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

For the common case of a 4×4 post in a 10 inch hole 24 inches deep with no gravel, about 0.87 ft³ of concrete, which is one and a half 80 lb bags — so two bags per hole in practice, since you cannot buy half a bag. Go up to a 12 inch hole 30 inches deep and it rises to roughly 1.46 ft³ per hole, or between two and three 80 lb bags. The hole diameter matters far more than the depth, because volume grows with the square of the diameter but only linearly with depth.

How deep should a post hole be?

Deep enough to clear the frost line, and at least a third of the height the post stands above ground. For a 6 foot fence that means about 2 feet buried as a minimum from the one-third rule, but if your frost line is at 36 or 48 inches then the frost line wins. Concrete set above the frost line gets heaved upward a little every time the ground freezes and expands, and over a few winters the posts visibly lift and the fence line goes crooked.

How wide should the hole be?

About three times the width of the post. A 4×4 post wants a 12 inch hole, a 6×6 post an 18 inch one. That leaves roughly a hand-width of concrete all the way round, which is what actually resists the post being levered sideways when wind loads the fence. Narrower than about two and a half times and the collar is too thin to do much; much wider than four times and you are buying concrete that adds very little strength for the money.

Do I need gravel at the bottom of the hole?

For timber posts, yes, and it is worth the small extra effort. Four to six inches of clean crushed gravel lets water drain away from the end grain of the post instead of collecting in a concrete cup around it. Standing water at the base is the single most common reason wooden posts rot out from the bottom while the visible part still looks sound. The gravel also gives the post something firm to bear on so it does not settle over time.

Should I subtract the volume of the post?

Yes, unless you are setting the post after the concrete has gone off. A 4×4 post in a 12 inch hole displaces around 12% of the total volume, and a 6×6 in the same hole displaces closer to 28% — enough to change how many bags you buy. The subtlety is that the post only displaces concrete over the length actually surrounded by it. If it stands on a 4 inch gravel base in a 30 inch hole, that is 26 inches of displacement, not 30.

Can I pour dry concrete into the hole and add water after?

It is a widely used method for fence posts and manufacturers sell fast-setting mixes specifically for it. Tip the dry mix in around the braced post, then add water on top and let it soak down. It is quicker and there is no mixing to do. The trade-off is that you have far less control over how thoroughly the mix is wetted, so the finished strength is more variable than properly mixed concrete. For a fence it is generally fine; for a structural deck post carrying real load, mix it properly.

How long before I can remove the bracing?

Fast-setting mixes hold a post firmly within 20 to 40 minutes and you can usually take the braces off after about four hours, but do not hang gates, panels or any load for at least 24 hours. Standard concrete mixes need longer — leave the bracing for 24 hours and wait a full week before applying real load. Concrete reaches only about two thirds of its final strength in the first week, and the rest arrives slowly over the following three.

Should the concrete finish above or below ground level?

Slightly above, and sloped away from the post. Crowning the top so water runs off is a small detail that meaningfully extends the life of a timber post, because a flat or dished top holds a puddle right against the wood every time it rains. Finishing below ground level is worse still, since soil then sits in the dip and stays permanently damp against the post.

How much wastage should I allow?

Ten percent is a sensible default for post holes, which is more than you would allow for a slab. Holes are rarely the neat cylinder you planned — soft ground caves slightly, augers wander, and a hole that ends up an inch wider than intended needs noticeably more concrete because the volume goes with the square of the diameter. Running one bag short with six posts braced and waiting is a genuinely bad afternoon.

Post Hole Concrete Calculator

Setting a fence post, a deck post or a mailbox looks like a simple volume problem until you actually stand at the merchant's counter trying to decide how many bags to load. A post hole is not a plain cylinder of concrete. The post itself sits in the middle of it and displaces a surprising amount, an...

THE HOLES

NUMBER OF POSTS

HOLE DEPTH (in)

HOLE SHAPE

HOLE DIAMETER (in)

POST SHAPE

POST WIDTH (in)

The hole is 3.0 times the post width, which is in the normal range of about three times.

GRAVEL BASE (in)

POST ABOVE GROUND (in)

30 in buried against 48 in standing proud satisfies the one-third rule.

WASTAGE %

BAG SIZE

CONCRETE TO BUY

17 bags

16.1 exact · 80 lb each · yields 0.600 ft³ per bag

PER HOLE

1.461 ft³

ALL 6 HOLES

8.766 ft³

+10% WASTE

9.642 ft³

Hole volume each

1.963 ft³

Less gravel base

0.262 ft³

Less post

0.241 ft³

Concrete each

1.461 ft³

Embedded post length

26.0 in

Gravel for all holes

1.571 ft³

Total concrete

9.642 ft³

Also equals

0.273 m³

SETTING THE POSTS

Dig or auger 6 holes at 12 in diameter, 30 in deep.

