Fence Post Concrete Calculator

How to use this calculator

You need four numbers: how many posts you are setting, what size the posts are, how wide you are digging the holes, and how deep. Pick your post from the list — the calculator uses the actual dimensions, so a “4×4” is treated as the 3½ inches it really measures, not 4 — and it subtracts that from the hole for you.

If you are putting gravel in the bottom of the hole for drainage, enter that depth too. The gravel displaces concrete, and most calculators ignore it, which is why their numbers come out high.

The formula, explained

A post hole is a cylinder with a post standing in the middle of it. The concrete fills what is left:

Hole area = π × (diameter ÷ 2)²
Concrete volume = (hole area − post cross-section) × concrete depth
Cubic inches ÷ 1,728 = cubic feet

The 1,728 is simply 12 × 12 × 12 — the number of cubic inches in a cubic foot. Concrete depth is the hole depth minus any gravel base, because the post sits on top of the gravel and the concrete starts there.

The part people skip is subtracting the post. On a 4×4 in a 10-inch hole the post takes up about 16% of the volume. Ignore it and you buy roughly one extra bag for every six posts.

Worked example: a 4×4 post in a 10-inch hole, 24 inches deep

1. Hole area: π × (10 ÷ 2)² = 78.54 square inches
2. Post cross-section: a 4×4 actually measures 3.5 × 3.5 = 12.25 square inches
3. Open area: 78.54 − 12.25 = 66.29 square inches
4. Volume: 66.29 × 24 = 1,591 cubic inches
5. In cubic feet: 1,591 ÷ 1,728 = 0.92 cubic feet
6. In bags: 0.92 ÷ 0.6 = 2 bags of 80 lb mix, or 0.92 ÷ 0.45 = 3 bags of 60 lb

For a 20-post fence that is 40 bags of 80 lb mix — about a pallet, and worth knowing before you get to the register.

How deep should a fence post hole be?

The standard rule, and the one Quikrete publishes, is to bury one third of the overall post length, then add about 6 inches to the hole for a gravel base. So a post with 6 feet showing above ground is a 9-foot post set 3 feet deep, in a 42-inch hole.

Height above ground	Post length needed	Set depth (⅓ of post)	Hole depth with 6 in gravel
3 ft	4 ft 6 in	18 in	24 in
4 ft	6 ft	24 in	30 in
5 ft	7 ft 6 in	30 in	36 in
6 ft	9 ft	36 in	42 in
8 ft	12 ft	48 in	54 in

Two things complicate that table in practice.

Posts come in even lengths. Lumberyards sell 6, 8, 10 and 12-foot posts, so a 6-foot fence usually means an 8-foot post set 24 inches, not a 9-foot post set 36. That is shallower than the one-third rule and it is what most residential fences are built on. It holds up fine in stable soil with a gate-free run; it is the corner posts, end posts and gate posts that need the extra depth.

Frost depth overrides everything. In a climate that freezes, the bottom of the hole has to sit below the local frost line or the ground will lift the post out over a few winters. That depth is set by your local building department, not by a rule of thumb, and in the northern United States it can be 42 to 48 inches. Call and ask before you dig — it is a two-minute phone call that decides your whole material order.

How wide should the hole be?

Three times the width of the post. That gives you enough concrete around the post to resist the sideways force the fence puts on it, and enough room to actually get the post plumb.

Post	Actual size	Hole diameter
2⅓ in metal post (chain link)	2.375 in diameter	8 in
4×4	3.5 × 3.5 in	10–12 in
4×6	3.5 × 5.5 in	12–16 in
6×6	5.5 × 5.5 in	16–18 in

Widening the hole costs you concrete fast, because volume goes up with the square of the diameter. Going from a 10-inch hole to a 12-inch one on the same 24-inch depth takes you from 0.92 to 1.40 cubic feet — more than half as much again for two inches. Dig the hole you need, not the widest one the auger will make.

How many bags of concrete per fence post?

