🕳️ Post Hole Concrete Calculator

How to Calculate Concrete for a Post Hole

A post hole is a cylinder with a post standing in the middle of it. The concrete you need is the cylinder minus the post — and skipping that second step is why most estimates come in high.

Diameter, post area and depth are all in inches; 1728 converts cubic inches to cubic feet. Concrete depth is the hole depth minus the gravel you put in the bottom, because the post sits on the gravel and the concrete starts above it.

Worked example

A 4 × 4 post in a 12 in hole, 30 in deep, with 4 in of gravel at the bottom:

• Concrete depth = 30 − 4 = 26 in
• Hole cross-section = π × 6² = 113.1 sq in
• Post cross-section = 3.5 × 3.5 = 12.25 sq in
• Net area = 113.1 − 12.25 = 100.85 sq in
• Volume = 100.85 × 26 ÷ 1728 = 1.52 cu ft per hole
• Eight holes plus 10% spillage = 13.4 cu ft = 30 bags of 60 lb

How much difference does the post make?

More than people assume, and it scales with how tight the hole is:

On a 30-post fence line, ignoring displacement on 6 × 6 posts in 12 in holes means buying about a quarter more concrete than you use. Round up per hole, though, not overall — you cannot pour three-quarters of a mixed bag into one hole and save the rest for the next.

Setting a whole fence? Our fence post concrete calculator is tuned for standard fence work, and the concrete calculator handles slabs, footings and pads.

How Deep and How Wide Should the Hole Be?

Two rules cover almost every post, and the deeper of the two always wins.

• Depth: one third of the above-ground height, minimum. A 6 ft fence needs at least 2 ft in the ground, which is why 8 ft posts are standard for 6 ft fences.
• Depth: below the local frost line. Non-negotiable in frost country. A post set above the frost line gets jacked out of the ground a little more every winter.
• Diameter: three times the post's width. A 3.5 in post wants a 10 to 12 in hole; a 5.5 in post wants 16 to 18 in.

Frost depth by region

These are typical ranges, not code. Your building department publishes the frost depth that governs your address, and it is the only number that matters if the job is inspected. Corner posts, gate posts and anything carrying a lateral load should go deeper and wider than line posts regardless — they take most of the force.

Deck and structural posts are a different problem

A hole full of concrete with a post in it is not a deck footing. Structural posts need a footing sized to the soil's bearing capacity and the load coming down the post — typically a 12 to 24 in diameter pad below the frost line, sized from the code tables. The post should sit on top of that footing on a galvanised post base, above grade, not be buried in the concrete. Buried wood in a concrete cup rots from the inside, and by the time you can see it the beam is already moving.

Bags of Concrete Per Hole

These tables assume the post runs to the bottom of the hole and no gravel base, so they are the maximum for that hole size. Adding 4 inches of gravel takes roughly 12 to 15% off. Bag yields are the standard ones: 80 lb = 0.60 cu ft, 60 lb = 0.45, 50 lb = 0.375, 40 lb = 0.30.

For a 4 × 4 post

For a 6 × 6 post

The 80 lb bag is the cheapest way to buy concrete by volume, and it is what to use if you can lift it — 45 of them make a cubic yard, against 60 sixty-pounders, 72 fifty-pounders or 90 forty-pounders. The smaller bags exist for handling, not economy.

Once the job passes about 1 cubic yard — roughly 45 × 80 lb bags — price a short load of ready-mix or rent a mixer. Mixing 45 bags by hand in a tub is a genuinely long day. Our concrete bag calculator covers the bag-versus-truck comparison for larger pours.

Gravel, Water and What Else the Hole Needs

Gravel in the bottom

Put 4 to 6 inches of #57 washed stone in the bottom of every hole before the post goes in. It does two things: it gets the end grain of a wood post out of standing water, and it gives water that runs down the post somewhere to go instead of sitting in a concrete bowl.

