How Many Bags of Concrete Per Fence Post at 10 Inch Diameter

The Short Answer: About 1.5 Bags per Post

For a 10-inch diameter post hole that is 30 inches deep, you need approximately 1.5 bags of 80 lb concrete per post. This assumes the hole is cylindrical and the post occupies some space. The exact number depends on hole depth and post size, but 1.5 bags is a reliable starting point for most 4x4 posts. Always round up to the nearest whole bag when ordering.

The math: volume of a cylinder is π times radius squared times depth. A 10-inch diameter hole has a 5-inch radius. In feet, that's 0.4167 ft. Depth of 30 inches is 2.5 ft. So volume = 3.1416 × (0.4167)^2 × 2.5 = 1.36 cubic feet. One 80 lb bag yields about 0.6 cubic feet, so 1.36 ÷ 0.6 = 2.27 bags. But wait—that's without subtracting the post. A 4x4 post is 3.5 inches square, taking up 0.085 cubic feet per foot. For 2.5 ft, that's 0.21 cubic feet. So net concrete volume = 1.36 - 0.21 = 1.15 cubic feet, which is 1.15 ÷ 0.6 = 1.92 bags. Hmm, that's closer to 2 bags. Let's recalculate carefully.

Step-by-Step Calculation for a 10-Inch Hole

First, convert all measurements to feet. A 10-inch diameter hole has a radius of 5 inches, which is 0.4167 ft. A 30-inch depth is 2.5 ft. The volume of the hole is π × (0.4167)^2 × 2.5 = 1.363 cubic feet. Next, subtract the volume of the post. A nominal 4x4 post actually measures 3.5 inches by 3.5 inches, so its cross-sectional area is 12.25 square inches, or 0.0851 square feet. Over 2.5 ft, the post displaces 0.213 cubic feet. Net concrete volume = 1.363 - 0.213 = 1.150 cubic feet. An 80 lb bag of concrete yields about 0.6 cubic feet, so you need 1.150 ÷ 0.6 = 1.92 bags. Round up to 2 bags per post.

If you use a different post size, adjust accordingly. For a 6x6 post (actual 5.5x5.5 inches), the displacement is larger, so you might need fewer bags. Always check the bag yield on the product label, as it can vary slightly by brand.

How Widening to 10 Inches Changes the Total for a Full Run

Suppose you have a 100-foot fence with posts spaced 8 feet apart. That gives 100 ÷ 8 = 12.5, so 13 posts (including end posts). If you use a 254 mm (10-inch) diameter hole, 762 mm (30 inches) deep, each post needs 2 bags of concrete (rounded up from 1.92). Total bags = 13 × 2 = 26 bags. If you had used a narrower 8-inch hole, the volume would be smaller: radius 4 inches = 0.333 ft, volume = π × (0.333)^2 × 2.5 = 0.873 cubic feet. Subtract post displacement 0.213 = 0.660 cubic feet, which is 1.1 bags, so 2 bags per post as well? Actually 0.660 ÷ 0.6 = 1.1, round up to 2 bags. So for 8-inch, you might still need 2 bags per post if you round up. But if you use a larger bag or buy by volume, the difference matters. For a 10-inch hole, you are safely at 2 bags per post. For a 12-inch hole, volume = π × (0.5)^2 × 2.5 = 1.963 cubic feet, minus 0.213 = 1.750 cubic feet, which is 2.92 bags, so 3 bags per post. So widening from 8 to 10 inches might not change bag count if you were already rounding up, but it increases concrete volume by about 56%.

Using the Fence Calculator to Get Exact Numbers

The fence calculator automates this math. You input your fence length, post spacing, hole diameter, and depth. It computes the number of posts, then for each post calculates the concrete volume using the cylinder formula minus the post displacement. It then divides by the bag yield (0.017 m³ per 80 lb bag, which is about 0.6 cubic feet) and rounds up to the nearest whole bag. It also totals the bags for the entire run and estimates cost based on your local price per bag.

For a typical US fence with 10-inch holes, the calculator will output the exact bag count, so you can order with confidence. It also accounts for gates, which may need larger footings.

Regional Notes and Where to Confirm

In the US, concrete bag yields are standardized by the manufacturer, but always check the bag for the yield in cubic feet. For other regions like the UK, Australia, New Zealand, Canada, Germany, and Russia, bag sizes and yields differ. For example, in the UK, bags are often 25 kg and yield about 0.01 m³. The calculator uses the regional bag yield you specify. Always confirm local building codes for footing depth and diameter with your local authority or supplier, as requirements vary by frost line and soil conditions.

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