For the most common combinations, assuming no gravel base and no waste allowance:

Post	Hole	Depth	Concrete	80 lb	60 lb	50 lb	40 lb
2⅓ in metal	8 in	24 in	0.64 cu ft	2	2	2	3
4×4	10 in	24 in	0.92 cu ft	2	3	3	4
4×4	10 in	30 in	1.15 cu ft	2	3	4	4
4×4	12 in	30 in	1.75 cu ft	3	4	5	6
4×4	12 in	36 in	2.10 cu ft	4	5	6	7
6×6	12 in	30 in	1.44 cu ft	3	4	4	5
6×6	16 in	36 in	3.56 cu ft	6	8	10	12
6×6	18 in	42 in	5.45 cu ft	10	13	15	19

The short version most people are looking for: a standard 4×4 fence post takes two 80 lb bags, or three 60 lb bags. Deeper holes for gate posts and corner posts push that to three or four.

Bag yields, by size

Bags are sold by weight, but concrete is bought by volume, so the yield printed on the bag is the number that matters.

Bag	Yield	Bags per cubic foot	Bags per cubic yard
80 lb	0.60 cu ft	1.7	45
60 lb	0.45 cu ft	2.2	60
50 lb	0.375 cu ft	2.7	72
40 lb	0.30 cu ft	3.3	90

These are the standard yields for concrete mix and fast-setting mix. They vary slightly by manufacturer and by how wet you mix it, so check the bag.

Do you need gravel in the bottom of the hole?

For a wood post, yes — 4 to 6 inches of compacted gravel. Quikrete's own instructions build it into the hole depth. The reason is drainage: concrete poured straight onto soil makes a bowl that holds water around the bottom of the post, which is exactly where rot starts. Gravel lets that water drain away.

It also saves you concrete. Six inches of gravel in a 10-inch hole is 0.23 cubic feet of volume you do not have to fill with mix — about a quarter of a bag per post, which on a 20-post fence is five bags.

Metal posts do not rot, so the gravel matters less, though it still helps with frost movement. Size the gravel for the whole job with the gravel calculator.

Dry pour or mixed?

For fence posts specifically, you can pour the mix in dry and add water afterwards — but only with a product made for it. Quikrete's Fast-Setting Concrete Mix is designed this way: set the post, fill the hole with dry mix to about 3 to 4 inches below ground level, then add water, roughly a gallon per 50 lb bag, and let it soak in. It sets in 20 to 40 minutes.

Two cautions. First, on holes deeper than about 2½ feet, Quikrete says to do it in lifts — fill and wet the first 2½ feet, wait for the water to soak all the way through, then add the rest. A single deep dump leaves dry pockets at the bottom. Second, this is a property of fast-setting mix, not of concrete in general. Standard concrete mix has to be mixed with water before it goes in the hole.

Because the mix stops 3 to 4 inches short of grade, the real volume on a dry pour is a little under what this calculator reports. If you want to match it exactly, enter a hole depth 3 inches shallower than you dug.

Why the calculator rounds up per post, not per job

Say each post needs 0.92 cubic feet and you have ten of them. That is 9.2 cubic feet in total, which divides into 16 bags of 80 lb mix. But you are not pouring one 9.2-cubic-foot block — you are pouring ten separate holes, and each one needs 2 bags, which is 20.

The gap is real waste, and you cannot avoid most of it. Mixed concrete starts setting immediately, so the leftover from post one is useless by the time you get to post three. The only way to recover it is to batch in a mixer and pour several posts from each load, working fast. If you are doing that, the calculator shows the batched figure in the breakdown.

Buy the per-post number. Running one bag short halfway down a fence line, with eight posts braced and waiting, is a far worse afternoon than having two bags left over.

Common mistakes

• Using nominal post dimensions. A 4×4 is 3½ inches and a 6×6 is 5½. Using 4 and 6 overstates the displacement and leaves you short.
• Forgetting that gate and corner posts are different. They carry far more load and are normally set deeper and in a wider hole — often a 6×6 where the line posts are 4×4. Calculate those separately.
• Digging a cone instead of a cylinder. A hand digger tends to flare the hole at the top, which quietly adds volume. If your holes are visibly wider at the surface, average the two diameters.
• Crowning the concrete and not counting it. Finishing the concrete slightly above grade so water runs away from the post is good practice, but it is extra volume the hole measurement does not include.
• Ignoring the frost line. A post set above frost depth in a cold climate will heave, and no amount of concrete fixes it.

