How this calculator works
Each form is a cylinder. Sonoco publishes nominal sizes and a concrete-volume chart; use actual inside dimensions when available. The default bag yield is approximately 14 L from a cited TARGET 30 kg concrete product, not bag mass divided by finished density. Site-batch mode splits a user-specified loose dry volume by a volume ratio; the default factor is an illustrative assumption, not a standardized yield or guaranteed strength.
Formula
Vnet = pi x inside diameter^2 x fill height x columns / 4; Vorder = Vnet x (1 + waste); bags = ceil(Vorder / printed mixed yield). Site ingredient volume = Vorder x specified dry-volume factor x ingredient parts / total parts.
Worked example
A 200 mm inside-diameter column filled 1 m high contains 0.0314159 m3 concrete. With no waste and 0.014 m3 mixed yield per bag, buy three bags.
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Frequently asked questions
Does bag weight determine concrete yield?
No. Use the printed mixed yield for the exact product. Different concrete mixes with the same bag mass can have different yields.
Do the site-batch ratios guarantee a strength grade?
No. Water-cement ratio, aggregate grading, moisture, cement and curing affect strength. Use an approved mix design and measured trial yield for structural concrete.
Does the form diameter size a footing?
No. This only calculates fill quantities for a chosen cylinder. Bearing capacity, frost depth and structural design determine the footing dimensions.