How to use it
Pick what you are pouring, then type its size. The answer is in cubic yards for ordering ready-mix, with cubic feet and the number of 80, 60 and 40 lb bags below it. Each figure is also shown with 10% extra. If you are pouring several of the same thing, such as a row of piers, put the count in How many.
Worked examples
- Slab. A 20' × 12' slab 4" thick is 80 cu ft, which is 2.96 cu yd, or 3.26 cu yd with 10% extra. In bags that is 134 of the 80 lb size.
- Footing. A strip footing 24' long, 2' wide and 1' 6" deep is 72 cu ft, which is 2.67 cu yd.
- Round piers. Six piers 12" across and 4' deep are 18.85 cu ft in total, which is 0.7 cu yd, or 32 bags of the 80 lb size.
The math
- A slab, footing or wall is length × width × thickness. A round column is π × radius² × height.
- Work in feet to get cubic feet, then divide by 27 for cubic yards. A 4" slab is 4 ÷ 12 of a foot thick.
- Bags are rounded up to the next whole bag.
Bags per cubic yard
Bag counts use the typical yield of premixed concrete. Go by the yield printed on the bag you buy.
| Bag | Yield | Bags per cu yd |
|---|---|---|
| 80 lb | 0.6 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.3 cu ft | 90 |
Typing measurements
Every box takes feet, inches and fractions the way you would write them: 9' 1 1/8", 7 1/2", 2.5' or 300mm. Put a space between whole inches and a fraction. A box also takes a whole calculation, so 14' 6" / 2 works anywhere.
Questions
How much concrete do I need for a 10 x 10 slab?
At 4 inches thick, a 10' × 10' slab takes 33.33 cubic feet, which is 1.23 cubic yards. That is 56 bags of the 80 lb size or 75 of the 60 lb size.
How many bags of concrete make a cubic yard?
About 45 bags of the 80 lb size, 60 of the 60 lb size or 90 of the 40 lb size, using typical yields of 0.6, 0.45 and 0.3 cubic feet a bag.
How do I calculate cubic yards of concrete?
Multiply length by width by thickness, all in feet, to get cubic feet, then divide by 27. The calculator does the feet-and-inches conversion for you.
Should I order extra concrete?
Most people do. The ground is rarely perfectly level, forms bow, and some is spilled, so the calculator shows every figure with 10% added alongside the exact amount. Running short in the middle of a pour is the expensive mistake.
How do I work out concrete for a round column or post hole?
Pick Round column and enter the diameter and the height, which for a post hole is its depth. One pier 12 inches across and 4 feet deep takes 3.14 cubic feet.
More calculators
- Feet & inches calculatorAdd, subtract, multiply and divide feet, inches and fractions. Free, nothing to install.
- Decimal feet to feet & inches converterType a length in any one box: decimal feet, feet and inches, inches or millimeters. The other three fill in.
- Roof pitch & rafter length calculatorEnter any two of pitch, rise, run and diagonal. The other two fill in.
- Stair calculatorEnter the floor-to-floor rise to get risers, treads, total run and stringer length.
- Circle & arc calculatorEnter a diameter for circumference and area. Add an angle for arc length and chord.
- Square footage & volume calculatorEnter length and width for area. Add a height for volume, cubic yards and wall area.
- Stud & on-center spacing calculatorEnter a length and a spacing to get the member count, the last bay and the equal spacing.
- Slope converterType a slope as inches per foot, percent, degrees or a ratio. The other three fill in.
- Gravel, sand, soil & mulch calculatorPick the material, type the length, width and depth, and get cubic yards and approximate tons.
- Drawing scale calculatorEnter a scale and a length to convert it, or a drawn length and its real size to find the scale.
Reference finders
- DSA forms & publications finderType a topic or a number, or paste a plan check comment, and see which DSA documents to open.
- OSHPD / HCAI CAN, PIN & forms finderType a topic or a number, or paste a plan review comment, and see which HCAI documents to open.