How Many Bags of Concrete Do You Need for a Fence Post?
Most fence posts need 1–2 bags of concrete each — but the right number depends heavily on your post size, hole depth, and soil. A standard 4x4 wood post usually takes one 80 lb bag; a 6x6 post can need four to six.
Here's how to figure out your exact number before you head to the store.
Quick Answer by Post Size
| Post Type | Typical Bags Needed (per hole) |
|---|---|
| 4x4 wood post | 1–2 (one 80 lb bag) |
| 6x6 wood post | 4–6 |
| 5x5 vinyl post | 2–3 |
| Round metal/steel post | 1–2 |
| Standard 6 ft fence post | ~2 bags |
These are per-hole estimates for a typical residential fence. Deeper holes, wider posts, or heavier-duty fencing (like a gate post) will need more.
How to Figure Out Your Exact Number
The amount of concrete you need is the volume of your hole, minus the volume of the post itself (since the post takes up space too). Two rules of thumb make this easy:
- Hole diameter: should be about 3 times the width of your post. A 4-inch post gets a 12-inch-diameter hole.
- Hole depth: typically one-third of the post's above-ground height, or deep enough to get below your local frost line — whichever is greater. Most residential fence posts use a 24–36 inch deep hole.
Once you know your hole's diameter and depth, the math is: (hole volume) − (post volume) = concrete volume needed. That's exactly what our Concrete Calculator does automatically — plug in your dimensions and skip the math.
One Bag or Two? Post Size Matters Most
- 4x4 wood posts (the most common fence post size): a 12-inch-diameter, 24-inch-deep hole needs about 1–2 cubic feet of concrete — right around one 80 lb bag, or two if you're going deeper.
- 6x6 wood posts (used for larger gates or heavier fencing): these need a wider hole (18+ inches diameter) and often 4–6 bags per post.
- Vinyl posts: a standard 5x5 vinyl post behaves similarly to a 4x4 wood post — plan on 2–3 bags.
- Steel/round posts: slightly less concrete than a square post of the same width, usually 1–2 bags.
Don't Forget Gravel at the Bottom
Add 2–3 inches of gravel to the bottom of the hole before pouring concrete. This isn't about saving concrete — it's for drainage. Without it, water can pool around the base of the post and cause wood posts to rot years earlier than they should.
Calculate Your Exact Amount
Post size, hole depth, and soil conditions all shift the number — the fastest way to get an exact answer for your project is to plug your measurements into our Concrete Calculator. It handles the hole-minus-post math for you and tells you exactly how many 60 lb or 80 lb bags to buy.
Setting more than one post? The Concrete Calculator can total up bags for multiple holes at once, so you buy exactly what you need — no extra trips to the store.