Concrete calculator

How to use the concrete calculator

Estimate the concrete volume for a slab, rectangular footing, or round column. Enter the dimensions, number of identical pours, and waste percentage to see cubic yards, cubic feet, and pre-mix bags.

1. Choose Feet / inches or Metres / cm.
2. Select Slab, Footing, or Column.
3. Enter the dimensions shown for that shape.
4. Enter the Number of pours and Waste %.
5. Review the concrete volume and pre-mix bag count.

What to enter

Unit system

Choose Feet / inches for US customary measurements or Metres / cm for metric measurements. The default is Feet / inches.

Shape

Choose Slab, Footing, or Column. The default is Slab.

Length

Enter the slab or footing length in feet, or metres in metric mode. The default is 10 ft.

plus inches

Add inches to Length in imperial mode, or centimetres in metric mode. The default is 0.

Width

For a slab, enter the width in feet or metres. The default is 10 ft. This field is hidden for footings and columns.

plus inches

Add inches to Width in imperial mode, or centimetres in metric mode. The default is 0. This field is hidden for footings and columns.

Thickness

For a slab, enter thickness in inches or centimetres in metric mode. The default is 4 in.

Footing width

For a footing, enter the rectangular footing width in inches or centimetres in metric mode. The default is 16 in.

Footing depth

For a footing, enter the rectangular footing depth in inches or centimetres in metric mode. The default is 8 in.

Diameter

For a column, enter the round column diameter in inches or centimetres in metric mode. The default is 12 in.

Column height

For a column, enter the height in feet or metres in metric mode. The default is 4 ft.

Number of pours

Enter the number of identical slabs, footings, or columns. The default is 1, and the value must be a whole number of at least 1.

Waste %

Add allowance for over-excavation and spillage. The default is 10%, and the allowed range is 0% to 50%.

Example: 12 ft × 10 ft slab at 4 in thick

Choose Slab, enter Length 12 ft, Width 10 ft, Thickness 4 in, Number of pours 1, and Waste 10%. Volume per pour is 12 × 10 = 120 sq ft, then 120 × (4 ÷ 12) = 40 cu ft. Net volume is 40 cu ft. Applying waste gives 40 × 1.10 = 44 cu ft. Cubic yards are 44 ÷ 27 = 1.6296, displayed as 1.63 cu yd. With the default 80 lb bag yield of 0.60 cu ft, 44 ÷ 0.60 = 73.333 bags, which rounds up to 74 bags.

How the concrete estimate is calculated

The calculator converts imperial or metric dimensions to internal feet and inches. For slabs, it calculates length × width × thickness in feet. For rectangular footings, it calculates length × footing width × footing depth in feet. For round columns, it calculates π × (diameter ÷ 24)² × height, where diameter is in inches and height is in feet. The result is multiplied by Number of pours and by 1 + Waste % ÷ 100. Cubic yards equal the total cubic feet divided by 27. Bag counts use the fixed default yield for 80 lb pre-mix bags: 0.60 cu ft per bag. The calculator rounds displayed measurements and volumes to two decimals and rounds the bag count up to a whole bag.

Reading the results

The main volume result shows concrete to order with waste: cubic yards in imperial mode or cubic metres in metric mode. The breakdown also shows net volume, cubic feet, cubic yards, cubic metres, exact bags before rounding, and the whole pre-mix bag count. Bag counts are rounded up because partial bags are not counted as complete bags.

Limitations and common mistakes

• The interface uses the default 80 lb pre-mix bag yield of 0.60 cu ft; it does not provide a bag-size selector.
• The estimate covers slab, rectangular footing, and cylindrical column geometry only. Irregular forms require separate calculations.
• The waste percentage is an allowance, not a measurement of actual over-excavation or spillage.
• Ready-mix ordering and bag suitability depend on the project and supplier. The tool's assumption recommends ordering ready-mix by the cubic yard above about 1 cu yd.

Frequently asked questions