What a 1:2:3 mix actually means
One part cement, two parts sand, three parts gravel by volume, with water at roughly half the cement weight, produces a general-purpose 3,000-psi-class concrete. The calculator scales those parts to your volume: per cubic yard expect about 6–7 bags of Portland cement, roughly a half-yard of sand and three-quarters of a yard of gravel.
When to use this instead of bagged mix
Site-mixing wins on price past about half a cubic yard and on remote sites where hauling 60 bags is impractical. It demands a real mixer (or a very strong friend), consistent measuring buckets, and quick placement — mixed concrete waits for no one. Structural elements like footings under beams should stick to engineered or bagged mixes with known strength.