Rectangular projects
Slabs and footings
Volume = length × width × depth
Use consistent units. When working in feet, divide thickness in inches by 12 before multiplying.
Concrete Calculator
Estimate ready-mix volume, full bags, waste, and optional cost for slabs, footings, columns, or round fence-post holes.
Choose a project shape and enter measurements below. For fence posts, select Post hole and enter the measured hole diameter, depth, and number of posts.
01 — Project setup
What are you pouring?
02 — Measurements
Enter project dimensions
ft
ft
in
Length and width use feet; slab or footing thickness uses inches.
03 — Site allowance
Waste allowance
Covers spills, uneven subgrade, and ordering margin.
04 — Bag purchasing
Select your bag and check product yield
ft³
Leave yield blank for the standard 0.60 ft³ estimate. If your product label states a different finished yield, enter it here; only the selected bag-size estimate will change. All counts round up to whole bags.
Add optional local prices +
$ / yd³
$ / m³
$ / bag
Your results update as dimensions change.
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Round projects
Fence post holes and columns
Volume = π × radius² × depth × holes
Radius is half the measured hole diameter. For a fence-post concrete estimate, multiply the cylindrical hole volume by the number of posts before adding waste and rounding to whole bags.
Example: 10 × 12 ft slab at 4 inches
Convert 4 inches to 1/3 ft. The measured volume is 10 × 12 × 1/3 = 40 cu ft. A selected 10% allowance gives 44 cu ft, or approximately 1.63 cu yd. These dimensions and allowance are an arithmetic example, not a slab design or supplier order minimum.
Published concrete guides
GuideConcrete Slab Cost: Volume, Bags, Ready-Mix, and Site ScopeCalculate slab volume for common sizes, then separate concrete material from site prep, reinforcement, finishing, delivery, and labor assumptions.Read guide →GuideHow Much Concrete Do I Need per Fence Post?Estimate concrete for fence-post holes with a cylinder formula, bag-yield checks, and links to the fence and concrete calculators.Read guide →
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Concrete questions
Before you order
01How do I calculate concrete for a slab?+
Multiply slab length by width by thickness, using consistent units. For imperial measurements, convert thickness from inches to feet before multiplying. The calculator handles this conversion and adds your selected waste allowance.
02How many bags of concrete do I need?+
Divide the total required cubic feet by the approximate yield of one bag, then round up. Typical yields are about 0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb, and 0.60 ft³ for 80 lb bags. Always confirm the package yield.
03Can I use this as a fence post concrete calculator?+
Yes. Choose Post hole, enter the round hole diameter, hole depth, and number of fence posts. The calculator applies the cylinder formula to all holes, adds the selected waste allowance, and converts the result into whole bag estimates using the bag yield you choose.
04How much concrete do I need per fence post?+
For a round fence-post hole, the concrete volume is pi times radius squared times hole depth. The quantity per post therefore depends on the actual hole diameter and depth, not only the post size. Enter one hole and one post to see the per-post amount, or enter the full post count for the project total.
05How do I convert cubic feet to cubic yards?+
Divide cubic feet by 27 because one cubic yard contains 27 cubic feet. For example, 54 cubic feet equals 2 cubic yards.
06How much waste should I add for concrete?+
Ten percent is a practical planning allowance for many small pours. A well-formed, level project may need less, while uneven excavation, spillage, or uncertain dimensions may justify 15 percent.
07How thick should a concrete slab be?+
Thickness depends on loading, reinforcement, base preparation, soil, and local code. Four inches is common for many residential walkways and patios, but vehicle slabs or structural work may require a different engineered specification.
08What units does this concrete calculator support?+
Use feet with inches for imperial projects or meters with centimeters for metric projects. Results are shown together in cubic feet, cubic yards, and cubic meters.