Tool
Concrete Calculator for Fence Posts
Tell this tool how many posts you are setting, how wide and how deep you are digging, and what size bag you buy, and it returns the concrete you need per post and for the whole job — in bags, cubic feet and pounds.
Concrete is the most common reason a fence project stalls halfway: two bags short, and the store run costs an afternoon. This calculator rounds up per post rather than per job, so the count is realistic and you finish with a little spare instead of a gap in the line.
Bag size
Concrete needed
28 bags
2 × 80 lb bag per post, 14 posts
- Concrete per hole
- 1.10 ft³
- Total concrete
- 15.5 ft³
- Total weight
- 2,240 lb
- Yield per bag
- 0.6 ft³
Bag counts are rounded up per post, so a few bags are left over for low spots. Dig below your local frost line first, and crown the concrete above grade so water runs away from the post.
Hole size and depth
Dig the hole about three times the width of the post: a 9 to 10 inch hole for a 4x4 post, 12 inches for a 6x6 post. Depth matters more than width. A shallow hole in soft ground lets the post lean after the first freeze and thaw, so the usual guidance is one third of the post's above-ground length below grade, and never less than the local frost depth.
That works out to 30 inches for a typical 6 foot fence in a mild climate, and 36 to 42 inches in regions with hard winters. The extra bags a deeper hole requires are cheaper than replacing a leaning fence.
Bag sizes and what they yield
- 80 lb bag: about 0.60 cubic feet of mixed concrete.
- 60 lb bag: about 0.45 cubic feet — the most common bag for fence work.
- 50 lb bag: about 0.38 cubic feet.
- 40 lb bag: about 0.30 cubic feet, useful for small touch-up jobs.
- Bag sizes differ by brand and mix, so check the yield printed on the bag if the count matters to you.
Pouring tips that save work
Set the post on a few inches of gravel or dry mix at the bottom of the hole so no dirt remains under it, then check it for plumb in two directions with a level before pouring. Fill the hole about two thirds, tamp it in with a spare piece of lumber to remove air pockets, finish filling, then slope the top away from the post so water sheds.
Water runs a post line faster when you keep a hose at the hole rather than hauling buckets. Mix to the consistency of thick oatmeal, not soup, and never pour dry mix into a wet hole expecting the same strength.
Common questions
- How many bags of concrete do I need per fence post?
- For a 9 inch hole 30 inches deep, about 2 bags of 80 lb concrete per post, or 3 bags of 60 lb. A narrower 8 inch hole at the same depth takes 2 bags of 60 lb. Deeper holes for frost, or wider holes for 6x6 posts, take more, which is why the calculator asks for the hole rather than assuming one.
- Should fence posts be set in concrete or gravel?
- Concrete holds a post rigid and is the standard for gates, corners and windy sites. Gravel drains better and lets a wood post dry out, which is why some builders use it for line posts in wet ground. Either way, crown the fill slightly above grade so water runs away from the post instead of pooling at its base.
- How long before I can hang a gate on a concrete-set post?
- Concrete sets enough to hold a post in about 24 hours and reaches useful strength in 2 to 3 days. Brace or stake gate posts and leave the gate off for at least 48 hours, longer in cold weather, because a gate is the heaviest load the post will ever carry.