Bags per post hole by hole size
The concrete is the hole minus the post. A 10 inch hole 30 inches deep is 1.36 cubic feet, a 4 × 4 post (3.5 inches actual) takes up 0.21 of it, and the 1.15 cubic feet left is just under two 80 lb bags. Round up per hole, not per job, because a part bag left in the barrow doesn’t carry to the next hole.
Bag yields are the manufacturers’ own. An 80 lb bag of standard mix gives 0.60 cubic feet, a 60 lb bag 0.45, and a 50 lb bag of Quikrete Fast-Setting Concrete Mix 0.375. In Canada the 30 kg bag gives 0.5 cubic feet and the 25 kg bag 0.41.
Concrete volume is the hole less the post (a 4 × 4 really measures 3.5 in). Fast-setting mix yields 0.375 cu ft per 50 lb bag. Standard mix gives 0.60 per 80 lb and 0.45 per 60 lb.
| Hole | Concrete | 80 lb bags | 60 lb bags | 50 lb fast-set |
|---|---|---|---|---|
| 8 in × 24 in | 0.53 cu ft | 1 | 2 | 2 |
| 10 in × 24 in | 0.92 cu ft | 2 | 3 | 3 |
| 10 in × 30 in | 1.15 cu ft | 2 | 3 | 4 |
| 12 in × 30 in | 1.75 cu ft | 3 | 4 | 5 |
| 12 in × 36 in | 2.10 cu ft | 4 | 5 | 6 |
| 16 in × 36 in | 3.93 cu ft | 7 | 9 | 11 |
A 6 × 6 post in a 16 inch hole 36 inches deep is the big one on the table at 3.93 cubic feet, which is seven 80 lb bags or eleven 50 lb fast-setting bags. Gate posts and corner posts usually get that treatment, and it’s the reason fence quotes carry a separate line for them.
How much Quikrete per post
Quikrete’s sheet for Fast-Setting Concrete Mix says two 50 lb bags will set a 3 inch square post in a 9 inch hole 30 inches deep. That’s less than the table above because the hole is narrower and the fill stops short. Quikrete’s method pours the dry mix to 3 or 4 inches below the top of the hole and backfills the rest with soil or sod.
Do the arithmetic the same way and the two agree. A 9 inch hole with 26 inches of concrete around a 3 inch post is 0.82 cubic feet, which is 2.2 bags at 0.375 cubic feet each. Quikrete calls that two. Widen the hole to 10 inches, use a real 3.5 inch 4 × 4 and fill to the top, and you’re at 1.15 cubic feet and four bags.
So when a bag says two per post and the calculator says four, neither is wrong. Buy against the hole you dig. If you’re digging 10 inch holes to the recommended three times the post width and filling them, it’s three to four 50 lb bags, and the lower count only holds for the narrower hole and the short fill.
Bags for a whole fence
Posts at 8 ft centres means a run divided by 8, plus one for the end. A 100 ft run is 14 posts. At two 80 lb bags each that’s 28 bags, or 56 bags of 50 lb fast-setting mix if every hole is filled, or 28 on Quikrete’s method. Add the gate posts and corners separately at the 6 × 6 rate.
Buy the per-hole count times the post count when you’re dry-pouring, because a dry bag can’t be split between holes. If you’re mixing in a barrow the pooled total works and you’ll save a few bags over a long run.
| Fence run | Posts at 8 ft | 80 lb bags | 50 lb fast-set, hole filled | 50 lb fast-set, Quikrete method |
|---|---|---|---|---|
| 24 ft | 4 | 8 | 16 | 8 |
| 50 ft | 8 | 16 | 32 | 16 |
| 100 ft | 14 | 28 | 56 | 28 |
| 150 ft | 20 | 40 | 80 | 40 |
| 200 ft | 26 | 52 | 104 | 52 |
| 300 ft | 39 | 78 | 156 | 78 |
At 6 ft spacing, for a taller fence or a windy site, a 100 ft run is 18 posts and the bag counts scale with it.
