How to Figure Concrete Weight and Bag Counts for Your Project

Estimate the wet volume your slab or footing needs, then turn that number into bag counts and total weight before you order anything.

Concrete math starts with volume, not weight. Once you know how many cubic feet you need, everything else falls out: bags for small jobs, cubic yards for a truck delivery, and a rough wet weight for planning labor and hauling. Two conversions do most of the work: 27 cubic feet make 1 cubic yard, and normal-weight concrete runs about 150 pounds per cubic foot. Bag yields vary a little by mix, so treat the numbers printed on the bag as the final word.

Step 1: Measure and get volume in cubic feet

Multiply length by width to get area in square feet, then multiply by thickness. Thickness is where people slip: 4 inches is 4 divided by 12, or 0.333 feet. Write every dimension in feet before you multiply anything.

Step 2: Convert to cubic yards if a truck is coming

Divide cubic feet by 27. Ready-mix concrete is sold by the cubic yard, and suppliers usually ask you to round up to a practical amount for the truck.

Step 3: Turn volume into bag counts

Divide your cubic feet by the yield listed on the bag. Common yields are about 0.60 cubic feet for an 80-lb bag, 0.45 for 60 lb, and 0.30 for 40 lb, but check the product label because mixes differ. Round up; a short pour is a real problem.

Step 4: Estimate the wet weight

Multiply cubic feet by about 150 pounds per cubic foot for normal-weight concrete. This is an estimating figure, not a specification, so confirm density with your supplier if the project is engineered.

Worked example 1: A 10 ft x 12 ft patio slab, 4 in thick

Area: 10 × 12 = 120 sq ft. Thickness: 4 ÷ 12 = 0.333 ft. Volume: 120 × 0.333 = about 40 cu ft. Yards: 40 ÷ 27 = 1.48, so order about 1.5 cu yd. Wet weight: 40 × 150 = 6,000 lb, roughly 3 tons.

Worked example 2: Bags for that same slab

Using 80-lb bags with a 0.60 cu ft yield: 40 ÷ 0.60 = 66.7, so buy 67 bags. Dry bag weight: 67 × 80 = 5,360 lb of mix to haul and mix. That is why mixer-and-wheelbarrow weekends exist, and also why trucks exist. With a 10 percent cushion the count becomes 40 × 1.1 ÷ 0.60 = 73.3, so buy 74 bags.

Worked example 3: One round footing, 12 in diameter and 24 in deep

Radius: 12 ÷ 2 = 6 in = 0.5 ft. Area: 3.14 × 0.5 × 0.5 = 0.785 sq ft. Volume: 0.785 × 2 ft = 1.57 cu ft. Bags: 1.57 ÷ 0.60 = 2.6, so 3 bags of 80-lb. Wet weight: 1.57 × 150 = 235.5 lb, about 236 lb.

Common mistakes

• Plugging 4 into the formula for a 4-inch slab instead of 0.333 feet, which overestimates by 12 times.
• Ordering exactly the calculated amount with zero margin, then running short mid-pour.
• Treating dry bag weight as wet concrete weight; they are two different things.
• Assuming every brand and bag size yields the same volume instead of reading the bag.

Footings, slabs that carry structure, and post bases usually have depth, strength, and mix requirements set by local code. Check with your building department before you pour and follow the mix strength printed on the product. Wet concrete is caustic, so wear waterproof gloves and eye protection.