How Much Concrete Do I Need for Fence Posts
Undersizing your post holes is the fastest way to end up with a fence that leans after the first hard frost. The math is simple once you know two rules of thumb — the hole should be about three times the post width and about one-third of the post's above-ground height deep — and then it is just cylinder volume times your post count. This guide gives you the formula, a worked example for a standard 4x4 post, and a bag-count table so you can order with confidence.
The hole-sizing rules of thumb
Before any math, settle the hole dimensions. Two rules keep almost every residential fence stable:
- Diameter: make the hole about 3 times the post width. A nominal 4x4 post is actually 3.5 inches across, so a 10- to 12-inch hole is the sweet spot.
- Depth: bury roughly one-third of the post's height above ground, and never less than your local frost depth. For a 6-foot fence, plan on 24-30 inches of buried post; in cold climates that can reach 36-48 inches.
A wider, deeper hole uses more concrete but resists uplift and rot far better. If you are setting a gate post or a corner, go one size larger than the run-of-the-mill line posts.
The per-hole volume formula
A post hole is a cylinder. The concrete fills the cylinder minus the space the post itself occupies:
Those divisors are the typical yields: a 50 lb bag makes about 0.375 cubic feet of cured concrete, a 60 lb bag about 0.45, and an 80 lb bag about 0.60. Yields vary slightly by mix, so check the bag.
Worked example: a standard 4x4 line post
Take a 3.5-inch post in a 12-inch hole, 30 inches (2.5 ft) deep:
So one typical line post takes about 1.75 cubic feet of mix. Most homeowners reach for 50 lb or 60 lb bags because they are easier to lug around the yard than 80 lb sacks.
Bag counts by hole size
Use this table to scale without re-running the formula for every post. Figures are net concrete for a 3.5-inch post:
| Hole diameter | Hole depth | Net concrete | 50 lb bags | 80 lb bags |
|---|---|---|---|---|
| 8 in | 24 in | 0.78 cu ft | 3 | 2 |
| 10 in | 28 in | 1.27 cu ft | 4 | 3 |
| 12 in | 30 in | 1.75 cu ft | 5 | 3 |
| 12 in | 36 in | 2.16 cu ft | 6 | 4 |
| 18 in (gate post) | 42 in | 4.45 cu ft | 12 | 8 |
Concrete for a whole run of fence
Multiply the per-post figure by the number of posts. A 6-foot wood privacy fence with posts every 8 feet over 100 feet needs about 13 posts. At 5 bags of 50 lb each, that is roughly 65 bags. The Concrete Footing Calculator handles irregular spacing automatically.
Bagged mix vs. a small ready-mix order
The break-even between bagged mix and a short ready-mix load is usually around 1 to 1.5 cubic yards. Under that, bags win on convenience: no pump, no cleanup window, and you can mix as you set each post. For most backyard fences (under about 150-200 linear feet) bags are the practical choice.
Common mistakes
- Skipping the post volume: forgetting to subtract the post overstates concrete by 10-15% on a narrow hole.
- Shallow holes: setting posts only 18 inches deep in a freeze climate lets frost heave tip them within a season.
- Wrong bag math: 50 lb and 80 lb bags do not have the same yield — do not assume "a bag is a bag."
- No gravel tip: a 2-3 inch crushed-stone base under each post improves drainage and slows rot at the ground line.
- Tamping dry: add water to the hole per the bag's ratio and tamp in lifts; pouring a dry bag into a wet hole never cures properly.
Get your exact count
Measure your post size, hole diameter, and depth, then enter them into the Concrete Footing Calculator for total cubic feet, bag counts for 50/60/80 lb mix, and the yard equivalent to take to a supplier. You can also browse every related tool in one place at All Concrete & Masonry Calculators.