Enter the length, width and thickness of your slab, a waste allowance and the volume one bag makes. The calculator shows the concrete you need in cubic yards or cubic metres, and how many bags that is. It is designed for flat rectangular slabs such as patios and pads.
This calculator needs JavaScript. The formula and a worked example are below.
How to use the concrete calculator
Measure the area to be poured and decide on the thickness. A 4 inch slab is common for walkways and patios, but heavier loads need more, so confirm the thickness with your plans or a professional.
Look at the bag for its yield. An 80 lb bag is commonly about 0.6 cu ft, but brands differ, so enter the figure from the bag you plan to buy.
The formula
Volume (cu ft) = length × width × thickness in inches ÷ 12. Add waste: volume × (1 + waste). Cubic yards = cubic feet ÷ 27. Bags = volume with waste ÷ bag yield, rounded up.
Worked example
A 10 ft by 10 ft slab is 4 inches thick. The volume is 100 × 4 ÷ 12 = 33.3 cu ft, or 1.23 cu yd. A 10% waste allowance makes it 36.7 cu ft, or 1.36 cu yd. With 0.6 cu ft bags that is 62 bags.
Bags or ready-mix?
Mixing dozens of bags by hand is hard work and the quality can vary between batches. For a pour of this size, get a ready-mix quote and compare it with the cost of the bags. Ready-mix is sold by the cubic yard, so use the figure above.
What this does not include
Gravel base, reinforcement, forms and finishing are separate. Sloped ground and thickened edges also add volume, so add extra when the sub-grade is not flat.
Frequently asked questions
How much waste should I add?
Ten percent is a common starting point because ground is rarely perfectly level and some concrete is spilled. Uneven ground may need more.
How many 80 lb bags make a cubic yard?
At 0.6 cu ft per bag, a cubic yard (27 cu ft) needs 45 bags. Check the yield printed on your bags.
Can I use this for footings or columns?
The volume of any rectangular shape works, but round columns need a different formula.
Related guides
Read aggregate concrete installation for finishes and installation. Landscaping around the slab? Try the mulch calculator.