Concrete Calculator — How Many Bags or Yards of Concrete

A concrete calculator turns a few measurements into the one number that matters at the store: how many bags of concrete to buy — or how many cubic yards to order from a ready-mix truck. Buy too little and your pour stops halfway with a cold joint; buy too much and you've paid for bags you'll never open. This tool does the volume math, adds a waste factor, and always rounds up to whole bags, because you can't buy half a bag.

Everything starts with volume in cubic feet: length × width × thickness, with every dimension in feet. The catch is that thickness is almost always given in inches, so convert it first — 4 inches = 4 ÷ 12 = 0.333 ft. A 10 ft × 10 ft slab poured 4 in thick is 10 × 10 × 0.333 = 33.3 cubic feet.

Two numbers come out of that cubic-foot figure:

• Cubic yards for ready-mix delivery: cubic feet ÷ 27. So 33.3 ÷ 27 = 1.23 cubic yards. Trucks sell by the yard, so this is what you quote the dispatcher.

• Bags for a smaller DIY pour. Each bag yields a fixed volume of mixed concrete:

• 40 lb bag → 0.30 cu ft
• 60 lb bag → 0.45 cu ft
• 80 lb bag → 0.60 cu ft

Bags = cubic feet ÷ bag yield, rounded up. That same 33.3 cu ft slab needs ceil(33.3 ÷ 0.60) = 56 bags of 80 lb concrete, or ceil(33.3 ÷ 0.45) = 75 bags of 60 lb. Bigger bags mean fewer trips and less mixing, which is why 80 lb is the contractor default for slabs.

This calculator supports the three shapes you'll actually pour: a slab or footing (length × width × thickness), and a round column or post hole (π × radius² × height, where radius is half the diameter). Enter a quantity to multiply identical pours — four deck footings, six fence-post holes — in one shot. A waste factor (default 10%) covers spillage, over-excavated holes, uneven subgrade and the mix you lose in the wheelbarrow. As a rough rule, about 60 bags of 80 lb concrete makes one cubic yard, so once a job climbs past 40–50 bags it's usually cheaper and faster to order ready-mix by the truck instead of mixing by hand.