TradesHow this works
Bagged concrete is priced by yield, so the whole game is turning your pour into a volume and then into bags. For posts, the concrete is the hole minus the post that sits in it, which is why a fat post in a snug hole barely takes a bag while a wide hole takes two or three. For a slab or a tube form it is straight geometry.
This gives the bag count in every size, with the total weight you will be lifting, so you can trade fewer heavy bags against more light ones. It also watches the size of the pour and tells you when you have crossed out of bag territory and into ready-mix, where a truck is cheaper and far less work.
Common questions
How many bags of concrete do I need per fence post?
It depends on the hole and the post, but a common case of a 10 inch wide hole about 30 inches deep with a 4 inch post takes roughly two 80 pound bags. The concrete is the hole volume minus the post that sits in it, so a bigger post or a narrower hole needs less. Enter your sizes and the tool gives it per post and for the whole run.
What does an 80 lb bag of concrete cover?
An 80 pound bag of concrete mix yields about 0.6 cubic feet of concrete. A 60 pound bag is about 0.45, a 50 pound bag about 0.375, and a 40 pound bag about 0.30. The tool uses these standard yields to give the bag count in every size so you can pick the one that suits the lift and the number of trips.
When should I order ready-mix instead of bagging it?
Bags make sense for posts and small pours, but the effort adds up fast. Once you are past roughly a third of a cubic yard, mixing dozens of bags by hand becomes a hard day, and a ready-mix truck is usually cheaper and far less work. The tool flags when your pour crosses that line and points you to the concrete-volume engine to compare.
Sources: Bag yields: standard concrete-mix published values (0.6/0.45/0.375/0.30 cu ft) · Post-hole volume is the hole cylinder minus the post (evergreen geometry). Engine version 1.0.0. Risk tier 1.