A note on accuracy

This calculator works from the hole you tell it about. Real holes are rarely perfect cylinders, soil collapses in, augers wander, and a dry pour stops short of grade. Treat the result as a close planning figure, add 5 to 10% if your holes are hand-dug, and round up when you buy. Bag yields are manufacturer figures and vary a little in practice, so check what is printed on the bag you are actually buying.

Where these figures come from

The hole sizing rules and the dry-pour method built into this page come from the published sources below. The arithmetic itself — cylinder volume, minus the post, converted to cubic feet and divided by the bag yield — is straightforward geometry, and you can check every step in the full working the calculator shows. Our worked example has been cross-checked against an independent calculator and agrees to three decimal places.

• Setting Posts in Concrete Source: Quikrete. The manufacturer's own method: hole three times the post diameter, depth one third of the post length plus 6 inches of gravel, and the dry-pour sequence for fast-setting mix.
• Post Hole Concrete Calculator Source: Inch Calculator. Independent implementation of the same hole-minus-post formula, used here to verify our worked example.
• Concrete in Practice (CIP) series Source: National Ready Mixed Concrete Association. Technical bulletins on placing, curing and cold-weather concreting.
• Working With Concrete Source: Portland Cement Association. Background on mix proportions, yields and curing.

Frequently asked questions

How many bags of concrete do I need per fence post?

Two 80 lb bags for a standard 4x4 post in a 10-inch hole dug 24 inches deep, which works out to 0.92 cubic feet of concrete. The same post needs three 60 lb bags or four 40 lb bags. Gate posts and corner posts are usually set deeper in a wider hole and take three or four 80 lb bags.

How deep should a fence post hole be?

Bury one third of the overall post length, then add about 6 inches for a gravel base. A 6-foot fence means a 9-foot post set 36 inches deep in a 42-inch hole. In practice most residential fences use an 8-foot post set 24 inches. In a freezing climate the bottom of the hole must sit below the local frost line, which your building department sets and which can be 42 to 48 inches.

How wide should a fence post hole be?

Three times the width of the post. A 4x4 wants a 10 to 12 inch hole, a 6x6 wants 16 to 18 inches, and a 2-3/8 inch metal chain link post wants about 8 inches. Going wider costs concrete quickly, because volume rises with the square of the diameter.

Do I subtract the post from the hole volume?

Yes. A 4x4 post takes up about 16% of a 10-inch hole, so ignoring it means buying roughly one extra bag for every six posts. Use the actual post size, not the nominal one: a 4x4 measures 3.5 inches and a 6x6 measures 5.5 inches.

Can I pour concrete dry into a fence post hole?

With fast-setting mix, yes. Quikrete's Fast-Setting Concrete Mix is made for it: set the post, fill the hole with dry mix to 3 to 4 inches below grade, then add about a gallon of water per 50 lb bag and let it soak in. On holes deeper than about 2.5 feet, do it in lifts so the bottom does not stay dry. Standard concrete mix must be mixed with water first.

Should I put gravel in the bottom of a post hole?

For wood posts, yes, 4 to 6 inches of compacted gravel. Concrete poured straight onto soil forms a bowl that traps water against the bottom of the post, which is where rot starts. Gravel drains it away, and it also saves you about a quarter of a bag of concrete per post.

How much concrete for a 6x6 post?

A 6x6 in a 16-inch hole 36 inches deep takes 3.56 cubic feet, or six 80 lb bags. In a 12-inch hole 30 inches deep it is 1.44 cubic feet, or three 80 lb bags. The 6x6 displaces 30.25 square inches of the hole, against 12.25 for a 4x4.

Why does the total round up per post instead of across the job?

Because each hole is mixed and placed on its own, and leftover concrete from one post has started setting before you reach the next. Ten posts at 0.92 cubic feet each need 2 bags apiece, which is 20 bags, not the 16 the raw total suggests. If you are batching in a mixer and pouring several posts per load you can approach the lower figure, which the breakdown shows.