Volume formula
π × (diameter ÷ 2)² × depth = ft³
ft³ ÷ 27 = yd³
Example: π × (5 in)² × 24 in = 1,885 in³ ÷ 1,728 ÷ 27 = 0.040 yd³ per hole.
Concrete planning estimate
Concrete Post Hole Calculator
Calculate concrete bags for fence posts, deck posts, mailbox posts, and sign posts. Enter hole diameter and depth to get the volume per hole and total bag count for your project.
What the post hole calculator estimates
The calculator models each post hole as a cylinder (round column). Enter the hole diameter in inches and depth in feet to get the volume and bag count per hole. Multiply by the number of holes for your full project.
- Concrete volume per hole in cubic yards or cubic meters.
- 40 lb, 60 lb, and 80 lb bag counts per hole.
- Total volume for a multi-post project.
- Cylinder volume formula with transparent inputs.
- Waste factor to account for irregular hole walls.
Post hole concrete formula
Post holes are calculated as circular cylinders.
Waste factor
Post holes are rarely perfect cylinders — use 10% waste to account for oversized or irregular holes bored by an auger. Round up bag counts — you cannot buy a partial bag.
Fast-setting concrete
Fast-setting mixes (Quikrete 50/80 lb bags) require no mixing bowl. Pour dry into the hole, add water, and wait 20–40 min before setting the post plumb. Standard mixes give more working time.
Common post hole bag estimates
Per-hole estimates with 10% waste. Use the calculator for your exact dimensions.
| Hole size | yd³ per hole | 80 lb bags/hole | 50 lb bags/hole |
|---|---|---|---|
| 8 in × 24 in deep | 0.028 | 2 | 3 |
| 10 in × 24 in deep | 0.044 | 3 | 4 |
| 10 in × 30 in deep | 0.056 | 3 | 4 |
| 12 in × 36 in deep | 0.096 | 5 | 7 |
| 16 in × 48 in deep | 0.228 | 11 | 17 |
Concrete post hole calculator FAQ
How many bags of concrete do I need for a fence post?
A typical 10 in diameter post hole that is 24 in deep needs about 3 bags of 80 lb fast-setting concrete per hole. Larger holes or deeper frost footings need more.
How deep should a fence post hole be?
A common guideline is one-third to one-half of the above-ground post height, or at least below the local frost line. A 6 ft fence post typically needs a 24–36 in deep hole. Check local frost depth requirements.
Should I use fast-setting or standard concrete for fence posts?
Fast-setting concrete mixes are popular for fence posts because you can pour dry concrete in the hole and add water — no mixing required. Standard concrete is fine too but needs a longer cure time before loading the post.
How wide should a post hole be?
A post hole should be 3 times the post diameter (or 2–3 times for smaller posts). A 4 × 4 post (3.5 in) typically needs an 8–10 in hole. A 6 × 6 post might need a 12 in hole.
Do I need concrete for fence posts?
Concrete provides the best long-term stability for fence posts. In very well-drained, compacted soils, gravel can work for light fences. Concrete is strongly recommended for gates, corners, and areas with high wind loads.
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Next planning steps
Planning estimates only
These are rough material estimates for planning purposes. They are not purchasing guarantees or engineering guidance. Call 811 before digging any post holes to have underground utilities marked. Structural posts, retaining walls, and deck supports require proper engineering review and permits.