Loose sand and aggregate have air gaps between the particles — mixing needs roughly 1.54 times the dry volume of materials to produce one unit volume of finished, settled concrete.
Skipping this dry-volume correction is the single most common reason people under-order concrete materials — the raw 1:2:4 ratio by itself doesn’t account for how much the loose ingredients compact once mixed and cured.
Estimate with the rule, then check it against the calculator that models it properly.
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