Concrete Bag Calculation for a Small Column or Post
Quick answer
For a single 4×4 fence post in a 12 in hole, 30 in deep: about 4 bags of 80 lb concrete, post volume subtracted. For a freestanding round column of the same dimensions, with no post inside: about 6 bags. The formula is nearly identical either way — the only difference is one extra subtraction step.
Whichever you’re pouring, the Bags of Concrete Calculator handles both column and post-hole shapes and gives you an instant bag count. This article walks through the math so you know exactly what the calculator is doing, and can spot the difference between the two calculations yourself.
Column or post — know which one you’re calculating
These get lumped together constantly, but they’re genuinely different pours.
| Freestanding column | Post set in a hole | |
|---|---|---|
| What it is | A standalone concrete pier, pillar, or footing — nothing embedded inside it | A wood or metal post set into a hole, with concrete filling the space around it |
| Volume calculation | Full cylinder or rectangle volume | Full hole volume, minus the post’s own volume |
| Typical use | Deck piers, mailbox bases, small structural columns | Fence posts, gate posts, deck support posts |
Skip the subtraction step on a post and you’ll over-order. Apply it to a freestanding column and you’ll come up short, since there’s nothing actually displacing that concrete.
Why the difference matters
These are small pours, so the dollar amounts involved are modest — but the percentage error isn’t. A 4×4 post can displace 10–15% of a small hole’s volume. On a single post that’s not much. On a project with a dozen deck piers or fence posts, that’s a real number of bags you either wasted buying, or fell short on mid-project.
The formulas
Where post volume is the post’s cross-section area × the depth it’s actually embedded in the ground (not its full above-ground height).
What to measure first
- Diameter or side lengthOf the hole or column, in feet.
- Depth or heightHow deep the hole goes, or how tall the column stands.
- Is there a post inside?If yes, note its actual dimensions — a “4×4” is really 3.5×3.5 in, a “6×6” is really 5.5×5.5 in.
- How manyFence lines and deck piers are almost always more than one — count them all before totaling.
Worked examples
Example 1 — fence post, 12 in hole × 30 in deep, 4×4 post, 80 lb bags
- Hole volume3.14 × 0.5² × 2.5 = 1.96 cu ft
- Post volume (3.5×3.5 in actual, 2.5 ft embedded)0.292 × 0.292 × 2.5 = 0.21 cu ft
- Net volume1.96 − 0.21 = 1.75 cu ft
- Add 10% waste1.93 cu ft
- Divide by yield (0.60)1.93 ÷ 0.60 = 3.2
- Round up4 × 80 lb bags
Example 2 — freestanding round deck pier, 10 in diameter × 4 ft tall, 80 lb bags
- Volume (no post to subtract)3.14 × 0.417² × 4 = 2.18 cu ft
- Add 10% waste2.40 cu ft
- Divide by yield (0.60)2.40 ÷ 0.60 = 4.0
- Result4 × 80 lb bags
Example 3 — square mailbox base, 8×8 in × 3 ft tall, 60 lb bags
- Convert to feet8 ÷ 12 = 0.667 ft
- Volume0.667 × 0.667 × 3 = 1.33 cu ft
- Add 10% waste1.47 cu ft
- Divide by yield (0.45)1.47 ÷ 0.45 = 3.26
- Round up4 × 60 lb bags
Get your exact bag count.
Enter your column or post-hole dimensions and get an instant bag count — post volume subtracted automatically.
Use the free Bags of Concrete CalculatorDon’t skip the waste allowance
| Situation | Add |
|---|---|
| Clean, formed hole or sonotube | 5–10% |
| Hand-dug post hole | 10–15% |
| Rocky or irregular ground | 15–20% |
Small pours like these feel forgiving, but a hand-dug hole is rarely a perfect cylinder — budget the extra 10-15% rather than being surprised mid-pour.
Sizing the hole, and when to use fast-setting mix
- Hole diameter rule of thumbAbout 3 times the post’s width — a 4×4 post typically wants a 10–12 in hole.
