How much concrete to buy?
Pick the shape, enter its size and how many you need. See the concrete to order in cubic yards, cubic feet and cubic metres, and how many premix bags that is.
- Concrete
- 1.3 yd³
You need 1.3 yd³ of concrete, or 59 bags.
- Cubic feet
- 35 ft³
- Cubic metres
- 0.991 m³
- Premix bags
- 59
- Weight of the concrete
- 4,900 lb
Concrete: 1.3 yd³. You need 1.3 yd³ of concrete, or 59 bags.
How to calculate
Computes the volume of concrete for a slab, a round column or a tube, with extra for waste, in cubic yards, cubic feet and cubic metres, and the number of premix bags.
Example with the default inputs (Shape Slab, footing or wall, Length 10 ft, Width 10 ft, Thickness 4 in, How many 1, Extra for waste 5%, Bag size 80 lb bag): You need 1.3 yd³ of concrete, or 59 bags.
Method: volume = shape volume × how many × (1 + waste ÷ 100); bags = ⌈volume ÷ bag yield⌉; cubic yards = cubic feet ÷ 27
- Slab: length × width × thickness. Round column: π × diameter² ÷ 4 × height. Tube: π × (outer² − inner²) ÷ 4 × height.
- Bag yields are from the QUIKRETE Concrete Mix data sheet: 40 lb 0.30 ft³, 50 lb 0.375 ft³, 60 lb 0.45 ft³, 80 lb 0.60 ft³, 90 lb 0.675 ft³.
- The weight uses the data sheet’s unit weight of mixed concrete, about 140 lb/ft³.
Worked examples
Each example is checked against the calculator on every build.
- Shape Slab, footing or wall, Length 10 ft, Width 10 ft, Thickness 4 in, How many 1, Extra for waste 0%, Bag size 80 lb bag gives Cubic feet 33.33 ft³, Premix bags 56, Weight of the concrete 4,667 lb.Source: hand calculation in content.mdx: 10 × 10 × 4/12 = 33.33 ft³ = 1.23 yd³; 33.33 ÷ 0.60 = 55.6, so 56 bags of 80 lb (QUIKRETE yield)
- Shape Slab, footing or wall, Length 12 ft, Width 20 ft, Thickness 6 in, How many 1, Extra for waste 10%, Bag size 60 lb bag gives Concrete 4.89 yd³, Premix bags 294.Source: hand calculation in content.mdx: 12 × 20 × 0.5 = 120 ft³; × 1.10 = 132 ft³ = 4.89 yd³; 132 ÷ 0.45 = 293.3, so 294 bags
- Shape Round column or post hole, Diameter 10 in, Height or depth 3 ft, How many 4, Extra for waste 0%, Bag size 50 lb bag gives Cubic feet 6.54 ft³, Premix bags 18.Source: hand calculation in content.mdx: π × (10/12)² ÷ 4 × 3 = 1.636 ft³ a hole; × 4 = 6.545 ft³; ÷ 0.375 = 17.5, so 18 bags
- Shape Round tube (hollow column), Diameter 24 in, Inner diameter 12 in, Height or depth 4 ft, How many 1, Extra for waste 0%, Bag size 80 lb bag gives Cubic feet 9.42 ft³, Premix bags 16.Source: hand calculation in content.mdx: π × (2² − 1²) ÷ 4 × 4 = 3π = 9.425 ft³; ÷ 0.60 = 15.7, so 16 bags
How it works
- Volume of one pour:
- Slab, footing or wall: length × width × thickness.
- Round column or post hole: π × diameter² ÷ 4 × height.
- Round tube (a hollow column): π × (outer diameter² − inner diameter²) ÷ 4 × height. The inner diameter must be less than the outer diameter.
- Volume to order = volume of one pour × how many × (1 + extra for waste ÷ 100).
- Cubic yards = cubic feet ÷ 27. Cubic metres use 1 ft = 0.3048 m exactly.
- Bags = volume to order ÷ the yield of one bag, rounded up to a whole bag. A count within 0.000000001 of a whole number counts as that whole number, so an exact fit is not rounded up. Each size can be up to 1,000 ft, and how many up to 10,000.
- Weight = volume to order × 140 lb/ft³.
Bag yields
These are the approximate yields of QUIKRETE Concrete Mix from its data sheet. Other brands of premix with the same bag weight are similar, but check the bag.
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
| 90 lb | 0.675 ft³ | 40 |
The volume is the volume of the form. It does not allow for a base that is lower than planned; the waste percent covers that.
Worked examples by hand
A 10 ft × 10 ft slab, 4 in thick, no extra, 80 lb bags. 4 in = 4/12 ft. 10 × 10 × 4/12 = 33.33 ft³, which is 33.33 ÷ 27 = 1.23 yd³. 33.33 ÷ 0.60 = 55.6, so 56 bags. It weighs 33.33 × 140 = 4,667 lb.
A 12 ft × 20 ft slab, 6 in thick, 10% extra, 60 lb bags. 12 × 20 × 0.5 = 120 ft³, and 120 × 1.10 = 132 ft³ = 132 ÷ 27 = 4.89 yd³. 132 ÷ 0.45 = 293.3, so 294 bags.
Four post holes, 10 in wide and 3 ft deep, 50 lb bags. One hole: π × (10/12)² ÷ 4 × 3 = 1.636 ft³. Four holes: 6.545 ft³. 6.545 ÷ 0.375 = 17.5, so 18 bags.
A tube 24 in across with a 12 in hollow, 4 ft tall, 80 lb bags. 24 in = 2 ft and 12 in = 1 ft. π × (2² − 1²) ÷ 4 × 4 = 3π = 9.425 ft³. 9.425 ÷ 0.60 = 15.7, so 16 bags.
Other questions people ask
How do I calculate concrete for a slab?
Multiply length × width × thickness, with every size in feet, to get cubic feet. Divide by 27 for cubic yards. A 10 ft × 10 ft slab 4 in thick is 10 × 10 × 4/12 = 33.33 ft³, or 1.23 yd³.
How many bags of concrete make a cubic yard?
A cubic yard is 27 cubic feet. An 80 lb bag of QUIKRETE Concrete Mix makes about 0.60 ft³, so a cubic yard takes 27 ÷ 0.60 = 45 bags. With 60 lb bags (0.45 ft³) it takes 60 bags.
How much extra concrete should I order?
The calculator adds the percent you type in Extra for waste. Order a little more than the exact volume: spills, a base that is lower in places and forms that bow all use concrete, and running short part way through a pour is worse than a little left over.
How much concrete does a post hole need?
Use the round column shape with the hole’s diameter and depth. A hole 10 in wide and 3 ft deep holds π × (10/12)² ÷ 4 × 3 = 1.64 ft³. The post itself takes up some of that space, so the answer is a little more than you need.
How much does a cubic yard of concrete weigh?
QUIKRETE gives the unit weight of its mixed Concrete Mix as about 140 lb/ft³. A cubic yard is 27 ft³, so it weighs about 27 × 140 = 3,780 lb.
Should I buy bags or ready-mix?
Bags suit small jobs such as posts, steps and pads you can mix by hand or in a small mixer. For a large slab the bag count grows fast: 2 cubic yards is 90 bags of 80 lb, so many people order ready-mixed concrete by the cubic yard instead.