Concrete Calculator
Ordering too little concrete stops a pour halfway; ordering too much wastes money you cannot get back. This free concrete calculator works as a concrete slab calculator, a concrete bag calculator and a cubic yards of concrete calculator in one: enter the length, width and thickness of your slab, add a waste allowance, and it tells you the exact volume in cubic feet and cubic yards plus how many 80 lb, 60 lb or 40 lb bags you need if you are mixing it yourself. It is built for patios, driveways, shed bases, fence posts and sidewalks — any flat pour where getting the amount right matters.
Frequently asked questions
How many bags of concrete do I need for a slab?
First work out the slab volume in cubic feet (length × width × thickness in feet), then divide by the yield of one bag: an 80 lb bag makes about 0.6 cubic feet of concrete, a 60 lb bag about 0.45 cubic feet and a 40 lb bag about 0.3 cubic feet. For example, a 10 × 10 ft slab at 4 inches thick is 33.3 cubic feet, so with 10% waste you need about 62 × 80 lb bags — which is why ready-mix delivery usually makes more sense for slabs that size. This concrete bag calculator does that maths for all three bag sizes at once.
How do I calculate cubic yards of concrete?
Measure the length and width in feet and the thickness in inches. Multiply length × width × (thickness ÷ 12) to get cubic feet, then divide by 27 to convert to cubic yards. A 10 × 10 ft slab at 4 inches thick is 100 × 0.333 = 33.3 cubic feet, which is 33.3 ÷ 27 = 1.23 cubic yards before waste. Ready-mix is sold by the cubic yard, so always round your order up to the nearest quarter or half yard your supplier offers.
Should I add extra concrete for waste?
Yes — adding about 10% extra is standard practice. Spillage, uneven ground under the slab, forms that bow slightly and material left in the wheelbarrow or truck all eat into your pour, and running short mid-pour leaves a weak cold joint. This calculator applies your waste percentage (10% by default, adjustable) to the volume before working out both the cubic yards to order and the bag counts, so the numbers you see already include that safety margin.
Related tools: planning the finish as well as the pour? Estimate wall coverage with our paint calculator, or check the margin on the job with the profit margin calculator.