Most residential fence posts need between 1 and 3 bags of concrete. The exact number depends on three measurements that almost no one gets right the first time: post size, hole diameter, and hole depth. Get those wrong and you either run out mid-pour or carry heavy bags back to the store. This guide gives you the formula, a full reference chart, and the exact bag counts for every common post size − so you buy the right amount on the first trip.
A standard 4×4 post in a 10-inch hole at 30 inches deep needs 2–3 bags of 80 lb (or 3 bags of 60 lb). A 6×6 post in a 12-inch hole at 36 inches deep needs 4 bags of 80 lb. Gate and corner posts use the next hole size up − usually 12 inches diameter and 36–42 inches deep.
The formula: hole volume minus post volume
Concrete fills the space around the post, not the whole hole. That is why bag-count charts differ from hole-volume charts − you have to subtract the volume of the post itself. The formula has three steps.
Step 1 − Hole volume = π × radius² × depth
Step 2 − Post volume = post width × post depth × embedment depth
Step 3 − Concrete = (Hole volume − Post volume) × (1 + waste %)
For a 4×4 post in a 10-inch hole at 30 inches deep:
- Hole radius = 5 inches. Hole volume = π × 5² × 30 = 2,356 in³
- Post volume = 3.5 × 3.5 × 30 = 368 in³
- Concrete = 2,356 − 368 = 1,988 in³ = 1.15 ft³
- Bags = 1.15 ÷ 0.60 = 2 bags of 80 lb (1.9 rounded up)
The post hole concrete calculator does this math for every post on your run at once − including the post-volume subtraction, a waste allowance, and the bag-size conversion.
Concrete bag yield: how much each bag actually covers
Bag counts depend on bag size, and bag sizes do not scale linearly. A 40 lb bag does not cover half of an 80 lb bag − it covers exactly half by weight, but the yield per pound is slightly different depending on the mix. Use the standard yields below; they are the same numbers the bag manufacturers publish.
| Bag size | Yield (cubic feet) | Yield (cubic inches) | Bags per cubic foot | Typical 2026 price |
|---|---|---|---|---|
| 40 lb | 0.30 ft³ | 518 in³ | 3.33 | $5–$7 |
| 50 lb | 0.375 ft³ | 648 in³ | 2.67 | $6–$8 |
| 60 lb | 0.45 ft³ | 778 in³ | 2.22 | $7–$9 |
| 80 lb | 0.60 ft³ | 1,037 in³ | 1.67 | $8–$11 |
An 80 lb bag is the best value per cubic foot in most markets − it costs about 15–20% more than a 60 lb bag but yields 33% more concrete. The trade-off is weight: 80 lb bags are harder to lift and pour into a narrow hole, especially on a ladder or slope. Many DIYers buy 60 lb bags for the handling, even at a slightly higher cost per cubic foot.
Bag count chart: every common post size and hole depth
This chart assumes a 4-inch compacted gravel base at the bottom of the hole and a 10% waste allowance. Bag counts are rounded up to the nearest whole bag − you cannot buy a partial bag, and running out mid-pour costs more than the extra bag.
| Post size | Hole diameter | Hole depth | Concrete per post | 40 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|---|---|---|
| 4×4 | 8 in | 24 in | 0.52 ft³ | 2 | 2 | 1 |
| 10 in | 24 in | 0.72 ft³ | 3 | 2 | 2 | |
| 10 in | 30 in | 0.91 ft³ | 4 | 3 | 2 | |
| 10 in | 36 in | 1.10 ft³ | 5 | 3 | 2 | |
| 6×6 | 12 in | 30 in | 1.36 ft³ | 5 | 4 | 3 |
| 12 in | 36 in | 1.63 ft³ | 6 | 4 | 3 | |
| 12 in | 42 in | 1.91 ft³ | 7 | 5 | 4 | |
| Gate / corner | 12 in | 36 in | 1.63 ft³ | 6 | 4 | 3 |
| 14 in | 42 in | 2.25 ft³ | 8 | 6 | 4 |
If your post count is already set, the concrete bag estimator multiplies these per-post counts by your full post count and adds waste − so you get a total bag count for the whole job, not just one hole.
