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Always add 5–10% for waste, spillage, and uneven subgrade. Ready-mix is typically ordered by the cubic yard; bagged concrete is calculated in cubic feet, where one 60 lb bag yields about 0.45 cubic feet.
Loose material compacts down once it's spread and rolled or tamped. If you need a 4" compacted base, you typically need to order 15–20% more material by volume than the final depth, since compaction reduces the loose volume.
Soil in the ground (bank measure) takes up less space than the same soil once it's dug up and loosened (swelled measure). This is why the dirt from a hole never fits back into the hole it came from — always estimate hauling based on the swelled volume, not the bank volume.
Rebar is typically placed in a grid pattern at a set spacing (commonly 12" or 18" on-center for slabs) running in both directions. Total linear footage combines both directions, and a waste allowance accounts for overlaps at splices and trimming at edges.
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