Concrete Calculator (Bags and Cubic Yards)
Enter the size of a slab, pad or footing, or round post holes, to estimate the concrete volume and the number of premixed bags.
How it works
Slab volume (cu ft) = length (ft) × width (ft) × thickness (in) ÷ 12. Round hole volume = π × (diameter ÷ 2)² × depth. Cubic yards = cubic feet ÷ 27. Bags = cubic feet with allowance ÷ bag yield, rounded up. Metric inputs are converted with 1 ft = 0.3048 m (NIST); 1 cu yd = 0.7646 m³.
Worked examples
- A 10 × 10 ft slab 4 in thick is 10 × 10 × 4 ÷ 12 = 33.3 cu ft = 1.23 cu yd. With a 10% allowance that is 36.7 cu ft, or 62 bags of 80 lb mix (36.7 ÷ 0.60 = 61.1, rounded up).
- Post holes: 8 holes, each 10 in across and 36 in (3 ft) deep. One hole is π × (10 ÷ 24)² × 3 = 1.64 cu ft, so 8 holes are 13.1 cu ft (0.48 cu yd). With 10% that is 14.4 cu ft, or 24 bags of 80 lb mix at 0.60 cu ft each. Any post volume is not subtracted, so this is on the safe side.
- Form depth: a 4 × 3 ft landing formed with 2 × 4 boards is 3½ in deep, not 4 in (PS 20: a dry 2 × 4 is 1½ × 3½ in). At 3½ in it is 3.5 cu ft, 6 bags of 80 lb; entered as 4 in it would be 4.0 cu ft, 7 bags.
How to measure
- Measure the inside of the forms, not the outside of the boards.
- Measure the length and width in two or three places. If they differ, use the largest figure.
- Measure the depth from the top of the forms down to the compacted base, in several spots. Low spots in the base take extra concrete.
- For round holes, measure the width across the top and the depth at the center. Holes dug wider at the bottom take more.
Common mistakes
- Using the nominal lumber size for form depth: a 2 × 4 form is 3½ in deep and a 2 × 6 is 5½ in (PS 20).
- Leaving depth in inches in a feet formula. 4 in is 4 ÷ 12 = 0.333 ft, not 4 ft. This calculator converts it for you.
- Ordering exactly the calculated volume. Uneven base, form bulging and spillage use more, so add an allowance.
- Assuming every bag yields the same. Yield depends on the product and bag size; QUIKRETE lists 0.60 cu ft for an 80 lb bag of its Concrete Mix. Use the yield on your bag.
- Treating this volume as a design. Thickness, reinforcement and base are set by the project and local codes; check local codes.
Assumptions
- Bag yields are QUIKRETE's published figures for its Concrete Mix (e.g. 80 lb ≈ 0.60 cu ft). Other brands and products (fast-setting, high-strength) can yield differently; check your bag.
- Flat, uniform thickness and straight sides. Uneven ground, sloped sites and form bulging use more concrete.
- Holes are perfect cylinders; any post volume is not subtracted.
- The waste allowance is an editable JobsiteCalc default, not an industry standard. Ask your supplier what extra they suggest for your job.
- This is a quantity estimate only. It does not cover structural design, thickness, reinforcement or permits; check local codes and product instructions.
Frequently asked questions
How many 80 lb bags of concrete make a cubic yard?
About 45. QUIKRETE says an 80 lb bag of its Concrete Mix yields approximately 0.60 cubic feet, and a cubic yard is 27 cubic feet (27 ÷ 0.60 = 45).
How many 60 lb bags are in a yard of concrete?
About 60. QUIKRETE lists 0.45 cubic feet per 60 lb bag, and 27 ÷ 0.45 = 60.
How do I convert cubic feet to cubic yards?
Divide by 27, because a cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet.
Related calculators
- Concrete slab calculator – adds thickened edges and a ready-mix cost estimate
- Cubic yards calculator – converts any volume to yards, feet and meters
- Gravel calculator – for the stone base under a slab
- Concrete block calculator – for block walls instead of poured walls
- Asphalt calculator – tons of hot mix for a driveway or patch instead of concrete
- Square footage calculator – to work out the area of odd shapes first
Sources
Formulas and reference values are from these sources. Page last reviewed 2026-10-19.