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Concrete Fence Post Calculator

Calculate how many bags of concrete you need per hole and for your full fence run. Enter hole size, depth, and number of posts.

Enter hole diameter in inches (e.g. 10 for a 10-inch post hole).

Your Ready-Mix Price (optional)

$ / yd³

Enter your supplier's quote for an exact cost estimate.

How to Use This Calculator

Enter your hole diameter in inches. For a standard 4×4 fence post, dig 10 to 12 inches across. For a 6×6 post, use 12 to 16 inches. Corner posts, end posts, and gate posts take more stress than line posts; use the wider size for those.

Set the depth in inches. The general rule: bury one-third of the total post length, plus 6 inches of gravel at the bottom for drainage. A 6-foot fence uses 8-foot posts buried 24 inches deep. An 8-foot fence uses 10-foot posts buried 36 inches deep. In frost-prone areas, the hole must reach below your local frost line.

Enter your post count. The result shows bags per hole and total bags for the full fence run, split by 80lb, 60lb, and 40lb bag sizes. The bag count includes a 10% waste factor for overpour and spillage.

How to Calculate Concrete for Fence Posts

Fence post holes are cylinders. The cylinder volume formula: V = π × r² × h. Convert diameter to radius by dividing by 2, then convert both radius and depth from inches to feet.

Formula: cubic feet per hole = π × (diameter ÷ 24)² × (depth ÷ 12)

Worked example: 10-inch diameter hole, 24 inches deep. Radius = 10 ÷ 2 = 5 inches = 0.417 feet. Volume = 3.1416 × (0.417)² × 2.0 = 3.1416 × 0.174 × 2.0 = 1.09 cubic feet. Bags of 80lb: 1.09 ÷ 0.60 = 1.82, round up to 2 bags per hole.

For a 12-inch hole at 30 inches deep: π × (0.5)² × 2.5 = 1.96 cubic feet. Bags of 80lb: 1.96 ÷ 0.60 = 3.27, round up to 4 bags per hole. Multiply by your post count and add 10% for waste. For 10 posts at 2 bags each: 10 × 2 = 20 bags, plus 10% = 22 bags to buy.

Fence Post Concrete Tips

Set posts plumb before the concrete goes in, not after. Use two levels on adjacent faces, or brace the post with scrap lumber staked into the ground at an angle. Once fast-setting concrete firms up in 20 minutes, adjusting is nearly impossible. Check plumb twice, pour once.

Add 6 inches of gravel to the bottom of each hole before placing the post. This layer drains water away from the post base and significantly extends the life of wood posts. Wood rot starts at the base where moisture pools. The gravel layer costs almost nothing and adds years of post life.

Use fast-setting concrete for all fence post work. Pour dry mix directly into the hole around the post, then add water per the bag instructions. No wheelbarrow, no mixing, no waiting. You can set 10 to 15 posts per day this way. Standard mix is fine for other concrete work, but fence posts are the one application where fast-setting concrete genuinely saves hours.

What to Buy

Quikrete Fast-Setting Concrete (50lb bags) is the most popular choice for fence posts. Pour dry into the hole, add water, done. Each 50lb bag yields 0.375 cubic feet. For 10-inch holes at 24 inches deep, plan on 3 bags of the 50lb size per hole. Quikrete 5000 (80lb bags, 0.60 cubic feet each) costs less per cubic foot and works well if you mix in a bucket.

Sakrete Fast Setting Concrete is a direct alternative to Quikrete and available at Home Depot, Lowe's, and most hardware stores. For gate posts or large-diameter holes where you are mixing more than 6 bags per hole, rent a small electric mixer. Hand-mixing that volume in a wheelbarrow is slow and tiring. For very large fence projects over 30 posts, compare the cost of a short-load ready-mix delivery against buying bags in bulk.

Frequently Asked Questions

How many bags of concrete do I need per fence post? +
For a standard 4×4 fence post in a 10-inch diameter hole at 24 inches deep, you need 2 bags of 80lb concrete per hole. A 6×6 post in a 12-inch hole at 30 inches deep needs 4 bags of 80lb. These are the two most common scenarios. Use this calculator to get an exact count for your specific hole size and depth.
How deep should fence post holes be? +
Bury at least one-third of the total post length, plus 6 inches for a gravel drainage layer at the bottom. A 6-foot fence with 8-foot posts needs a 2-foot hole. An 8-foot fence with 10-foot posts needs a 3-foot hole. In cold climates with frost heave, the hole bottom must sit below the local frost line, which ranges from 12 inches in the South to 48 inches or more in northern states.
How wide should the hole be for a fence post? +
Dig the hole 3 times the post width. For a 4×4 post (3.5 inches actual), a 10-inch diameter hole is standard. For a 6×6 post, use a 12 to 16-inch hole. The wider the hole, the more concrete you pour, but also the stronger the post. Gate posts and corner posts handle much more stress than line posts; use the wider end of the range for those.
Can I use fast-setting concrete for fence posts? +
Yes, and it is the preferred method for most fence installations. Quikrete Fast-Setting Concrete and Sakrete Fast Setting Concrete can be poured dry into the hole around the post and then watered in place. No mixing required. The concrete sets firm in 20 to 40 minutes, which means you can set multiple posts in a day without waiting for each one to cure. Follow the bag instructions for water volume.
How long before I can hang fence panels after setting posts in concrete? +
With standard concrete mix, wait at least 24 to 48 hours before applying any load to the posts. With fast-setting concrete (Quikrete Fast-Setting, Sakrete Fast Setting), wait 4 hours before attaching panels and 24 hours before hanging a gate. Full cure takes 28 days, but working strength is achieved much sooner. Do not hang heavy gates before the 24-hour mark regardless of mix type.
How many bags of Quikrete do I need for 10 fence posts? +
It depends on your hole size. For 10 posts in 10-inch holes at 24 inches deep, you need 2 bags of 80lb Quikrete per post, or 20 bags total. Add 10% for waste and spillage: order 22 bags. For 10-inch holes at 30 inches deep, you need 3 bags per post, so 30 bags plus 10% = 33 bags. Always use the calculator with your exact hole dimensions for the most accurate count.
Should I use concrete for every fence post? +
Concrete is best for corner posts, end posts, and gate posts, which take the most lateral load. For line posts in stable, well-draining soil, tamped gravel alone can work for light privacy fences. However, concrete is the standard and the most reliable method. If you are installing a wood privacy fence, use concrete for all posts. The material cost is low and the stability difference is significant.

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