🪵 Post Hole Concrete Calculator
By ToolNimba Construction Team · Updated 2026-06-19
Concrete per hole = pi × (diameter ÷ 2)² × depth, minus the buried post volume. The square post volume uses the actual side length times the depth. Bag yields are typical mixed volumes (50 lb ≈ 0.375 ft³, 60 lb ≈ 0.45 ft³) and are rounded up to whole bags after the waste allowance.
Setting fence posts, deck posts or a mailbox and not sure how many bags of concrete to grab? This calculator works out the concrete needed for each cylindrical post hole, then subtracts the space the post itself takes up, so you get a realistic number rather than the volume of an empty hole. Enter the hole diameter, the hole depth, the post size and how many posts you are setting, and it shows the total in cubic feet and cubic yards, plus how many 50 lb and 60 lb bags to buy with a waste allowance built in.
What is the Post Hole Concrete Calculator?
A post hole is a cylinder, so the concrete around the post is the volume of that cylinder minus the volume of the post that sits inside it. The cylinder volume is pi times the radius squared times the depth, where the radius is half the hole diameter. A square wooden post displaces its own side length squared times the buried depth. Subtracting the post volume matters more than people expect: a 4x4 post in a 12 inch hole takes up roughly a tenth of the hole, and a chunky 6x6 in a narrow hole can take up a third or more, so ignoring it makes you over-buy.
The one trap is units. Hole diameter, hole depth and post size are almost always given in inches, but concrete volume only makes sense in feet (and bags are rated in cubic feet). This tool converts every inch measurement to feet before it multiplies, so a 12 inch diameter becomes 1 foot and a 24 inch depth becomes 2 feet. If you do the maths by hand, divide each inch figure by 12 first, or you will be out by a factor of 1,728.
Bagged concrete is the usual choice for post setting because the holes are small and scattered. A standard 50 lb bag of mix yields about 0.375 cubic feet once water is added, and a 60 lb bag yields about 0.45 cubic feet. Always round up to whole bags and add a waste margin: holes are rarely perfect cylinders, the auger or digging bar leaves an oversized cavity at the bottom, and some mix sticks to the bucket. A 10 percent allowance (the default here) is sensible, and for a long fence run buying one or two spare bags is cheaper than a second trip to the store mid-pour.
When to use it
- Estimating how many bags of concrete to buy for a fence with a known number of posts.
- Sizing the concrete for deck footings, mailbox posts, sign posts or a pergola.
- Comparing how much more concrete a wider or deeper hole needs before you dig.
- Checking the concrete difference between a 4x4 and a 6x6 post in the same hole.
How to use the Post Hole Concrete Calculator
- Enter the hole diameter in inches (the width of the hole you are digging).
- Enter the hole depth in inches (how deep each hole goes).
- Pick the post size, or choose custom to type an exact square side, or no post to fill the whole hole.
- Enter the number of posts you are setting.
- Adjust the waste allowance if you want, then read the total concrete in cubic feet and yards and the bag counts.
Formula & method
Worked examples
Six fence posts, each in a 12 inch diameter hole 24 inches deep, using 4x4 posts (3.5 in actual side), with a 10 percent waste allowance.
- Convert to feet: diameter 12 in = 1 ft, depth 24 in = 2 ft, post side 3.5 in = 0.291667 ft
- Hole volume = pi × (1 ÷ 2)² × 2 = 3.14159 × 0.25 × 2 = 1.570796 ft³
- Post volume = 0.291667² × 2 = 0.085069 × 2 = 0.170139 ft³
- Concrete per hole = 1.570796 − 0.170139 = 1.400657 ft³
- Six holes = 1.400657 × 6 = 8.4039 ft³
- Add 10% waste = 8.4039 × 1.10 = 9.2443 ft³ (about 0.342 cubic yards)
- 50 lb bags = 9.2443 ÷ 0.375 = 24.65, rounded up to 25 bags
Result: About 9.24 ft³ (0.342 cubic yards), or 25 bags of 50 lb (21 bags of 60 lb).
Four heavier posts in 10 inch diameter holes 30 inches deep, using 6x6 posts (5.5 in actual side), with no waste allowance.
