How to Calculate Concrete for a Post Hole
The concrete in a post hole fills the gap between the post and the soil, not the whole hole. The accurate method is hole volume minus the embedded post volume. Here is the step-by-step math with the unit conversions that trip people up.
Step 1: Find the hole volume
Most post holes are cylindrical. The volume of a cylinder is π × radius² × depth. Work in feet so the result is in cubic feet. For a 12-inch hole, the radius is 6 inches, or 0.5 feet. For a 24-inch depth, that is 2 feet. Hole volume = π × 0.5² × 2 = π × 0.25 × 2 = 1.571 cubic feet.
The common mistake here is mixing inches and feet. Convert diameter and depth to feet before squaring and multiplying, or convert the final cubic-inch result to cubic feet by dividing by 1728 (the number of cubic inches in a cubic foot).
Step 2: Find the embedded post volume
The post displaces concrete, so subtract the buried part of the post. For a square 4×4 post (actually about 3.5 inches, but use the nominal or actual size you prefer) embedded 24 inches: post volume in cubic inches = 4 × 4 × 24 = 384; in cubic feet = 384 ÷ 1728 = 0.222. For a rectangular post, use width × thickness × embedded depth. For a round post, use π × (post diameter ÷ 2)² × embedded depth, converting inches to feet or dividing cubic inches by 1728.
Step 3: Subtract and total
Concrete per hole = hole volume − embedded post volume. For the example: 1.571 − 0.222 = 1.349 cubic feet per hole. Multiply by the number of holes and add a waste allowance (10% is common) for spillage and uneven hole walls. For 10 holes: 1.349 × 10 × 1.10 = 14.84 cubic feet. Convert to cubic yards by dividing by 27: 14.84 ÷ 27 = 0.55 cubic yards.
Step 4: Convert to bags
Bag yields are approximate: a 40 lb bag yields about 0.30 cubic feet, a 50 lb bag about 0.375, a 60 lb bag about 0.45, and an 80 lb bag about 0.60. Divide the total cubic feet by the yield and round up. For 14.84 cubic feet in 60 lb bags: 14.84 ÷ 0.45 = 33 bags. Always confirm the yield printed on your specific bags — it varies by product.
| Bag size | Approx. yield |
|---|---|
| 40 lb | 0.30 cu ft |
| 50 lb | 0.375 cu ft |
| 60 lb | 0.45 cu ft |
| 80 lb | 0.60 cu ft |
Worked example
10 holes, 12 in diameter, 24 in deep, 4×4 square posts embedded the full depth, 10% waste, 60 lb bags. Hole volume = π × 0.5² × 2 = 1.571 cu ft. Post volume = (4 × 4 × 24) ÷ 1728 = 0.222 cu ft. Per hole = 1.349 cu ft. Total = 1.349 × 10 × 1.10 = 14.84 cu ft = 0.55 cu yd. Bags = 14.84 ÷ 0.45 = 33 bags of 60 lb mix.
Practical cautions
- Convert all dimensions to the same unit before calculating — inches-to-feet errors are the most common mistake.
- Hole walls are rarely perfectly cylindrical; the waste allowance covers the over-excavation.
- If you skip subtracting the post, you over-order — fine as a safety margin, but not the accurate figure.
- For the full fence plan, pair this with the Fence Calculator and read How Much Concrete Do I Need for Fence Posts?
The Post Hole Concrete Calculator performs all of these steps — hole minus post, waste, unit conversions, and bag counts — in one result.