Concrete calculator: volume and bags

Quick answer

An 80 lb bag of Quikrete concrete mix makes 0.60 cu ft, so one cubic yard (27 cu ft) takes 45 bags before any allowance. In Australia, Boral states 108 bags of 20 kg for one cubic metre. A 10 ft × 10 ft slab 4 in thick needs 1.23 cubic yards, or 62 bags of 80 lb with 10 percent extra.

What this calculator does

It calculates the volume of four common shapes: a rectangular slab, a strip footing, a round column such as a Sonotube form or a post hole, and a flight of solid steps. It adds an allowance for uneven ground and spillage and converts the volume into bags of dry concrete mix, with the yield of common bag sizes filled in.

For about one cubic yard (0.76 m³) or more, a note below the result shows how much dry mix the bags weigh and suggests asking a ready-mix supplier for a price too. That threshold is a rule of thumb of this site, not a rule of suppliers.

It calculates quantity only. The thickness of a slab, the size of a footing, the reinforcement and the strength of the concrete depend on the load, the soil and your building code.

How to use it

1. Choose metric or US units.
2. Choose the shape.
3. Slab: enter length, width and thickness. Strip footing: enter the total length of the trench, its width and its depth. Round column: choose a Sonotube size or Other, then the height or depth. Steps: enter the number of steps, their width, the height of each step and the depth of each tread.
4. Enter how many identical pieces you pour, for example the number of columns.
5. Choose the bag. This fills in the yield stated by a manufacturer, which you can change to the yield printed on your bag.
6. Set the allowance.
7. Select Calculate.

For fence posts, use the fence post concrete calculator: it subtracts the post and a gravel base. This calculator fills the whole hole.

In US units you can type fractions and feet and inches, such as 3 1/2 or 4' 6".

How the result is calculated

• Slab: volume = length × width × thickness.
• Strip footing: volume = length × width × depth.
• Round column: volume = π × (diameter ÷ 2)² × height, with π = 3.14159.
• Steps: volume = width × tread depth × step height × n × (n + 1) ÷ 2 for n steps, plus width × extra top depth × n × step height. The steps are treated as solid concrete on the ground.

1. Volume = volume of one piece × number of pieces.
2. Concrete to order = volume × (1 + allowance ÷ 100), rounded up to 0.001 m³ or 0.01 cubic yard.
3. Bags = concrete to order ÷ yield of one bag, rounded up to a whole bag.

Thickness, width, depth and diameter are entered in millimetres or inches and converted before the multiplication. One cubic metre is 1000 litres. One cubic yard is 27 cubic feet.

Worked example

A slab is 3 m long, 3 m wide and 100 mm thick. Volume: 3 × 3 × 0.1 = 0.9 m³. With 10 percent allowance: 0.9 × 1.1 = 0.99 m³.

A 20 kg bag with a yield of 9.3 L makes 0.0093 m³. Bags: 0.99 ÷ 0.0093 = 106.5, rounded up to 107 bags. That is more than two tonnes of dry mix to carry and mix, so compare the bags with the price of ready-mixed concrete.

In US units: a slab of 10 ft by 10 ft and 4 in thick has 10 × 10 × (4 ÷ 12) = 33.3 cu ft, which is 1.23 cubic yards. With 10 percent allowance that is 36.7 cu ft or 1.36 cubic yards. With 80 lb bags that yield 0.6 cu ft each: 36.7 ÷ 0.6 = 61.1, rounded up to 62 bags.

A post hole of 300 mm diameter and 1.2 m depth has 3.14159 × 0.15² × 1.2 = 0.085 m³. With 10 percent allowance that is 0.094 m³, or 11 bags of 9.3 L.

Three solid steps 1.2 m wide, each 175 mm high with 280 mm treads: 1.2 × 0.28 × 0.175 × 3 × 4 ÷ 2 = 0.353 m³, with 10 percent allowance 0.389 m³, or 42 bags of 9.3 L. The lowest step is 1 step high, the middle one 2 and the top one 3, which is where the 3 × 4 ÷ 2 = 6 step blocks come from.

Bags per Sonotube

The table shows the number of 80 lb bags for one round tube, filled to the depth shown, without allowance. It uses the Quikrete yield of 0.60 cu ft per bag and the same formula as the calculator. Add your allowance, or use the calculator with your own bag.

Sonoco lists its Sonotube round forms from 6 to 36 in in diameter; its Builder's Tube line comes in 6, 8, 10 and 12 in, in lengths of 4, 8 and 12 ft. The volume per foot in the table agrees with the figures Sonoco prints for the Builder's Tube sizes.

About the example values

The yield of a bag depends on its weight and on the mix. The bag presets come from the data sheets of manufacturers.

The metric example of 9.3 L is 1000 L divided by the 108 bags of 20 kg that Boral states for one cubic metre. The US example of 0.6 cu ft is the Quikrete value for an 80 lb bag. The rapid-set 20 kg preset uses the 10 L stated by Easy Mix. Use the yield on the bag you buy.

The allowance of 10 percent is an example. Ground that is not level, formwork that bows and concrete left in the mixer all take material.

Common mistakes

• Entering the thickness in the wrong unit. A slab of 100 mm is 0.1 m, and a slab of 4 in is one third of a foot. The calculator expects millimetres or inches for thickness.
• Measuring the thickness at the edge only. If the ground is lower in the middle, the slab is thicker there.
• Using the weight of the bag instead of its yield.
• Ordering the exact volume of ready-mixed concrete without any allowance.
• Assuming that any thickness is sufficient. Driveways, footings and structural slabs have to follow the building code.