Post Hole Concrete Calculator

How to Calculate Concrete for Post Holes?

Each post hole is a cylinder. The volume formula is pi x radius squared x depth. A hole 12 inches in diameter (6-inch radius) and 36 inches deep has a volume of 3.14 x 0.25 x 3 = 2.36 cubic feet. Subtract the volume of the post inside the hole (a 4x4 post at 3.5 x 3.5 inches occupies about 0.26 cubic feet in a 36-inch hole). The net concrete per hole is approximately 2.1 cubic feet. Multiply by the number of posts and add a waste factor. The calculator above performs this math for any combination of post count, hole diameter, and hole depth.

How Many Bags of Concrete Per Post?

For a standard 4x4 post in a 10-inch diameter hole at 36 inches deep, you need approximately 2 bags of 80-pound concrete or 3 bags of 50-pound fast-setting mix. For a 6x6 post in a 12-inch hole at 36 inches deep, plan on 3 bags of 80-pound or 4 bags of 50-pound. The exact quantity depends on the hole diameter, depth, and post size. Larger holes and deeper settings consume more concrete. It is always better to have one extra bag on hand per every 5-6 posts than to run short during a pour and make a second trip to the store.

What Size Hole Should I Dig for Each Post Type?

The hole diameter should be three times the post width. A 4x4 post (3.5 inches actual) needs a hole at least 10-12 inches in diameter. A 6x6 post (5.5 inches actual) needs a 16-18 inch hole. This surrounding concrete provides adequate bearing area to resist lateral forces from wind and impact. The depth should be one-third of the total post length above and below ground. A 6-foot fence post (8 feet total including buried portion) goes 24-30 inches deep minimum. In frost zones, extend the hole below the frost line (36-48 inches in northern states) to prevent frost heave from lifting the post out of alignment.

Fast-Setting vs Regular Concrete for Posts?

Fast-setting concrete (like Quikrete Fast-Setting) hardens in 20-40 minutes and achieves enough strength to accept fence rails and hardware within 4 hours. The dry mix is poured directly into the hole around the post, then water is added on top. The water percolates through the mix and initiates the chemical reaction without the need for mixing in a wheelbarrow. Regular concrete mix requires premixing with water to a thick consistency before pouring into the hole. It takes 24-48 hours before the post can accept loads. Fast-setting is more convenient for DIY fence projects. Regular mix is preferred for structural posts that need precise placement before the concrete sets.

How to Set a Post in Concrete Properly?

Dig the hole to the specified diameter and depth. Add 4 inches of gravel at the bottom for drainage (water pooling at the base of a post accelerates rot). Place the post in the hole and brace it plumb using temporary 2x4 supports nailed to the post and staked to the ground. Check plumb with a level on two adjacent faces. Fill the hole with concrete to within 2-3 inches of ground level. Slope the top of the concrete away from the post so rainwater drains outward rather than pooling against the wood. For fast-setting concrete, pour the dry mix around the post to the desired level, then add water per the bag instructions. Do not disturb the post until the concrete reaches initial set.

Should I Use Concrete for Every Fence Post?

Concrete is strongly recommended for all end posts, corner posts, and gate posts because these bear the highest lateral forces. Line posts in a standard fence experience less force and can sometimes be set in compacted gravel or tamped soil, though concrete provides a more durable result. In sandy or loose soil, concrete is essential for all posts because the soil alone cannot resist leaning. In dense clay soil, some builders set posts in tamped crushed stone, which allows water to drain away from the post base and can actually extend post life by reducing moisture contact. The tradeoff is that non-concrete post settings may shift over time, especially in freeze-thaw regions.

Post Hole Digging Methods

A manual post hole digger (clamshell digger) works for a small number of holes in soft soil but becomes exhausting quickly. A gas-powered two-person auger drills holes in seconds and is the standard tool for fence contractors. One-person augers are lighter but less powerful. Renting a skid-steer-mounted auger makes sense for projects with 30+ holes. Before digging, call 811 (the national dig line) to have underground utilities marked at no cost. Hitting a buried gas line, electric cable, or water main is dangerous and expensive. Wait for all utilities to be marked and keep all post holes at least 24 inches from any marked utility line.

How to Prevent Post Rot at Ground Level?

The most vulnerable zone for post rot is right at the ground line where the post alternates between wet and dry. Use pressure-treated posts rated for ground contact (marked UC4A or UC4B). Apply additional brush-on wood preservative to the buried portion before setting. The gravel drainage layer at the bottom of the hole keeps the post base from sitting in standing water. Crowning the concrete above grade directs surface water away from the post. Some builders wrap the ground-line portion with self-adhering bitumen membrane for extra protection. Even with all precautions, pressure-treated posts in concrete typically last 15-25 years before the buried end deteriorates enough to require replacement.

Frequently asked questions