How Many Bags of Concrete for a Fence Post?
A 10 inch wide, 30 inch deep post hole holds about 1.36 cubic feet. That is 2.3 bags of 80 lb mix, so buy three, or two if you allow for the volume the post takes up.
Try your own numbers
Last checked 11/10/2026. Sources: Quikrete: Concrete Mix 1101 data sheet (80 lb bag yields about 0.60 cu ft); 811: Call before you dig (US).
The post takes up room
The hole is a cylinder: π × (5 in)² × 30 in = 2,356 cu in, which is 1.36 cu ft. A 4 × 4 post is really about 3.5 × 3.5 in, so the part buried in a 30 in hole takes up about 0.21 cu ft. That leaves about 1.15 cu ft of concrete, or 1.9 bags of 80 lb mix: two bags.
The calculator gives the whole hole (three bags) because it cannot know your post. Two bags is the tight answer, three leaves some spare. With a fence of several posts, set the number of holes and the tool adds them up; spare bags are cheaper than a second trip.
Before you dig, call 811 so utilities can mark buried lines. How deep and how wide the holes should be depends on the fence height, the soil and local rules, so check with your building department.
Common questions
How many bags for 10 posts?
Set the number of holes to 10. At the same hole size that is 13.6 cu ft, or 23 bags of 80 lb mix before any allowance for the posts.
Should the hole be wider than the post?
Commonly about three times the post width, but your fence maker’s instructions are the better guide.