How many bags of concrete do I need for a 4x4 fence post?

For the common 10 inch hole, 30 inches deep: 3 bags of 80 lb, 3 of 60 lb or 4 of 50 lb fast-setting mix, including 10% waste. An 8 inch hole 24 inches deep gets away with 2 bags of 50 lb.

How many bags of concrete for a 6x6 post?

A 6x6 in a 12 inch hole 36 inches deep needs 4 bags of 80 lb or 6 of 50 lb; widen the hole to 16 inches and it is 7 bags of 80 lb. Gate posts are worth the extra bags because the gate pulls on them every day.

How deep should a fence post hole be for concrete?

The usual rule is one third of the post length in the ground, with a minimum of 24 inches (600 mm) and always below the frost line where the ground freezes. A 6 ft fence on 8 ft posts is set about 30 to 36 inches deep. Check your local code; some areas set the depth for you.

Can I pour dry concrete into a post hole?

Fast-setting post mix is made for that: pour it dry around the braced post, then add the amount of water printed on the bag. Regular concrete mix should be mixed first. The bag count is the same either way because it is the volume of the hole that decides it.

Do steel and vinyl posts need the same amount of concrete as wood?

Slightly more, because a round steel post or a hollow vinyl post displaces less concrete than a solid 4x4. Use the table row for the hole size and add the half bag the wooden post would have taken up; the concrete calculator gives the full hole volume, which is the safe figure for any post.

How much gravel do I need in the bottom of a post hole?

About 3 to 4 inches (75 to 100 mm) of gravel so the end of the post drains: for a 10 inch hole that is roughly a quarter of a cubic foot per post, or one small bag for every two posts. It is separate from the concrete and not included in the tables.

How many bags of concrete per fence post?

A 4x4 fence post set 30 inches deep in a 10 inch hole takes about 1.27 cubic feet of concrete once the post itself is allowed for: 3 bags of 80 lb, 3 of 60 lb or 4 of 50 lb fast-setting mix, with 10% waste. Smaller holes need less, 6x6 posts and deeper holes need more; the tables below cover the usual combinations in bags and in litres.

Open the concrete calculator

Bags per 4x4 post

The hole is treated as a cylinder and the post as a square prism that stays inside it, so the concrete is the hole minus the post, plus 10% for the hole that is never quite round. Bag yields are the figures printed on typical bags: 0.60 cubic feet for 80 lb, 0.45 for 60 lb and 0.375 for 50 lb fast-setting mix. Yields vary a little by brand, so check the bag and round up.

4x4 post (3.5 x 3.5 in actual), 10% waste
Hole	Depth	Concrete (cu ft)	80 lb bags	60 lb bags	50 lb bags
8 in	24 in	0.58	1	2	2
10 in	24 in	1.01	2	3	3
10 in	30 in	1.27	3	3	4
12 in	30 in	1.93	4	5	6
12 in	36 in	2.31	4	6	7

Bags per 6x6 post

Gate posts, corner posts and anything carrying a heavy fence are usually 6x6 in a wider, deeper hole. The post displaces more concrete, but the hole grows faster than the post does.

6x6 post (5.5 x 5.5 in actual), 10% waste
Hole	Depth	Concrete (cu ft)	80 lb bags	60 lb bags	50 lb bags
12 in	30 in	1.58	3	4	5
12 in	36 in	1.9	4	5	6
14 in	36 in	2.83	5	7	8
16 in	36 in	3.91	7	9	11

Getting the hole size right

Two separate rules decide the hole. The diameter is about three times the post width: 10 to 12 inches for a 4x4, 12 to 16 inches for a 6x6. The depth is a third of the post length, at least 24 inches, and below the frost line in cold climates. Mixing the two up is how people end up with a 12 inch hole that is only 18 inches deep: stiff at the top, loose at the bottom.

Bags for the whole fence

With posts on 8 ft centres, a fence of a given length needs one more post than it has panels. The figures use the typical 4x4 hole above (10 inches wide, 30 inches deep). The concrete calculator multiplies any hole size by the number of posts and lets you set the waste.

Fence with 4x4 posts at 8 ft centres, 10 in hole, 30 in deep
Fence length	Posts	80 lb bags	60 lb bags	50 lb bags
50 ft	7	21	21	28
100 ft	13	39	39	52
150 ft	19	57	57	76
200 ft	26	78	78	104

Beyond about 30 posts you are carrying and mixing well over a ton of bags. A ready-mix truck can fill post holes from a wheelbarrow, but the holes must all be dug and the posts braced before it arrives, so most fencing crews stay with bags and a mixer.

