How Many Bags of Concrete per Fence Post?
October 25, 2026 · 6 min read
Bags of fast-setting or regular concrete for 4×4, 6×6 and steel fence posts at 2–3 ft deep, how wide and deep to dig, and totals for a whole fence.
The quick answer
A standard 4×4 fence post set 2 feet deep in a 10-inch hole needs about 0.92 cubic feet of concrete. That's roughly 2½ bags of 50 lb fast-setting concrete, or about 1½ bags of 80 lb regular concrete mix. Deeper holes and bigger posts take more: a 6×6 post set 3 ft deep takes nearly 5 bags of fast-setting mix.
Planning a whole fence? Our fence calculator counts the posts for your length, corners and gates, then adds up the post concrete along with the rails, pickets or panels.
Bags per post by post size and depth
The concrete fills the hole around the post, so the amount depends on the hole width, the depth and the size of the post itself:
| Post | Hole | Depth | Concrete per post | 50 lb fast-setting | 80 lb concrete mix |
|---|---|---|---|---|---|
| 4×4 wood | 10″ | 2 ft | 0.92 cu ft | 2.5 bags | 1.5 bags |
| 4×4 wood | 10″ | 2½ ft | 1.15 cu ft | 3.1 bags | 1.9 bags |
| 4×4 wood | 10″ | 3 ft | 1.38 cu ft | 3.7 bags | 2.3 bags |
| 6×6 wood | 12″ | 2 ft | 1.15 cu ft | 3.1 bags | 1.9 bags |
| 6×6 wood | 12″ | 3 ft | 1.73 cu ft | 4.6 bags | 2.9 bags |
| 2⅜″ steel | 8″ | 2 ft | 0.64 cu ft | 1.7 bags | 1.1 bags |
Don't round up post by post. Partial bags carry over to the next hole, so add up the whole fence and round up once at the end. That's how the fence post calculator does it.
How the math works
Concrete volume per post = hole volume − post volume. For a 4×4 (actually 3½ × 3½ inches) in a 10-inch hole, 2 ft deep:
- Hole: π × 5² × 24 = 1,885 cubic inches.
- Post: 3.5 × 3.5 × 24 = 294 cubic inches.
- Concrete: 1,885 − 294 = 1,591 cubic inches, or 0.92 cubic feet (divide by 1,728).
- Bags: a 50 lb bag of Quikrete Fast-Setting Concrete Mix yields about 0.375 cu ft (Quikrete data sheet), so 0.92 ÷ 0.375 = 2.5 bags.
These figures fill the hole right to the top, so they include a small margin. Quikrete's own instructions stop the dry mix 3–4 inches below ground level and set the post on a gravel base, and its data sheet says two 50 lb bags will set a 3-inch square post in a 9-inch hole 30 inches deep. In practice, expect 2 to 3 bags of fast-setting mix per 4×4 post. Unopened spare bags can usually be returned.
How wide and how deep to dig
Quikrete's post-setting guide says to make the hole about three times the width of the post, with a depth of one-third of the overall post length, plus 6 inches for a gravel base. For a 6 ft fence on 8 ft posts, that means about 2½–3 ft of post in the ground.
- Gravel first. Tamp the bottom of the hole and add 6 inches of gravel or crushed stone so water drains away from the end of the post.
- Frost line. In cold climates, posts that support a structure, such as a deck, must go at least 6 inches below the local frost line. Ask your building department for the frost depth where you live.
- Call 811 before you dig. It's the free national call-before-you-dig service, and it covers fences and mailboxes as well as bigger projects. Contact 811 a few business days before you start, and the utility companies will mark buried lines for free.
Concrete for a whole fence
A typical 100 ft backyard fence with posts every 8 ft, two corners and one gate needs 16 posts. With 4×4 posts set 2 ft deep in 10-inch holes, that's 14.7 cu ft of concrete, or 40 bags of 50 lb fast-setting mix. With posts every 6 ft instead, it's 20 posts and 50 bags.
Don't forget the gravel: a 6-inch base in a 10-inch hole is about a quarter of a cubic foot per post, or roughly 4 cu ft for those 16 posts. Our gravel calculator converts that into bags or tons. Enter your own fence length, post spacing and post size in the fence materials calculator to get the posts, concrete, rails and pickets in one list.
Fast-setting or regular concrete?
- Fast-setting mix is the easiest for fence posts. You pour it into the hole dry, then add water, about 1 gallon per 50 lb bag according to Quikrete, and it sets in roughly 20–40 minutes. Wait about 4 hours before hanging heavy gates or other loads on the post.
- Regular concrete mix gives more volume per bag and suits big holes, gate posts and corner posts. You mix it first, in a wheelbarrow or mixer, and the post needs bracing for longer while it sets.
- Holes deeper than about 2½ ft: Quikrete recommends filling in layers. Add mix up to 30 inches deep, let the water soak all the way in, then add the rest.
Tips for posts that stay straight
- Brace before you pour. Plumb each post on two adjacent faces with a level and hold it with temporary braces until the concrete sets.
- Set the corner and end posts first, run a string line between them, then line up the posts in between.
- Crown the top. Slope the concrete or soil away from the post at the surface so rainwater runs off instead of pooling around the wood.
- Use ground-contact rated posts. Pressure-treated posts rated for ground contact, or galvanized steel posts, last far longer in the ground.
Pouring a slab or a footing instead of post holes? See how much concrete you need for slabs, footings and other shapes.
Sources & further reading
- QUIKRETE Fast-Setting Concrete Mix No. 1004 – Data Sheet — QUIKRETE
- Setting Posts (project guide) — QUIKRETE
- 811 – Know What's Below. Contact Before You Dig. — Common Ground Alliance