- Depth rule of thumbRoughly a third to half the post’s above-ground height, or down to your local frost depth, whichever is deeper.
- Fast-setting concreteSets in 20–40 minutes and doesn’t need mixing in a wheelbarrow — pour it dry into the hole around the post, then add water. A genuine time-saver for a handful of posts.
7 mistakes with columns and posts
- Not subtracting the post’s volumeOver-orders material on every single post hole in the project.
- Subtracting a post that isn’t thereApplying the post-hole formula to a genuinely freestanding column underestimates it.
- Using nominal post size instead of actualA “4×4” is really 3.5×3.5 inches — using 4×4 exactly slightly overstates how much volume the post displaces.
- Digging too shallowBelow your local frost depth is a code requirement in most areas, not just a suggestion.
- Rounding a small bag count down3.2 bags means 4, not 3 — there’s no such thing as buying 0.8 of a bag.
- Forgetting to multiply by the number of postsOne post’s volume isn’t the whole fence line if you’re setting a dozen.
- Buying a full 80 lb bag for a job that needs a quarter of oneFor a single small post, a 40 lb or 50 lb bag (or a small fast-set tube) wastes far less material.
When several posts add up to a bigger decision
A single post or two is always a bags job — there’s no practical reason to call a ready-mix truck for that. But a long fence line or a deck with a dozen piers can quietly climb toward 40-plus bags once you total every hole. If your project’s grand total crosses that mark, it’s worth a quick check against ready-mix pricing, the same way you would for a slab or footing.
Quick reference: freestanding round columns (80 lb bags)
| Diameter | Height | Bags |
|---|---|---|
| 8 in | 2 ft | 2 |
| 8 in | 4 ft | 3 |
| 10 in | 4 ft | 4 |
| 12 in | 3 ft | 5 |
| 12 in | 4 ft | 6 |
Quick reference: post holes, post volume subtracted (80 lb bags)
| Hole size | Post size | Bags per post |
|---|---|---|
| 12 in × 24 in deep | 4×4 | 3 |
| 12 in × 30 in deep | 4×4 | 4 |
| 16 in × 36 in deep | 6×6 | 7 |
| 18 in × 42 in deep | 6×6 | 10 |
What can change the number
- Post sizeA 6×6 post displaces noticeably more volume than a 4×4, at the same hole size.
- Local frost depthColder regions require deeper holes, directly increasing volume.
- Soil bearing capacitySofter soil sometimes calls for a wider hole to spread the load.
- Fast-set vs. standard mixYields are similar, but check the specific bag’s label — they can differ slightly by brand.
Calculate it in five steps
- Decide: column or post? This determines whether you subtract anything.
- Measure diameter/side and depth or height, in feet.
- Find the full volume using the cylinder or rectangle formula.
- If it’s a post, subtract the post’s actual (not nominal) volume.
- Add 10–15% waste, divide by bag yield, and round up.
Frequently asked questions
How many bags of concrete for one fence post?
A 4×4 post in a 12 in hole, 30 in deep, needs about 4 bags of 80 lb concrete, once the post’s own volume is subtracted.
Do I need to subtract the post’s volume?
Yes, for an accurate number — a 4×4 or 6×6 post genuinely displaces concrete inside the hole. Skipping this step means buying a bit more than you actually need.
How deep should a post hole be?
Roughly a third to half the post’s above-ground height, or down to your local frost depth — whichever requirement is deeper.
What’s the difference between calculating a column and a post?
A freestanding column uses the full cylinder or rectangle volume. A post set into a hole uses that same volume minus the post’s own volume, since the post takes up space the concrete won’t fill.
Can I use fast-setting concrete for posts?
Yes — it’s a common choice for posts specifically, since it sets in 20-40 minutes and doesn’t require pre-mixing in a wheelbarrow. Pour it dry around the post, then add water.
How many bags for a single deck pier?
A typical 10-12 in diameter pier, 3-4 ft deep, needs about 4-6 bags of 80 lb concrete, with no volume subtracted since there’s nothing embedded inside it.
Related reading
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