How many bags of concrete do you need per fence post?
A 4×4 on an 8 ft post takes 3 bags of 80 lb concrete (about 1.42 cu ft), following QUIKRETE's fence-post method.
The method, from the QUIKRETE Concrete Mix No. 1101 data sheet
“Start by digging the post hole about 3 times the diameter of the post. Hole depth should be 1/3 the overall post height. Place 6 in (150 mm) of dry concrete mix in the bottom of the hole.”
“For load-bearing applications, follow local building codes for proper footing specifications.”
Concrete per post
Assumptions: hole diameter is 3 × the actual post width; depth is ⅓ of the post length; the post stands on 6 in of mix; the hole is filled to grade around the post. Bags are rounded up, with no waste.
| Post | Post length | Hole (dia × depth) | Concrete (cu ft) | 80 lb | 60 lb | 50 lb | 40 lb |
|---|---|---|---|---|---|---|---|
| 4×4 (3.5 in) | 6 ft | 10.5 × 24 in | 1.08 | 2 | 3 | 3 | 4 |
| 4×4 | 8 ft | 10.5 × 32 in | 1.42 | 3 | 4 | 4 | 5 |
| 4×4 | 10 ft | 10.5 × 40 in | 1.76 | 3 | 4 | 5 | 6 |
How we worked it out
- Concrete = hole cylinder (π × r² × depth) minus the post's volume above the 6 in base.
- A nominal 4×4 is 3.5 × 3.5 in actual, per the Southern Pine Inspection Bureau. Other post sizes are not covered, because no primary source for their actual size was found (6×6: no data).
- Bag yields are from the QUIKRETE data sheet.
- The bag calculator's post-hole mode fills the full cylinder without deducting the post, so it gives a little more. That is a safe upper figure.
- Your hole size and local code may differ, so measure your actual hole.
Questions
How many 50 lb bags of concrete do I need for a fence post?
For a 4×4 on an 8 ft post, using the method above: 4 bags (3.78 rounded up).
How much concrete is needed for a 4x4 post?
About 1.08–1.76 cu ft, for 6–10 ft posts. That is 2–3 bags of 80 lb mix.
Is 2 ft deep enough for fence posts?
This page doesn't give footing advice. For reference only: QUIKRETE's own instructions size the hole at one third of the post's overall length (24 in for a 6 ft post), and say to “follow local building codes” for load-bearing work. Check your local code.
Related pages
Sources
QUIKRETE Concrete Mix yields; other products differ — check your bag.
Values marked with a link come from the linked source, which opens in a new tab. Unit conversions and our own defaults are listed separately. Every value the site uses is in the methodology source register.
Where the numbers come from 8
- Fence post hole diameter, times the post width
- 3 × post width
- QUIKRETE Concrete Mix No. 1101 data sheet (revised October 2022): "about 3 times the diameter of the post" (opens in a new tab)quikrete.com · PDF
- Fence post hole depth, share of the post length
- ≈ 0.333333 of post length
- QUIKRETE Concrete Mix No. 1101 data sheet (revised October 2022): "Hole depth should be 1/3 the overall post height" (opens in a new tab)quikrete.com · PDF
- Dry mix under a fence post
- 6 in
- QUIKRETE Concrete Mix No. 1101 data sheet (revised October 2022): "Place 6 in (150 mm) of dry concrete mix in the bottom of the hole" (opens in a new tab)quikrete.com · PDF
- Actual side of a nominal 4 × 4 post
- 3.5 in
- Southern Pine Inspection Bureau, Nominal vs actual lumber sizes (cites PS 20-20) (opens in a new tab)blog.spib.org
- 80 lb concrete bag yield
- 0.6 cu ft/bag
- QUIKRETE Concrete Mix No. 1101 data sheet (revised October 2022) (opens in a new tab)quikrete.com · PDF
- 60 lb concrete bag yield
- 0.45 cu ft/bag
- QUIKRETE Concrete Mix No. 1101 data sheet (revised October 2022) (opens in a new tab)quikrete.com · PDF
- 50 lb concrete bag yield
- 0.375 cu ft/bag
- QUIKRETE Concrete Mix No. 1101 data sheet (revised October 2022) (opens in a new tab)quikrete.com · PDF
- 40 lb concrete bag yield
- 0.3 cu ft/bag
- QUIKRETE Concrete Mix No. 1101 data sheet (revised October 2022) (opens in a new tab)quikrete.com · PDF
Unit conversions and our defaults3
- Circle constant π
- ≈ 3.14159 unitless
- Circle geometry, as specified in the owner brief
- Inches per foot
- 12 in/ft
- Unit definition: one foot contains twelve inches
- Cubic inches per cubic foot
- 1728 cu in/cu ft
- Geometry: 12 × 12 × 12 inches