Field calculator
Concrete bags calculator (bags from volume)
For a small pour, the question is how many bags to buy, and the answer is the volume divided by what one bag yields. Bags needed equals the concrete volume in cubic feet divided by the yield per bag, plus a little waste for spillage and overpack. Enter the volume, the bag yield (the cubic feet of mixed concrete one bag makes), and a waste percentage. Typical yields are about 0.60 cubic feet for an 80 pound bag, 0.45 for a 60 pound bag, and 0.375 for a 50 pound bag, but the figure is printed on the bag and varies by mix, so use the real number. To get the volume, multiply length by width by thickness in feet, remembering a 4 inch slab is 0.333 feet thick and a 6 inch slab is 0.5 feet. Bagged mix is the right call for posts, equipment pads, footings, and repairs; once the job passes roughly one cubic yard (27 cubic feet) ready-mix delivery is usually cheaper and far less labor.
Result
Concrete bag count: bags = volume needed (cubic feet) / the yield per bag, plus a waste allowance. Enter the volume, the bag yield (cubic feet of mixed concrete per bag), and a waste percentage. Common yields are about 0.60 cu ft for an 80 lb bag, 0.45 for 60 lb, and 0.375 for 50 lb, but confirm the figure printed on the bag since it varies by mix. To find the volume, multiply length × width × thickness in feet (a 4 in slab is 0.333 ft thick). Bagged mix suits small jobs like posts, pads, and repairs; once you pass roughly one cubic yard (27 cu ft) ready-mix is usually cheaper and faster.
Worked example
A small pad needs 6 cu ft of concrete. How many 80-lb bags (0.6 cu ft yield each)?
- Volume needed6 cu ft
- Bag yield0.6 cu ft (80 lb)
- Waste5%
- Base bags = volume ÷ yield = 6 ÷ 0.6 = 10 bags.
- Add 5% waste: 10 × 1.05 = 10.5, round up to 11 bags.
11 × 80-lb bags. Each 80-lb bag yields ~0.6 cu ft; 60-lb bags yield ~0.45, so you would need more of those.
anvilfield.com/calculators/concrete-bags-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
More Concrete calculators
Comparing your options?
Concrete bags FAQ
How thick should a slab on grade be?
Slab on grade thickness comes from the load, the subgrade stiffness, and the concrete's flexural strength, not a single number. Residential floors are commonly 4 in, light commercial around 5 in, and industrial floors carrying lift trucks 6 in or more. The structural engineer sizes it to the real loads and the project specification.
Does a slab on grade need rebar?
Not always. A slab on grade gets its strength from thickness and the subgrade, not steel, so plain jointed concrete can carry load fine. Reinforcement does not prevent cracks, it holds them tight after they form. Whether you need it, and what kind, depends on the joint strategy and the engineer's design.
Do you need a vapor barrier under a slab?
Yes for any floor getting a coating or a moisture-sensitive covering, or in a conditioned space, where ground moisture would ruin the finish. Use a sheet that meets ASTM E1745, not ordinary poly. ACI 302 places it directly under the slab on the base. Exterior or uncovered slabs often skip it.
What is the modulus of subgrade reaction?
The modulus of subgrade reaction, k, is the stiffness of the soil under a slab, measured as pressure per unit of deflection in pounds per cubic inch (pci). It ranges from about 50 pci for soft soil to 400 pci or more for compacted granular support. Uniform support matters more than raw stiffness.
Why does my slab on grade keep cracking?
Most slab cracks come from a soft or non-uniform subgrade, shrinkage with joints spaced too far apart or cut too late, or curling, not from weak concrete. Steel laid on the ground controls nothing. Check the base, the joint spacing and timing, and the mix before you blame the concrete strength.