How much concrete do I need for a fence post?
A standard 4×4 post in a 10″ hole, 30″ deep, needs about 1.5 bags of 80 lb fast-setting concrete – buy 2 to be safe.
The concrete volume is the hole volume minus the buried post: V = π × r² × depth − post area × depth. That subtraction is exactly what this calculator does, which is why the result is usually lower than the "fill the whole hole" calculators.
How many bags of concrete per fence post?
2 bags of 80 lb for a 4×4 in a 10″×30″ hole; 3–4 bags for a 6×6 in a 12″×36″ hole.
| Post | Hole (dia × depth) | 80 lb bags |
|---|---|---|
| 4×4 | 8″ × 24″ | ≈ 1 |
| 4×4 | 10″ × 30″ | ≈ 2 |
| 4×4 | 12″ × 36″ | ≈ 3 |
| 6×6 | 12″ × 36″ | ≈ 3 |
| 6×6 | 12″ × 48″ | ≈ 4 |
How big should a fence post hole be?
Roughly 3 times the width of the post: 10–12″ for a 4×4, 12–18″ for a 6×6.
Wider is only marginally better than deep. Depth does most of the work for frost resistance and stability, so dig deeper before you dig wider.
How deep should a fence post be buried?
At least one third of the post length. A 6 ft post → 24″ deep; an 8 ft post → 30–36″.
In cold climates the hole must extend below the frost line, which in northern states can be 36″ or deeper. Posts below the frost line are far less likely to heave.
Do you have to use concrete for fence posts?
Concrete is the strongest and most common option, but gravel, expanding foam or tamped earth also work.
Concrete holds best against frost heave when the hole is wider at the bottom than the top (a bell shape). DIY fences do well with fast-setting concrete or a dedicated quick-set post mix.
How much concrete for a 4×4 fence post?
About 1.5–2 bags of 80 lb for a 4×4 post in a 10″ hole, 30″ deep.
A 4×4 is actually 3.5″ wide. Over a 30″ buried length that is roughly 0.26 ft³ of post occupying the hole – subtracting it saves a meaningful amount of concrete across a whole fence.
- Hole volume:
V_hole = π × (d/2)² × depthin cubic feet. - Post volume: square post
V_post = width² × depth; round postV_post = π × (p/2)² × depth. - Fill volume:
V = V_hole − V_postper post; total × number of posts. - Bag yields: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 50 lb ≈ 0.38 ft³, 40 lb ≈ 0.30 ft³ (mixed).
- Rounding: bags are rounded up – you cannot buy a fraction of a bag.
- Nominal lumber sizes (4×4, 6×6) use actual widths (3.5″, 5.5″).
- Results are estimates. Dig holes deeper than wider in cold climates, and confirm local frost depth and mix requirements with a professional.
Home Alone DIY · Free estimator only · Not professional engineering advice. Mix designs, densities and bag yields vary by brand; always verify with your supplier.