- Convert to feet: diameter 10 in = 0.83333 ft, depth 30 in = 2.5 ft, post side 5.5 in = 0.458333 ft
- Hole volume = pi × (0.83333 ÷ 2)² × 2.5 = 3.14159 × 0.173611 × 2.5 = 1.363538 ft³
- Post volume = 0.458333² × 2.5 = 0.210069 × 2.5 = 0.525174 ft³
- Concrete per hole = 1.363538 − 0.525174 = 0.838365 ft³
- Four holes = 0.838365 × 4 = 3.3535 ft³ (about 0.124 cubic yards)
- 50 lb bags = 3.3535 ÷ 0.375 = 8.94, rounded up to 9 bags
Result: About 3.35 ft³ (0.124 cubic yards), or 9 bags of 50 lb.
Concrete per hole (cubic feet) before waste, for common hole sizes with a 4x4 post (3.5 in)
| Hole diameter | Depth 18 in | Depth 24 in | Depth 30 in |
|---|---|---|---|
| 8 in | 0.40 | 0.53 | 0.66 |
| 10 in | 0.69 | 0.92 | 1.15 |
| 12 in | 1.05 | 1.40 | 1.75 |
| 14 in | 1.48 | 1.97 | 2.46 |
Approximate yield and bags per cubic foot for common pre-mixed concrete bags
| Bag size | Yield (cubic feet) | Bags per ft³ | Bags per cubic yard |
|---|---|---|---|
| 40 lb | 0.30 | 3.33 | 90 |
| 50 lb | 0.375 | 2.67 | 72 |
| 60 lb | 0.45 | 2.22 | 60 |
| 80 lb | 0.60 | 1.67 | 45 |
Common mistakes to avoid
- Forgetting to convert inches to feet. A hole 12 inches across and 24 inches deep is 1 foot by 2 feet, not 12 by 24. Multiply inch dimensions together and you overstate the volume enormously. Always divide each inch measurement by 12 before working out cubic feet.
- Not subtracting the post volume. The concrete only fills the gap around the post, not the post itself. Ignoring the post inflates your bag count, especially with chunky 6x6 posts in narrow holes where the post can take up a third of the hole.
- Using nominal lumber sizes instead of actual. A 4x4 is really 3.5 by 3.5 inches and a 6x6 is 5.5 by 5.5 inches. Using the nominal 4 or 6 inch figure for the post overstates how much concrete the post displaces and leaves you short.
- Buying the exact calculated amount. Augered holes flare at the bottom, soil sloughs in, and some mix is lost to the mixing tub. Round up to whole bags and keep a 10 percent margin so a single short bag does not stop the job.
Glossary
- Post hole
- The cylindrical hole dug for a fence, deck or sign post, usually filled with concrete to hold the post firm.
- Diameter
- The full width of the round hole across its center. Half the diameter is the radius used in the volume formula.
- Nominal vs actual size
- Lumber is sold by nominal size (4x4) but the real dimensions are smaller (3.5 by 3.5 inches) after milling and drying.
- Yield
- The volume of mixed concrete a single bag produces once water is added, for example about 0.375 cubic feet for a 50 lb bag.
- Cubic yard
- The unit ready-mix concrete is sold by in the US. One cubic yard equals 27 cubic feet.
Frequently asked questions
How much concrete do I need per fence post?
It depends on the hole size and post size. As a rough guide, a 4x4 post in a 12 inch diameter hole dug 24 inches deep needs about 1.4 cubic feet of concrete, which is roughly four 50 lb bags or three 60 lb bags per post. Enter your own numbers above for an exact figure.
How deep should a fence post hole be?
A common rule is to bury about one third of the post length, or at least below the local frost line. For a typical 6 foot fence that means a hole around 24 to 30 inches deep. Deeper holes and wider diameters both increase the concrete needed.
Why does the calculator subtract the post volume?
Concrete fills only the space around the post, not the post itself. Subtracting the post volume (its side squared times the buried depth) gives the real amount of concrete and stops you over-buying, which matters most with thick 6x6 posts.
How many 50 lb bags of concrete fill a post hole?
A 50 lb bag yields about 0.375 cubic feet mixed. Divide the concrete needed per hole by 0.375 and round up. For example 1.4 cubic feet per hole works out to about four 50 lb bags. The calculator does this for you across all your holes.
Should I use 50 lb or 60 lb bags?
Either works. 60 lb bags yield more per bag (about 0.45 cubic feet versus 0.375), so you carry fewer of them, but each is heavier to lift. The calculator shows both counts so you can pick whichever is easier to buy and handle.
Do I need to mix the concrete or can I pour it dry?
For permanent posts most builders mix the concrete with water before pouring, which gives full strength. Some quick-setting products are designed to be poured dry into the hole and then wetted. Follow the bag instructions, and note that bag yields here assume normal mixing.