Washed stone specifically — not crusher run, not sand. The fines in crusher run clog the voids and defeat the drainage. Sand holds water and gives no lateral support at all. Tamp the gravel before setting the post so the post cannot settle into it.

Volume is small but easy to forget: a 12 in hole with 4 in of gravel takes 0.26 cu ft each, so eight holes is about 2.1 cu ft — five 0.5 cu ft bags, or a corner of a bulk pile if you are already ordering. Our gravel calculator covers the grades.

Mixing water

Standard bagged concrete needs roughly 3 pints of water per 40 lb, 4.5 pints per 60 lb, and 6 pints (0.75 gallon) per 80 lb. Start with about 90% of that and add the rest slowly. A wet, soupy mix is the single easiest way to end up with weak concrete — it looks easier to work and cures noticeably softer.

Crown the top

Bring the concrete 1 to 2 inches above grade and slope it away from the post on all sides. A flat or dished top holds a ring of water against the post for days after every rain, which is what actually rots fence posts — not ground moisture. Finish the crown with a trowel before the concrete stiffens.

If the post is a wood post, do not let the concrete form a cup below grade either. The aim is a plug that sheds water outward, sitting on gravel that drains downward.

Fast-setting or standard mix?

For fence posts, fast-setting mix poured dry is a legitimate method and it is what most fence crews use: post in, plumb and braced, dry mix poured in around it, then water added per the bag. It saves an enormous amount of mixing on a long fence line. For anything structural, mix it properly — see our dry pour concrete guide for where the method holds up and where it does not.

Setting a Post, Step by Step

1. Call 811 before you dig. Free, and the locate takes a couple of days, so do it first. Hitting a gas or fibre line with an auger is a very expensive afternoon.
2. String the line and mark every hole before digging any of them. Post spacing that drifts is impossible to hide once the rails go on.
3. Dig the hole to full depth and diameter, keeping the sides straight. A bell shape that is wider at the bottom actually resists frost heave better; a cone that is wider at the top is the worst possible shape.
4. Add 4 to 6 in of #57 stone and tamp it so the post cannot settle later.
5. Set the post and brace it. Two braces at 90 degrees, screwed to stakes. Do not rely on backfill to hold plumb.
6. Plumb it on two adjacent faces with a post level, and check the string line for alignment and height before any concrete goes in.
7. Pour the concrete in lifts, rodding each lift with a stick to release air pockets. For dry-pour, fill in 12 in lifts and water each one.
8. Crown the top 1–2 in above grade, sloping away from the post on every side.
9. Re-check plumb immediately. You have a couple of minutes with fast-setting mix and a couple of hours with standard mix.
10. Leave the braces on. Four hours minimum for fast-setting, 24 to 48 for standard, before hanging rails, panels or a gate.

Mistakes to Avoid

• Setting the post shallower than the frost line. It will lift, and no amount of concrete prevents it.
• A hole that flares wider at the top. That shape gives frost the perfect wedge to lift the whole plug.
• No gravel at the bottom. The hole becomes a bucket and the post end sits in water.
• A flat or dished concrete top. Water pools against the post. Crown it.
• Too much mixing water. Soupy concrete is weak concrete. Follow the bag.
• Hanging a gate the same day on standard mix. Give it 48 hours.
• Buying by total volume instead of per hole. Round up hole by hole — leftover mixed concrete is waste, not stock.
• Burying a structural post in concrete. Deck and pergola posts belong on a footing, on a galvanised base, above grade.

Building the fence next? Our fence calculator works out post spacing, rails and pickets for the whole run.

Frequently Asked Questions

How much concrete do I need for a post hole?

Work out the cylinder and subtract the post. A 4 × 4 in a 12 in hole 30 in deep, with 4 in of gravel underneath, needs 1.52 cubic feet — about three 80 lb bags or four 60 lb bags. The formula is (π × radius² − post area) × concrete depth ÷ 1728, all in inches.

How many bags of concrete do I need per post?

