exact volume. The forms are never exactly the size on the drawing, the sub-base is never perfectly flat, and some of it ends up on the ground. Adding 5 to 10 percent is common practice on site, and running short mid-pour costs far more than a little spare.
- Measuring the form instead of the hole. For footings and post holes, what you pay for is the hole you actually dug, and a hand-dug hole flares wider than its nominal size.
- Forgetting the posts, the pipes and the blockouts. Anything already in the hole is concrete you do not buy.
For the plain geometry behind all of this, the volume calculator handles any shape in metric or imperial units, and the area calculator covers the surface first if you are working from a sketch.
Method and sources. Volumes are plain geometry and the conversions are exact by definition: 1 cubic yard = 27 cubic feet, and 1 foot = 12 inches. Every figure in the tables was computed from those formulas rather than copied. Bag yields, the 2 inch minimum pour and the cured unit weight of approximately 140 lb per cubic foot, tested to ASTM C138, are from the QUIKRETE Concrete Mix (no. 1101) product data sheet, which also gives 4,000 psi at 28 days tested to ASTM C39. The calculator reports weight at 145 lb per cubic foot, the value used for normalweight concrete up to 5 ksi in AASHTO bridge design; ACI 301 defines normalweight concrete as typically 135 to 160 lb per cubic foot, so treat the weight as an estimate and ask your supplier for the figure for the mix they deliver. Dressed lumber sizes are from Table 3 of Voluntary Product Standard PS 20-25, the American Softwood Lumber Standard. Waste allowances are common site practice, not a standard. Slab thickness, strength and reinforcement for your project are set by the building code that applies to it and by the loads it carries, not by this page.
Concrete Questions
How many bags of concrete are in a cubic yard?
45 bags of 80 lb, 60 bags of 60 lb, 72 bags of 50 lb or 90 bags of 40 lb. A cubic yard is 27 cubic feet and an 80 lb bag yields about 0.60 cubic feet, so 27 divided by 0.6 is 45. Every bag size works out to 3,600 lb of dry mix per cubic yard, because they all yield 0.0075 cubic feet per pound.
How much concrete do I need for a 10 by 12 patio?
At 4 inches thick, 40.00 cubic feet, which is 1.481 cubic yards. Add 10 percent for waste and you are ordering 1.630 cubic yards, or buying 74 bags of 80 lb. The math is 10 × 12 × (4 ÷ 12) = 40 cubic feet, then 40 ÷ 27 for the yards. At 6 inches thick the same patio takes 60.00 cubic feet, or 2.222 cubic yards before waste.
How much does a cubic yard of concrete weigh?
About 3,900 lb, close to two short tons. At 145 lb per cubic foot, the value used for normalweight concrete in bridge design, a cubic yard is 3,915 lb. ACI 301 defines normalweight concrete as typically 135 to 160 lb per cubic foot, which puts a cubic yard between 3,645 and 4,320 lb, so the exact weight depends on the mix. The bagged mix tests at about 140 lb per cubic foot, or 3,780 lb per cubic yard.
Is it cheaper to use bags or order ready mix?
That depends on your supplier's rate sheet, not on arithmetic, because plants set their own full load size, minimum order and charge for a part load. What the arithmetic does tell you is the work, and the work is the part people underestimate: one cubic yard is 45 bags of 80 lb and 3,600 lb of dry material, lifted into a mixer and then into the forms. Even half a cubic yard is 23 bags and 1,840 lb. Work out your volume first, then take that figure both to the plant and to the shelf price, because those two numbers are the only ones that settle it.
Do I subtract the post when filling a post hole?
Yes. The post occupies part of the hole, so the concrete only fills what is left. A dry 4x4 is actually 3.5 by 3.5 inches and a dry 6x6 is 5.5 by 5.5 inches, under the American Softwood Lumber Standard. In a hole 10 inches across and 30 inches deep, a 4x4 displaces 0.213 of the 1.364 cubic feet, so six such holes need 12 bags of 80 lb rather than 14. Select the post size in the calculator and it is taken off automatically.
How thick does a concrete slab have to be?
That is set by what the slab carries and by the building code your project falls under, so check with your local building department rather than with a general figure. Two things are fixed no matter the project: the bagged mix is designed for pours of 2 inches or more, and thickness drives the quantity directly, so a 6 inch slab takes exactly half again as much concrete as a 4 inch one over the same area.
How much extra concrete should I order for waste?
Adding 5 to 10 percent is common practice on site, and this calculator defaults to 10 percent. It is practice rather than a rule, so treat it as a judgement about your own pour: forms that bulge, a sub-base that is not quite level and ground that is not quite flat all take more than the drawing says. Running short part way through a pour leaves a cold joint where you did not want one, which costs far more than the spare.