How Much Concrete Do You Need Per Fence Post?
The bag count for fence posts is easy to get wrong in both directions. Guess low and you are driving back to the store with wet holes waiting. Guess high and you are storing a dozen bags of concrete that will harden in a damp garage. The fix is to calculate the hole volume, subtract the post, and divide by the yield printed on the bag.
Step 1: Choose the hole size
Diameter. A common rule of thumb is a hole about three times the width of the post.
| Post | Actual size | Typical hole diameter |
|---|---|---|
| Wood 4×4 | 3.5 × 3.5 in (89 mm) | 10–12 in (25–30 cm) |
| Wood 6×6 | 5.5 × 5.5 in (140 mm) | 12–16 in (30–40 cm) |
| Vinyl 5×5 | 5 × 5 in (127 mm) | 10–12 in (25–30 cm) |
| Chain-link line post (round) | commonly 1⅝–1⅞ in for residential | 6–8 in (15–20 cm) |
| Chain-link terminal post (round) | commonly 2–2⅜ in for residential | 8–10 in (20–25 cm) |
Depth. Bury about one-third of the post’s total length, or at least one-third to one-half of the above-ground height. Go below the frost line if your local code requires it. Many installers also dig 4–6 in deeper and put gravel in the bottom so water drains away from the end of the post.
Step 2: Calculate the concrete volume
Concrete fills the hole minus the space the post takes up.
Hole volume (in³) = 3.1416 × (diameter ÷ 2)² × concrete depth
Post volume (in³) = post width × post depth × concrete depth (for round posts, use 3.1416 × radius²)
Concrete (ft³) = (hole volume − post volume) ÷ 1,728
There are 1,728 in³ in a cubic foot (12 × 12 × 12).
In metric, work in metres and the result is in cubic metres; multiply by 1,000 for litres.
Step 3: Divide by the bag yield
Typical yields for standard concrete mix, as printed on the bags of major US brands:
| Bag | Typical yield | Litres |
|---|---|---|
| 40 lb (18 kg) | about 0.30 ft³ | about 8.5 L |
| 60 lb (27 kg) | about 0.45 ft³ | about 12.7 L |
| 80 lb (36 kg) | about 0.60 ft³ | about 17 L |
Fast-setting post mixes yield less per pound; a 50 lb bag of fast-setting mix is commonly listed at about 0.375 ft³. Always check the label of the product you buy.
Per-post quick reference (4×4 wood post)
Concrete column depth is the depth filled with concrete, not counting any gravel below.
| Hole diameter | Concrete depth | Concrete per post | 80 lb bags | 60 lb bags | 50 lb fast-set bags |
|---|---|---|---|---|---|
| 8 in | 24 in | 0.53 ft³ | 0.9 | 1.2 | 1.4 |
| 10 in | 24 in | 0.92 ft³ | 1.5 | 2.0 | 2.5 |
| 10 in | 30 in | 1.15 ft³ | 1.9 | 2.6 | 3.1 |
| 10 in | 36 in | 1.38 ft³ | 2.3 | 3.1 | 3.7 |
| 12 in | 24 in | 1.40 ft³ | 2.3 | 3.1 | 3.7 |
| 12 in | 30 in | 1.75 ft³ | 2.9 | 3.9 | 4.7 |
| 12 in | 36 in | 2.10 ft³ | 3.5 | 4.7 | 5.6 |
Notice how quickly volume rises with diameter: going from a 10 in to a 12 in hole adds about 50% more concrete at the same depth.
Worked example: a 31-post backyard fence
From our fence post counting guide: 29 standard 4×4 posts plus 2 gate posts.
Standard posts: 10 in hole, 24 in of concrete, 6 in of gravel below (30 in total dig). - Hole: 3.1416 × 5² × 24 = 1,885 in³ - Post: 3.5 × 3.5 × 24 = 294 in³ - Concrete: (1,885 − 294) ÷ 1,728 = 0.92 ft³ per post - 29 posts: 26.7 ft³
Gate posts: 6×6 post, 12 in hole, 30 in of concrete. - Hole: 3.1416 × 6² × 30 = 3,393 in³ - Post: 5.5 × 5.5 × 30 = 908 in³ - Concrete: 1.44 ft³ per post; 2 posts: 2.9 ft³
Total: 26.7 + 2.9 = 29.6 ft³.
- 80 lb bags: 29.6 ÷ 0.60 = 49.3 → 50 bags
- 60 lb bags: 29.6 ÷ 0.45 = 65.8 → 66 bags
- 50 lb fast-set: 29.6 ÷ 0.375 = 78.9 → 79 bags
Buy a few extra bags for holes that come out wider than planned; augers and shovels rarely cut a perfect cylinder. Unopened bags can often be returned, though store policies vary.
Gravel: 6 in in the bottom of each hole is about 0.27 ft³ for a 10 in hole and 0.39 ft³ for a 12 in hole, so about 8.7 ft³ in total, or roughly 18 bags of 0.5 ft³.
Metric check: a 25 cm hole with 61 cm of concrete around an 89 mm post holds about 0.026 m³ (26 L) of concrete, or about 1.5 bags of 36 kg standard mix.
Rounding per post vs. for the whole job
If you round up for every post (2 bags × 29), you would buy 58 bags for the standard posts alone. Calculating the total first gives 45. Mixing in a wheelbarrow lets leftover concrete from one hole go into the next, so total-then-round is usually the better method.
Mixing and setting tips
- Follow the water ratio on the bag. Too much water weakens concrete.
- Crown the top. Slope the concrete away from the post just above grade so rainwater runs off instead of pooling against the wood.
- Brace before it sets. Check plumb on two adjacent faces and brace each post with stakes.
- Respect cure times. Don’t hang rails or gates until the concrete has reached the strength the bag specifies.
Common mistakes
- Not subtracting the post. On a 4×4 in a 10 in hole, the post takes up about 15% of the hole volume.
- Using the full dig depth. If 6 in of the hole is gravel, it is not concrete.
- Assuming every bag yields the same. An 80 lb bag of standard mix and a 50 lb bag of fast-setting mix are not interchangeable in the math.
- Oversized holes. A 14 in hole where 10 in would do nearly doubles the concrete.
- Skipping the utility locate. In the US, call 811 a few days before digging.
Calculate your fence
The fence calculator estimates posts, rails and materials for your fence length. Combine its post count with the per-post figures above for your concrete order.
FAQ
How many 80 lb bags of concrete per fence post?
For a 4×4 post in a 10 in hole with 24 in of concrete, about 1.5 bags. A 12 in hole at the same depth needs about 2.3 bags.
Can I use fast-setting concrete for fence posts?
Yes, it is made for this. It yields less per bag than many standard mixes, so you will need more bags for the same holes.
Should concrete go all the way to the bottom of the post?
Many installers set the post on a few inches of gravel so the end grain can drain, and fill around it with concrete. Follow your fence manufacturer’s instructions for vinyl and metal systems.
Do line posts need concrete on a chain-link fence?
Terminal posts (ends, corners, gates) are almost always set in concrete. Line posts are usually set in concrete too, though practices vary by region and soil.