For a typical 4 × 4 fence post in a 10 in hole 30 in deep, two 80 lb bags or three 60 lb bags. A 6 × 6 in an 18 in hole 36 in deep needs eight 80 lb bags. Bigger post, wider hole, deeper hole — each one moves the number a lot, so use the tables above rather than a single rule of thumb.

Does the post itself reduce how much concrete I need?

Yes, and by more than most people expect. A 4 × 4 fills 16% of a 10 in hole and 24% of an 8 in hole. A 6 × 6 fills 27% of a 12 in hole. Ignoring it on a long fence line means buying a fifth to a quarter more concrete than you actually use.

How deep should a post hole be?

At least one third of the post height above ground, and always below the local frost line — whichever is deeper. A 6 ft fence needs 2 ft minimum, and 30 in is common. In northern climates the frost line rules: 42 to 54 in across the Upper Midwest and New England, and more in northern Minnesota or North Dakota.

How wide should a post hole be?

About three times the post width. A 4 × 4 (actually 3.5 in) wants a 10 to 12 in hole; a 6 × 6 (5.5 in) wants 16 to 18 in. Narrower than that and there is not enough concrete around the post to resist lateral load. Corner and gate posts should go wider and deeper than line posts.

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

Yes — 4 to 6 inches of #57 washed stone, tamped. It keeps the end of a wood post out of standing water and gives water running down the post a route away instead of a concrete bowl to sit in. Use washed stone, not crusher run (the fines clog it) and not sand (it holds water and gives no support).

Should the concrete come above ground level?

Yes. Bring it 1 to 2 inches above grade and slope it away from the post on every side. A flat or dished top holds a ring of water against the post after every rain, and that is what rots fence posts — far more than ground moisture does. Trowel the crown before the mix stiffens.

Can I pour concrete dry into a post hole?

For fence posts, yes, and it is what most fence crews do. Set and brace the post, pour fast-setting dry mix in around it in 12 in lifts, and add the water each bag calls for. It saves a huge amount of mixing on a long run. For structural footings, mix it properly — dry pour is much less predictable when strength actually matters.

Should I use fast-setting or standard concrete for posts?

Fast-setting for fence, mailbox and sign posts — it sets in 20 to 40 minutes and takes load in about 4 hours, so you can finish in a day. Standard mix for structural footings, where you want the control that comes with mixing it and are not in a hurry. Both reach about 4,000 psi; fast-setting just gets there sooner.

How long before I can put load on the post?

About 4 hours with fast-setting mix, 24 to 48 hours with standard concrete before hanging rails, panels or a gate. Full design strength takes 7 days for fast-setting and 28 days for standard. Leave the braces on for the whole waiting period — a post nudged out of plumb at hour three stays out of plumb.

Can I set a deck post in concrete?

You can, but you should not. A structural post belongs on top of a properly sized footing, on a galvanised post base, above grade. Wood buried in a concrete cup rots from the inside out, and the footing under a deck post has to be sized to the soil bearing capacity and the load — which a post hole full of concrete never is.

How many 80 lb bags of concrete are in a cubic yard?

45 bags. An 80 lb bag yields 0.60 cubic feet and a cubic yard is 27 cubic feet. It is 60 bags at 60 lb, 72 at 50 lb, and 90 at 40 lb. Once a job passes about a cubic yard, price a short load of ready-mix or rent a mixer — 45 bags by hand is a long day.

Do post holes need rebar?

Not for fence, mailbox or sign posts — the concrete is acting as a mass to resist tipping, not as a beam, so there is nothing for reinforcement to do. Structural footings are a different matter and may require reinforcement or a vertical dowel by design. If an inspector is involved, the drawing decides, not a rule of thumb.

Can I set posts without concrete?

Yes — compacted angular gravel, added in 4 in lifts and tamped hard each time, is a sound method in well-drained soil. It drains better than concrete, it can be re-tamped if the post loosens, and the post comes out in future without jackhammering. It gives less lateral resistance though, so keep concrete for gate posts, corner posts and anything carrying a load.