The dry-pour method, from the data sheet
Fast-setting mix is designed to go in dry. Dig the hole about three times the post width and a third of the post height deep. Stand the post, brace it plumb, and pour the dry mix in to 3 or 4 inches below grade. Add about a gallon of water per 50 lb bag and let it soak through. For holes deeper than 30 inches, fill to 30, let the water soak in, then add the rest.
It sets in 20 to 40 minutes and Quikrete says to wait 4 hours before putting any strain on the post. At 24 hours it’s at 1,000 psi, and 4,000 at 28 days. Standard mix should be mixed wet before it goes in, and that’s the one to use when the holes are big enough that the water can’t reach the bottom of a dry pour.
Put 6 inches of gravel in the bottom first. It lets water drain off the end grain instead of sitting in a concrete cup, and it’s the cheapest thing you can do for a wooden post.
How big to dig the hole
Three times the post width across, and a third of the post below ground. A 6 ft fence on 8 ft posts wants a hole 10 to 12 inches wide and 2 ft deep. In frost country the bottom of the hole goes below the frost line whatever the post height says, which means 36 to 48 inches across the northern states and 42 inches and more in most of Canada.
Deeper holes take more concrete than the one-third rule suggests, so check the table at the depth your frost line sets, not the depth the post height would give you.
Sources
Bag yields and the two-bags-per-post figure are from Quikrete’s Fast-Setting Concrete Mix No. 1004 data sheet and Sakrete’s product page. The hole volumes are the post hole calculator’s own arithmetic.
Quikrete Fast-Setting Concrete Mix product page ↗Quikrete Fast-Setting Concrete Mix No. 1004 data sheet (PDF) ↗Sakrete Fast-Setting Concrete Mix (bag yield) ↗IRC R507.3 deck footings (UpCodes) ↗
Run your numbers
Hole diameter, depth, post size and bag size in, bags per hole and for the whole job out. Then the fence calculator for the posts, rails and pickets of the run.
Common questions
How many bags of concrete for a 4x4 fence post?
Two 80 lb bags for a 10 inch hole 30 inches deep, filled to the top. In 50 lb fast-setting bags it’s four for the same hole, or two on Quikrete’s method with a 9 inch hole and the fill stopped 3 to 4 inches short.
How many 50 lb bags of Quikrete per fence post?
Quikrete says two 50 lb bags set a 3 inch post in a 9 inch hole 30 inches deep. For a 10 inch hole filled to grade around a real 3.5 inch 4 × 4 it’s four. Three is the usual compromise for a 10 inch hole with the fill stopped short.
How much concrete for a 6x6 post?
A 16 inch hole 36 inches deep takes 3.93 cubic feet once the post is subtracted, which is seven 80 lb bags, nine 60 lb or eleven 50 lb fast-setting bags. Gate and corner posts usually get this size.
How many bags of concrete for 12 fence posts?
About 24 bags of 80 lb mix for 10 by 30 inch holes with 4 × 4 posts. Dry-pouring fast-setting mix, buy 48 of the 50 lb bags for filled holes, or 24 on Quikrete’s shorter fill.
Can I pour Quikrete dry into a post hole?
Yes, with the fast-setting mix. That’s the method on the bag: brace the post, pour the dry mix to 3 or 4 inches below grade, add a gallon of water per 50 lb bag and wait 4 hours before loading the post. Standard mix should be mixed wet first.
How long before a post set in fast-setting concrete can take a load?
Quikrete sets in 20 to 40 minutes and says to wait 4 hours before any strain. It reaches 1,000 psi at 24 hours. Hang the gate or the panels the next day to be safe.
Should I fill the post hole to the top with concrete?
Quikrete stops the fill 3 to 4 inches below grade and backfills with soil, which hides the concrete and uses fewer bags. Filling to the top and crowning it so water runs off keeps the post drier. Either way, put gravel in the bottom.