Concrete and masonry calculators

Every calculation on this page ends the same way: a volume, then that volume divided by whatever one bag makes. Concrete is unusual among building materials in being sold two entirely different ways — by the cubic yard from a truck, and by dry weight in a sack — so the number you measure is almost never the number you buy.

Which tool you want depends on the shape you are filling. A flat pour of even depth is the slab calculator. A cylinder with a timber standing in the middle of it is the post hole calculator, and the post has to come out of the hole before you order. If you already know your volume and only need it turned into sacks, the bag calculator does that in both directions.

Two mistakes account for most wrong orders across all three. The first is leaving a thickness or a depth in inches while the other dimensions are in feet, which overstates the answer twelvefold and usually announces itself by looking absurd. The second is quieter: rounding a bag count up on each element and then multiplying. Six post holes needing one and a half sacks each is nine sacks bought as one job, not twelve. Round once, at the end.