Concrete Calculator
Calculate concrete volume and bag count for slabs, footings, columns, and walls. Your work stays on this device whenever possible.
How to use Concrete Calculator
- Choose your pour shape: Slab / Footing, Circular Slab, Round Column / Tube, Wall, or Stairs.
- Enter dimensions and select length/width units (feet, inches, meters, yards) and thickness units (in, cm, mm, ft, m).
- Select a Waste Allowance (0% exact geometric volume, or 5% to 20% for subgrade unevenness and spillage).
- Select a Bag Size (40 lb, 60 lb, 80 lb with label yields, or Metric bag with custom m³ yield), and optionally enter ready-mix and bag prices.
- Press "Calculate Concrete Needed" for instant volume and cost breakdowns, or use the Multi-Pour Schedule below to combine multiple pours and export to CSV.
Worked example
A 10 ft by 10 ft patio slab at 4 inches thickness equals 33.33 cubic feet (1.235 cubic yards, 0.944 cubic meters), needing 56 bags of 80 lb. With a 10% waste allowance, volume becomes 36.67 cu ft (1.358 cu yd), needing 62 bags.
When to use this tool
- Deciding between bagged concrete and a ready-mix truck delivery for home slabs, posts, and patios.
- Planning a multi-pour job combining footings, retaining walls, and steps into an exportable CSV schedule.
Important limitations
Bag-count estimates use nominal label yields (0.30 cu ft for 40lb, 0.45 cu ft for 60lb, 0.60 cu ft for 80lb, or custom m³ yield for metric bags). Waste allowance is configurable from 0% to 20% for uneven subgrade and spillage.
Frequently asked questions
How many bags come out of one cubic yard?
One cubic yard is 27 cubic feet. At label yields, one cubic yard requires 45 bags of 80 lb (0.60 cu ft each), 60 bags of 60 lb (0.45 cu ft each), or 90 bags of 40 lb (0.30 cu ft each) before waste allowance.
Does the calculation include waste allowance?
You can select your exact waste factor allowance from 0% (exact geometric net volume) up to 20%. The tool displays both the exact net volume and total volume with waste, and calculates bag requirements based on the waste allowance.
What thickness units are supported?
Thickness supports inches (in), centimeters (cm), millimeters (mm), feet (ft), and meters (m), automatically converting to your chosen system so 100 mm or 10 cm accurately evaluates to the same volume as 3.937 inches.
How are metric bags calculated?
Because bagged concrete yields vary by cement density and manufacturer, selecting 'Metric bag' lets you enter your bag's exact stated label yield in cubic meters (e.g. 0.0115 m³ for standard 25 kg bags) without relying on invented conversions.
Can I calculate multiple pours together?
Yes. The Multi-Pour Project Schedule allows you to add multiple slabs, round tubes with quantities, walls, and steps to a single consolidated estimate with grand totals and one-click CSV export.