Post hole calculator
Post hole concrete calculator
For post holes, calculate the round hole volume from diameter, concrete depth, and quantity. Ten 8 inch diameter fence post holes at 30 inches deep need about 0.32 cubic yards before waste, or about 16 80-lb bags with a 10% buying allowance.
Quick answer
| Common line-post hole | 8 in diameter x 30 in deep |
|---|---|
| Corner or gate posts | Often wider or deeper than line posts |
| 10 line-post example | 0.32 yd3 before waste / 16 80-lb bags with 10% waste |
| Concrete depth | Do not include gravel depth below the concrete |
Concrete for common post hole groups
The table assumes round holes and does not subtract post displacement. Rough hand-dug holes usually offset much of that displacement.
| Scenario | Dimensions | Base volume | 80-lb bags | Ready-mix with 10% | Bagged with 10% |
|---|---|---|---|---|---|
| One 4x4 line post Useful for a single repair post. | 8 in dia x 30 in deep x 1 | 0.04 yd3 | 2 bags before waste | 0.04 yd3 / $6 | 2 bags / $10 |
| Ten 4x4 line posts Common small fence run planning example. | 8 in dia x 30 in deep x 10 | 0.33 yd3 | 15 bags before waste | 0.36 yd3 / $54 | 16 bags / $80 |
| Two gate posts Gate and end posts often need larger holes. | 10 in dia x 36 in deep x 2 | 0.13 yd3 | 6 bags before waste | 0.15 yd3 / $21 | 6 bags / $30 |
| Four deck piers Use the deck plan and frost depth for final size. | 12 in dia x 48 in deep x 4 | 0.47 yd3 | 21 bags before waste | 0.52 yd3 / $78 | 24 bags / $120 |
| Six heavy deck piers Volume rises fast as diameter increases. | 16 in dia x 48 in deep x 6 | 1.25 yd3 | 56 bags before waste | 1.37 yd3 / $206 | 62 bags / $310 |
Interactive calculator
Post hole concrete calculator
Enter the inside hole diameter, concrete depth, and quantity. If gravel is placed at the bottom of the hole, subtract that gravel depth before entering the concrete depth.
- Recommended order
- 0.36 yd³
- Exact volume
- 0.32 yd³
- With waste before rounding
- 0.36 yd³
- Equivalent
- 9.60 ft³ / 0.27 m³
Bag counts use the recommended order volume and round up to whole bags. Check the product label for exact yield.
- 40 lb bags
- 33
- 50 lb bags
- 26
- 60 lb bags
- 22
- 80 lb bags
- 17
Formula: cubic feet = length x width x thickness in feet; cubic yards = cubic feet / 27. Recommended order = exact volume x (1 + waste %) and then rounded up.
Bag estimate = recommended order volume / bag yield, rounded up. Bag yields: 40 lb = 0.30 ft³, 50 lb = 0.375 ft³, 60 lb = 0.45 ft³, 80 lb = 0.60 ft³.
Planning note: this estimate is for material planning only. It does not replace local building codes, supplier guidance, engineering design, or contractor judgment.
How to decide what to buy
Group posts by size
Line posts, corner posts, gate posts, and deck piers often use different diameters or depths. Calculate each group separately instead of averaging every hole.
Diameter changes volume quickly
Round hole volume grows with radius squared. Moving from an 8 inch to a 12 inch hole more than doubles the concrete for the same depth.
Code and frost depth still control footings
The calculator gives volume. Deck piers, structural posts, frost zones, and poor soil may require a larger or deeper footing than a generic post-hole rule.
Material checklist
- Concrete bags for each post group
- Extra bags for rough holes, collapsed sides, and spillage
- Drainage gravel if used, kept separate from concrete depth
- Post bases, bracing, level, and inspection requirements for structural posts
Cost assumptions
The table uses $150 per cubic yard for ready-mix and $5 per 80-lb bag. It is a planning comparison, not a delivered quote.
- Post holes are usually bagged because total volume is small and spread across the site.
- Ready-mix can make sense for many large deck piers, but access and short-load minimums matter.
- Buying one or two extra bags is often cheaper than stopping a fence or deck pour while a post is braced.
Common mistakes
- Using the post width instead of the hole diameter.
- Counting gravel depth as concrete depth.
- Using line-post quantities for gate posts or deck piers.
- Forgetting that hand-dug holes are rarely perfect cylinders.
Formula and methodology
Volume in cubic feet equals length x width x thickness in feet. Cubic yards equal cubic feet / 27. An 80-lb premix bag is estimated at 0.60 cubic feet. Ready-mix order size and bagged cost use a 10% buying allowance for field variation.