Concrete Calculator

About Concrete Calculator

Concrete Calculator computes the volume of concrete needed for a slab or pour based on length, width, and depth. Results are shown simultaneously in metric (m³, 25 kg and 40 kg bags) and imperial (cubic yards, cubic feet, 60 lb and 80 lb bags). A configurable waste percentage is added to the volume before calculating bag counts, defaulting to 10%. The tool is useful for any size of concrete pour, from a small garden path to a full driveway or foundation slab. Bag counts for both 25 kg and 40 kg metric bags and 60 lb and 80 lb imperial bags are shown together, making it easy to compare options at your local supplier.

How to use Concrete Calculator

  1. Choose metric or imperial input
  2. Enter length, width, and depth of your slab
  3. See volume and bag count in both metric and imperial instantly

Frequently Asked Questions

How is the volume calculated?
Volume equals length times width times depth. For metric input, depth is entered in centimetres and converted to metres. For imperial, depth is in inches and converted to feet, then to cubic yards and metres.
Why is waste added?
Concrete pours always require a buffer for spillage, uneven forms, and mixing inefficiency. The default 10% waste factor means if your slab needs 1 m³, the calculator recommends buying enough for 1.1 m³. You can adjust the waste percentage to 0 or higher.
How many bags equal one cubic metre?
A 25 kg bag covers approximately 0.012 m³ and a 40 kg bag covers approximately 0.020 m³. These are standard values for pre-mixed concrete. Actual coverage can vary slightly by brand.
How many bags of concrete do I need for a 3m x 3m slab?
A 3 m x 3 m slab at 10 cm depth needs 0.9 m³ of concrete, plus 10% waste = 0.99 m³. That is approximately 83 bags of 25 kg concrete mix. Enter the exact dimensions to get precise bag counts for your project.
What depth should a concrete slab be?
A standard residential patio or path slab is 10 cm (4 inches) thick. A driveway bearing vehicle weight should be at least 15 cm (6 inches). A structural foundation typically requires 20 cm (8 inches) or more depending on load.
Is Concrete Calculator free and private?
Yes. Concrete Calculator is completely free with no account required. All calculations run in your browser and nothing is uploaded or stored.
Does Concrete Calculator work on mobile?
Yes. Concrete Calculator is fully responsive and works in all modern browsers on desktop, mobile, and tablet.

Related Tools