Tip 4 in of clean gravel into the bottom of each and tamp it flat. This keeps standing water away from the end of the post.

Stand the post on the base, brace it plumb, and check it against the string line before any concrete goes in. Concrete gives you about twenty minutes.

Fill with 17 bags total. Crown the top so water runs away from the post rather than sitting against it.

THE HOLE

Why this is not just a cylinder. The hole holds 1.963 ft³, but the post occupies 0.241 ft³ of that and the gravel base another 0.262 ft³. What you actually buy is the remainder, 1.461 ft³ per hole. Note the post stands on the gravel, so it displaces concrete over 26.0 in, not the full depth.

Bag yields are the finished concrete volume after water is added, not the dry weight — an 80 lb bag makes about 0.600 ft³. Buy whole bags; a part bag left open goes off.

Live calculation · updates as you type

Created with❤️byeaglecalculator.com

HOW TO USE

1
Enter how many posts you are setting and how deep the holes are. Depth should reach below the frost line for your area, and be at least a third of the height the post will stand above ground.
2
Choose the hole shape and size. Augered holes are round; hand-dug ones are usually closer to square. A diameter of about three times the post width is the normal target.
3
Set the post shape and size so the calculator can deduct what the post itself occupies. Choose "Ignore post" if you want the raw hole volume instead — useful when the post goes in after the concrete has set.
4
Add a gravel base depth if you are using one, pick your bag size, and read off the number of bags. Adjust the wastage allowance if the ground is soft and the holes are likely to over-dig.

THE FORMULAS

Round hole volumeπ × (d/2)² × depth

Square hole volumewidth² × depth

Gravel base volumehole area × gravel depth

Embedded post lengthdepth − gravel depth

Post displacementpost area × embedded length

Concrete per holehole − gravel − post

Total concreteper hole × posts × (1 + waste)

Bags requiredtotal volume / bag yield

BAG YIELD REFERENCE

Bag	Yield (ft³)	Yield (m³)	Bags per ft³	Bags per m³
40 lb	0.300	0.00850	3.33	117.7
50 lb	0.375	0.01062	2.67	94.2
60 lb	0.450	0.01274	2.22	78.5
80 lb	0.600	0.01699	1.67	58.9
20 kg	0.331	0.00936	3.02	106.8
25 kg	0.413	0.01171	2.42	85.4
30 kg	0.496	0.01405	2.02	71.2
40 kg	0.661	0.01873	1.51	53.4

Yield is the volume of finished concrete a bag makes once water is added, which is why an 80 lb bag produces only 0.6 ft³ rather than the larger figure its weight might suggest. Brands vary by a few percent; check the bag if you are ordering close to the line.

FREQUENTLY ASKED QUESTIONS

RELATED CALCULATORS

Concrete Slab Volume Calculator

Calculate instantly →

Concrete for 10m × 10m Slab Calculator

Calculate instantly →

Concrete for 4m × 4m Slab Calculator

Calculate instantly →

Concrete for 12m × 12m Slab Calculator

Calculate instantly →

MORE CONCRETE CALCULATORS

Concrete Slab Volume Calculator

Calculate instantly →

Concrete Footing Volume Calculator

Calculate instantly →

Concrete Bags Calculator

Calculate instantly →

Concrete Patio Calculator

Calculate instantly →

Concrete Driveway Calculator

Calculate instantly →

Concrete Mix Ratio Calculator

Calculate instantly →

Concrete Steps Calculator

Calculate instantly →

Rebar Quantity Calculator

Calculate instantly →

Cement Render Calculator

Calculate instantly →

Ready-Mix Concrete Cost Calculator

Calculate instantly →

Concrete Wall Volume Calculator

Calculate instantly →

Concrete Path Calculator

Calculate instantly →

Pool Deck Concrete Calculator

Calculate instantly →

Concrete for 3m × 3m Slab Calculator

Calculate instantly →

Concrete for 4m × 4m Slab Calculator

Calculate instantly →

Concrete for 5m × 5m Slab Calculator

Calculate instantly →

Concrete for 6m × 6m Slab Calculator

Calculate instantly →

Concrete for 8m × 8m Slab Calculator

Calculate instantly →

Concrete for 10m × 10m Slab Calculator

Calculate instantly →

Was this calculator helpful?

Last updated: July 29, 2026 · Formula verified · Eagle-eyed accuracy for every calculation.