BuildCalc Pro

Post Hole Concrete Calculator

Calculate exactly how many bags of concrete you need for fence posts, deck posts, mailboxes, and more. Enter your hole size and number of posts for instant results with cost estimates.

Rule of thumb: 3× the post width

1/3 of post length or below frost line

Drainage layer at bottom

How to Use This Calculator

1. Select your measurement unit (Imperial or Metric). 2. Choose your post type — this helps estimate the post volume displaced inside the hole. 3. Enter the hole diameter. Rule of thumb: hole should be 3× the post width (e.g., 10" hole for a 4×4 post). 4. Enter the hole depth — typically 1/3 of the total post length, or below your local frost line. 5. Enter the number of posts. 6. Choose your bag size (50, 60, or 80 lb). 7. Set the gravel base depth (3" is standard for drainage). 8. Optionally enter cost per bag for a total estimate. 9. Click "Calculate" for detailed results. Tip: Fast-setting concrete (like Quikrete Fast-Setting) can be poured dry and then soaked with water — no mixing required. Perfect for post holes.

How Many Bags of Concrete Per Fence Post? (Exact Calculator)

Setting fence posts, deck posts, or mailbox posts in concrete is one of the most common DIY projects — and one of the most frequently miscalculated. Buy too few bags and you are making a second trip to the store mid-project. Buy too many and you have leftover bags that harden in your garage.

The amount of concrete per post depends on three factors: hole diameter, hole depth, and the size of the post itself. A standard 4×4 fence post set in a 10-inch diameter hole that is 24 inches deep requires approximately 0.6 cubic feet of concrete — that is roughly 1 to 2 bags of 50-lb fast-setting mix per post.

The rule of thumb for hole size is three times the post width: a 4×4 post (actually 3.5 inches) gets a 10-inch hole, while a 6×6 post (5.5 inches) gets a 12-inch hole. Hole depth should be one-third of the total post length, or below your local frost line — whichever is deeper.

Fast-setting concrete (like Quikrete Fast-Setting Mix) is the preferred product for post holes because you can pour it dry directly into the hole and add water — no mixing required. The post can support weight in 20-40 minutes and reaches full strength in 24-48 hours.

Always place 3 inches of gravel at the bottom of the hole before adding concrete. This gravel layer provides critical drainage that prevents the post base from sitting in water, which causes premature rot in wood posts and frost heaving in cold climates.

Frequently Asked Questions

How many bags of concrete do I need for a fence post?

A standard 4×4 fence post in a 10" diameter × 24" deep hole requires 1-2 bags of 50-lb fast-setting concrete. For 6×6 posts in 12" holes, plan for 2-3 bags per post. Always buy 10% extra for waste.

How deep should a fence post be set in concrete?

Fence posts should be set at least 24 inches deep, or one-third of the total post length, whichever is deeper. In cold climates, the hole must extend below the frost line (36-48 inches in northern states) to prevent frost heaving.

Can I use fast-setting concrete for fence posts?

Yes, fast-setting concrete is ideal for fence posts. Pour it dry into the hole around the post, then add water. No mixing needed. The post is stable in 20-40 minutes and fully cured in 24-48 hours. It is the most popular method for DIY post installation.

Should I put gravel at the bottom of a post hole?

Yes, always place 3 inches of gravel at the bottom of every post hole. This drainage layer prevents water from pooling around the post base, which causes wood rot and frost heaving. Use crushed gravel, not pea gravel — it compacts better.