How this calculator works
For each hole we calculate the volume of a cylinder (π × radius² × depth), subtract the space taken by the post, and subtract the gravel base if you use one. That leaves the volume of concrete. We multiply by the number of posts, add your waste allowance, and divide by the volume one bag yields: 0.60 ft³ for an 80 lb bag, 0.45 ft³ for 60 lb and 0.375 ft³ for 50 lb.
The 1,728 converts cubic inches to cubic feet.
Worked example: a 10 in hole, 30 in deep, with 6 in of gravel and a 4×4 post: the concrete depth is 24 in. Hole volume is 78.5 × 24 = 1,885 in³, minus the post's 12.25 × 24 = 294 in³, which leaves 1,591 in³ or 0.92 ft³. With 10% waste that is 1.01 ft³, about 1.7 bags of 80 lb per post.
Tips before you buy
- Rapid-set concrete is popular for posts: pour it dry into the hole and add water, with no mixing. It sets in about 20 to 40 minutes, but follow the bag directions and check the post stays plumb.
- Slope the top of the concrete away from the post so rain runs off, instead of sitting against the wood.
- Wood posts last longer with a gravel base for drainage. Do not set the end grain directly in a puddle of concrete.
- Bury roughly one third of the fence height, with a 2 ft minimum. A 6 ft fence typically uses 8 ft posts with about 2 ft in the ground, and an 8 ft fence uses 11 to 12 ft posts with about 3 ft buried. Go deeper where soil freezes.
Tools & materials for this project
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Frequently asked questions
How many bags of concrete do I need per fence post?
For a 4×4 post in a 10 in hole about 24 in deep, roughly 1½ bags of 80 lb, or 2 to 3 bags of 50 lb. Enter your numbers above for an exact count.
How deep should fence posts be?
Plan on burying about one third of the fence height, with a minimum around 24 in, and below the frost line in cold climates. A 6 ft fence on an 8 ft post is typically set 24 to 30 in deep. Follow local code and the fence manufacturer's guidance.
How wide should a post hole be?
About three times the post width, so 10 to 12 inches for a 4×4. A wider hole resists wind load better but needs more concrete.
Do I need gravel under the post?
Many installers add 4 to 6 inches of gravel for drainage so the post bottom is not sitting in water. This calculator lets you subtract that depth from the concrete volume.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.