How much concrete for a 4×4 post?

The 4×4 is the standard wood fence post, and 3 ft is a common depth for a 9 ft post holding a 6 ft fence (Quikrete’s one-third rule). The post itself takes up 12.25 of the hole’s 78.5 square inches, so it only trims the concrete by about 16%. These figures assume no gravel in the bottom; 4 in of gravel trims the concrete by about a ninth. Either way, crown the top of the concrete so rain runs away from the post.

2 feet or 3 feet deep

Depth drives the bag count almost one-for-one. At 2 ft, the same hole takes about 1 cu ft with the extra — 2 bags of 80 lb or 3 of 50 lb fast-setting. At 30 in (about right for an 8 ft post, which sits a third of its length in the ground), it is 1.27 cu ft, a bit over 2 bags of 80. At 3 ft it is 1.52 cu ft. Where frost depth is deeper than that, the hole gets deeper still, and local rules may say how deep a fence post must go.

A 10 in hole is about three times the 3½ in post, which is Quikrete’s rule of thumb. A 12 in auger is common too; it adds about 50% more concrete (variants below), so don’t go bigger than you need.

Buying for a whole fence

For a 100 ft fence with posts every 8 ft you need 13 or 14 posts. At 2.5 bags of 80 lb per post that is about 33–36 bags if you share batches, or 42 if every hole gets its own 3. Fast-setting is easier for a fence line because it is poured dry around a braced post and sets in 20–40 minutes, so you can move on to the next hole; Quikrete still says to wait 4 hours before putting strain on the post. It costs more per cubic foot. Gate posts usually go bigger — see how much concrete for a 6×6 post — a single post out front is on how many bags of concrete for a mailbox post, and steel chain-link posts on how much concrete for a metal fence post.