Fence Post Concrete Calculator

How Many Bags of Concrete Per Fence Post?

The number of bags per post comes down to the hole size, because the concrete only fills the ring around the post. Here are typical per-post bag counts for standard 4×4 line posts:

• 8 in hole × 24 in deep — about 0.70 ft³ of concrete → 1–2 bags of 60 lb per post
• 10 in hole × 30 in deep — about 1.36 ft³ → 2–3 bags of 60 lb per post
• 12 in hole × 36 in deep — about 2.36 ft³ → 4–5 bags of 60 lb per post
• 12 in hole × 42 in deep (frost line) — about 2.75 ft³ → 5–6 bags of 60 lb per post

One 80 lb bag yields roughly 0.60 ft³ mixed and a 60 lb bag about 0.45 ft³, so 80 lb bags cut your bag count by about a third for the same hole. Multiply the per-post figure by your number of posts to size the order.

How to Estimate Concrete for a Whole Fence Run

Enter the hole diameter, hole depth, and number of posts above and the calculator totals the concrete and bags for the entire fence — and it subtracts the buried post volume so you are not paying for bags you will never use. To set a single post or work in a different post shape, use the post hole calculator; to lay out spacing and panel counts first, start with the fence calculator.

Net-Minus-Post — Why It Changes Your Bag Count

Each hole is a cylinder, but the post sitting in it displaces concrete. The honest volume per post is the hole volume minus the buried post volume:

Concrete per post = (π × (hole radius)² × depth) − (post width × post thickness × depth)

Example: 10-Post Fence

1. 10 holes, 8-inch (0.667 ft) diameter, 2 ft deep, 4×4 posts
2. Hole volume each: π × 0.333² × 2 = 0.698 ft³
3. Post volume each: 0.292 × 0.292 × 2 = 0.171 ft³
4. Net per post: 0.698 − 0.171 = 0.527 ft³
5. Total for 10 posts: 5.27 ft³ ÷ 0.45 ≈ 12 bags of 60 lb

Skipping the subtraction here would have you buying about 16 bags instead of 12 — four wasted bags on just ten posts.

Gravel Base and Crowning the Top

Two finishing details decide how long a concrete-set post lasts:

• Gravel base — drop 3–6 inches of compacted gravel or crushed stone in the bottom of each hole and set the post on it. Water drains down through the gravel instead of pooling against the buried end grain, which is the main cause of rot.
• Crowned top — slope the top of the concrete up to the post on all sides so rain sheds away instead of collecting in a ring against the wood.
• Collar above grade — finish the crown an inch or two above the surrounding soil so the post base never sits in wet ground.
• Ground-contact lumber — use UC4B-rated pressure-treated wood for the buried portion; above-ground-rated lumber will not hold up in soil.

Line Posts vs Corner and Gate Posts

Not every post in a run uses the same amount of concrete. Corner, end, and gate posts carry the heaviest lateral loads and need a bigger footing:

• Line posts — the standard posts along a straight run; an 8–10 inch hole at 24–30 inches deep (2–3 bags of 60 lb) is typical.
• Corner and end posts — go one hole size larger and 6 inches deeper than line posts; budget one to two extra bags each.
• Gate posts — the most heavily loaded of all because of the gate's weight and swing; a 12-inch hole at frost-line depth can take 4–6 bags of 60 lb each.

For a typical 100-foot privacy fence with posts every 8 feet — about 14 posts — figure 28–42 bags of 60 lb for the line posts plus a few extra for each corner and gate. Buying by the pallet (around 56 bags) is usually cheaper once you pass 40 bags.