How Many Bags of Concrete for a Fence Post?

You're standing in the Quikrete aisle holding a fence post in one hand and a phone in the other, trying to figure out how many 50-pound bags you need for ten holes. The answer depends on the post size, the hole diameter, the depth, and which bag size you grab. This guide gives you the full math — and a calculator to skip it.

The short answer

For a standard 4×4 wood post in a 10-inch-wide hole, 30 inches deep: plan on two 50-lb bags of fast-setting concrete per post. For a 6×6 post in a 12-inch hole at the same depth, plan on three 50-lb bags. Round metal posts in 8-inch holes usually take one to one-and-a-half bags.

Always buy 10–15% extra. Bags are cheap; an extra trip to the store on a Saturday afternoon is not.

How the math actually works

Concrete needs are calculated by volume, not by post count. The formula is straightforward:

1. Calculate hole volume (cylinder = π × r² × depth)
2. Subtract post volume (the post displaces concrete)
3. Divide by bag yield (each bag fills a known cubic-foot volume)

A 50-lb bag of fast-setting concrete yields about 0.375 cubic feet. A 60-lb bag yields 0.45 cubic feet. An 80-lb bag yields 0.60 cubic feet. Manufacturers print these on the side of the bag, but the numbers above are the working defaults.

Hole sizing rules

The standard rule of thumb: hole diameter should be three times the post width. A 4×4 post (3.5 inches actual) gets a 10–12 inch hole. A 6×6 post (5.5 inches actual) gets a 16-inch hole. Round posts get sized similarly — a 4-inch round metal post gets an 8-inch hole minimum.

Depth depends on frost line. In the South, 24 inches is fine. In the Midwest, 36 inches. In Maine and Minnesota, 42–48 inches isn't overkill. Check your local building code — when in doubt, go deeper.

Bag size economics

80-lb bags are cheaper per cubic foot, but 50-lb bags are easier to carry from the truck to the back fence line. If you're setting more than ten posts, the savings on 80-lb bags add up. If you're setting three posts in your back yard, grab the 50s and save your back.

Fast-set vs standard mix

Fast-setting concrete (Quikrete Q•Max, Sakrete Fast Setting) is poured dry into the hole, then water is added on top. It sets in 20–40 minutes and reaches full strength in a few hours. This is the right choice for most fence projects — you can hang the rails the same day.

Standard concrete mix needs to be mixed in a wheelbarrow with water first, takes 24 hours to set, and 7 days to reach full strength. Use it only if you're already mixing concrete for something else nearby.

Common mistakes

• Holes too narrow. A skinny hole means not enough concrete around the post to anchor it. Frost will heave it within two winters.
• Skipping the gravel base. Two inches of gravel at the bottom of every hole helps drainage. Wet bottoms rot wood posts and rust metal ones.
• Rushing the cure. Even fast-set needs 4–6 hours before you stress the post with rails or panels.

Key takeaways

• 4×4 post, 30" deep, 10" hole: 2 × 50-lb bags
• 6×6 post, 36" deep, 12" hole: 3 × 50-lb bags
• Round metal post, 30" deep, 8" hole: 1–1.5 × 50-lb bags
• Fast-set saves a day. Use it.
• Buy 10–15% extra. Always.