Free tool Calculate total concrete bags for your whole fence Enter your post count, hole size, and depth for exact cubic feet and 40/50/60/80 lb bag counts.Hole size and depth: the measurements that matter most
Two rules determine the hole dimensions, and both come from the same logic: the hole has to be big enough to hold enough concrete to resist the lateral load on the post.
Diameter: 3× the post width
A 4×4 post (3.5 in actual) gets a 10-inch hole. A 6×6 post (5.5 in actual) gets a 12-inch hole. This is the most common rule, and it is what most bag-count charts assume. Some manufacturers recommend 4× the post width for steel posts − a 2.5-inch steel post in a 10-inch hole − because steel posts have a smaller footprint and need more concrete to resist the same load.
Depth: one-third of the post length, below the frost line
A 6 ft finished fence uses a 9 ft post. One-third of 9 ft is 3 ft, so the hole is 36 inches deep. That is the standard. But in cold climates, the hole must also go below the frost line − often 36–48 inches − even if that means a deeper hole than the one-third rule requires. Frost heave moves posts more reliably than wind does.
| Finished fence height | Post length | Hole depth (one-third) | Hole depth (frost climate) | Typical hole diameter |
|---|---|---|---|---|
| 4 ft | 6 ft | 24 in | 30–36 in | 8–10 in |
| 5 ft | 7.5 ft | 30 in | 36 in | 10 in |
| 6 ft | 9 ft | 36 in | 36–42 in | 10 in |
| 6 ft (heavy) | 10 ft | 40 in | 42–48 in | 10–12 in |
| 8 ft | 12 ft | 48 in | 48–54 in | 12 in |
Concrete cost per post in 2026
Bagged concrete is one of the few fence materials whose price is predictable. A standard 80 lb bag runs about $8–$11 at a big-box retailer, depending on region and brand. The 4×4 post in a 10-inch hole at 30 inches deep needs 2 bags, so the concrete cost per post is about $16–$22.
| Project size | 80 lb bags | Concrete cost (DIY) | Installed cost (pro) |
|---|---|---|---|
| 1 post | 2 | $16–$22 | $60–$150 |
| 10 posts (4×4) | 20 | $160–$220 | $300–$500 |
| 14 posts (100 ft at 8 ft) | 28 | $224–$308 | $400–$700 |
| 20 posts (150 ft at 8 ft) | 40 | $320–$440 | $600–$1,000 |
| 20 posts (including 2 gate posts) | 46 | $368–$506 | $700–$1,200 |
Professional installation adds labor, but it also bundles the concrete into the total − most fence installers do not quote concrete as a separate line. If you are comparing a DIY concrete cost against a professional quote, the professional number above includes digging, mixing, pouring, and cleanup, not just the bags.
Common concrete mistakes that cost a second trip
- Forgetting the gravel base. A 4–6 inch gravel base at the bottom of the hole drains water away from the post and gives the concrete a level surface to cure on. Without it, posts sit in standing water and rot or rust from the bottom up.
- Filling to the top of the hole. Leave 2–3 inches below grade and slope the concrete away from the post so water drains. A flush pour traps water against the post.
- Not subtracting the post volume. The hole volume is not the concrete volume. A 4×4 post displaces about 0.2 cubic feet over a 30-inch depth − roughly half a bag. Ignoring it over-orders slightly; it rarely causes a shortfall, but it adds up over 14 posts.
- Using the wrong bag size for the hole. A 10-inch × 36-inch hole needs 2 bags of 80 lb, not 2 bags of 40 lb. The bag count is the same number but the yield is different. Always match the bag size to the chart column.
- Skipping the waste allowance. Uneven holes, over-digging, and spillage add 5–10%. Buy the extra bag − returning a $9 bag is cheaper than a second trip.
- Mixing too much at once. Fast-setting concrete has a 20–30 minute working time. Mix one hole at a time, or two at most, until you know your pace.
- Not bracing the post. Concrete holds the post in position, but only after it cures. Brace every post plumb before pouring, and check it again after pouring.