How much concrete do I need per fence post?
A typical 4×4 post set in a 10-inch-wide hole, 2 feet deep, takes about 2 bags of 80-lb concrete (roughly 0.9 cubic feet once you subtract the post). Dig wider or deeper and you'll need more — here's how to work it out for your exact holes.
The formula
The concrete fills the hole around the post, so you calculate the hole's volume and subtract the part the post takes up:
Concrete = hole volume − post volume
A round hole is a cylinder, so its volume is π × radius² × depth (all in feet). The post is a square column: width × width × depth. Subtract the second from the first and you have the concrete you need, in cubic feet. Divide by the yield per bag — an 80-lb bag makes about 0.60 cu ft, a 60-lb bag about 0.45, and a 50-lb fast-set bag about 0.375 — then round up to whole bags.
A worked example
For a 4×4 post (actually 3.5 in) in a 10-inch-wide hole, 2 feet deep:
- Hole radius: 5 in = 0.42 ft. Hole volume: π × 0.42² × 2 = 1.09 cu ft
- Post: 3.5 in = 0.29 ft. Post volume: 0.29 × 0.29 × 2 = 0.17 cu ft
- Concrete: 1.09 − 0.17 = 0.92 cu ft
- Bags: 0.92 ÷ 0.60 = 1.5 → round up to 2 bags of 80-lb mix
Skip the arithmetic
The concrete calculator does the cylinder-minus-post math and bag counts for you, and the fence calculator tells you how many posts your run needs in the first place.
Open the Concrete CalculatorBags per hole (4×4 post)
| Hole size | Concrete | 80-lb bags | 60-lb bags |
|---|---|---|---|
| 8″ wide × 2 ft deep | 0.53 cu ft | 1 | 2 |
| 10″ wide × 2 ft deep | 0.92 cu ft | 2 | 3 |
| 10″ wide × 2.5 ft deep | 1.15 cu ft | 2 | 3 |
| 12″ wide × 2 ft deep | 1.40 cu ft | 3 | 4 |
| 12″ wide × 3 ft deep | 2.10 cu ft | 4 | 5 |
Setting bigger 6×6 posts? Figure roughly 3 bags of 80-lb mix for a 12-inch hole 3 feet deep, and 6–8 bags for a wide 16–18-inch hole at that depth. Multiply whichever figure fits by your number of posts, then add a spare bag or two for the run.
How wide and how deep?
Two rules of thumb size almost every hole:
- Width ≈ 3× the post. A 4×4 wants a 10–12-inch-diameter hole; a 6×6 wants about 16–18 inches. Wider holes grip better but eat concrete fast — the volume rises with the square of the diameter.
- Depth ≈ ⅓ of the above-ground height, or below the frost line — whichever is deeper. A standard 6-foot fence usually means a hole about 2 feet deep; in cold climates the frost line can push that to 3 feet so winter heave doesn't lift the post.
Gate and corner posts carry more load, so it's worth digging them a little deeper and wider than the line posts in between. Our how many fence posts guide covers spacing and the post count.
Don't forget the gravel base
Before any concrete goes in, drop about 6 inches of gravel in the bottom of each hole and set the post on it. That gives water somewhere to drain instead of pooling against the post base, which is the single biggest thing you can do to slow rot on a wood post or rust on a metal one. Size it with the gravel calculator if you're doing a long run.
Fast-set or standard mix?
For a handful of posts, fast-setting post mix is the easy choice — brace the post plumb, pour the dry mix into the hole, then add water on top. No mixing tub, and it sets in about 20–40 minutes so you can move on. For a long fence with many posts, plain concrete mix is cheaper per hole, but you'll need to mix each batch with water first. Either way, fill to just above grade and crown the top so water sheds away from the post.
A few setting tips
- Brace before you pour. Hold each post plumb with two stakes and check it on two faces with a level — concrete won't fix a leaning post once it sets.
- Buy a spare bag. Holes are never perfectly uniform, and coming up short mid-pour means a hardware run. A bag or two extra per run is cheap insurance.
- Let it cure before adding panels. Give the concrete at least a day (longer in cold or damp weather) before hanging rails, panels, or a gate.