Metric: 100 mm post in a 300 mm hole

A 100 x 100 mm fence post (about 95 mm actual) is usually set 600 to 750 mm deep in a 250 to 350 mm hole. Bags of 25 kg yield about 12 litres of concrete and 20 kg bags about 10 litres; post mix sold in 20 kg bags is in the same range, but check the bag.

100 mm post, 10% waste
Hole	Depth	Concrete (L)	25 kg bags	20 kg bags
250 mm	600 mm	26	3	3
300 mm	600 mm	41	4	5
300 mm	750 mm	51	5	6
350 mm	750 mm	72	6	8

Before you dig

Call the utility locating service before digging post holes; it is free in most countries and the lines are shallower than you think.
Put 3 to 4 inches (75 to 100 mm) of gravel in the bottom of the hole so the end grain of the post drains. That gravel is not concrete and is not in the tables; the gravel calculator will do it by the hole.
Keep the top of the concrete just above ground level and slope it away from the post so water runs off rather than pooling against the wood.
Brace each post plumb before the concrete sets: fast-setting mix gives you 20 to 40 minutes, regular mix a few hours. Hang the panels the next day.
Pouring a slab as well? The 10x10 slab guide uses the same bag yields.

Frequently asked questions

How many bags of concrete do I need for a 4x4 fence post?: For the common 10 inch hole, 30 inches deep: 3 bags of 80 lb, 3 of 60 lb or 4 of 50 lb fast-setting mix, including 10% waste. An 8 inch hole 24 inches deep gets away with 2 bags of 50 lb.
How many bags of concrete for a 6x6 post?: A 6x6 in a 12 inch hole 36 inches deep needs 4 bags of 80 lb or 6 of 50 lb; widen the hole to 16 inches and it is 7 bags of 80 lb. Gate posts are worth the extra bags because the gate pulls on them every day.
How deep should a fence post hole be for concrete?: The usual rule is one third of the post length in the ground, with a minimum of 24 inches (600 mm) and always below the frost line where the ground freezes. A 6 ft fence on 8 ft posts is set about 30 to 36 inches deep. Check your local code; some areas set the depth for you.
Can I pour dry concrete into a post hole?: Fast-setting post mix is made for that: pour it dry around the braced post, then add the amount of water printed on the bag. Regular concrete mix should be mixed first. The bag count is the same either way because it is the volume of the hole that decides it.
Do steel and vinyl posts need the same amount of concrete as wood?: Slightly more, because a round steel post or a hollow vinyl post displaces less concrete than a solid 4x4. Use the table row for the hole size and add the half bag the wooden post would have taken up; the concrete calculator gives the full hole volume, which is the safe figure for any post.
How much gravel do I need in the bottom of a post hole?: About 3 to 4 inches (75 to 100 mm) of gravel so the end of the post drains: for a 10 inch hole that is roughly a quarter of a cubic foot per post, or one small bag for every two posts. It is separate from the concrete and not included in the tables.

How many bags of concrete for a 10x10 slab?
Bags of 80, 60 and 40 lb concrete for a 10 x 10 ft slab at 4, 5 and 6 inches with 10% waste, plus the metric 3 x 3 m version in 25 kg and 20 kg bags.
How many bags of concrete in a cubic yard?
80, 60, 50 and 40 lb bags per cubic yard and per cubic foot, 25 kg and 20 kg bags per cubic metre, what a yard of bags weighs, and when a ready-mix truck makes sense.
How much concrete for a 12x12 slab? (plus 16x16, 20x20, 24x24)
Cubic yards and bags of concrete for 12x12, 16x16, 20x20 and 24x24 ft slabs at 4 and 6 inches with 10% waste, the 4 x 4 m metric slab, and the point where ready-mix wins.
How many bags of concrete for a sonotube? 8, 10, 12 and 16 inch
Bags of 80, 60 and 50 lb concrete for 8, 10, 12 and 16 inch sonotubes at 2, 3 and 4 ft, concrete per foot of tube, and 200 to 300 mm forms in 25 kg bags.

Last updated 2026-09-12. Figures are planning estimates from typical product data: check the yield, size or coverage printed on what you buy and round up.