How Much Concrete per Fence Post? Hole Size and Bag Chart
Most 4×4 fence posts take 2 to 3 bags of 50 lb fast-setting concrete, depending on the hole. Here’s the chart, the geometry behind it and the manufacturer’s hole guidance.
Start with the hole, not the post
The amount of concrete depends almost entirely on the hole you dig. QUIKRETE’s post-setting instructions give a rule of thumb. Dig the hole about three times the width of the post. Make it 1/3 to 1/2 of the post’s above-ground height deep, plus 6 in. Put 6 in of gravel in the bottom for drainage, and stop the concrete 3–4 in below ground so soil or sod can cover it. Local frost depth and building rules can require a deeper hole, so check them first.
For a 4×4 post, which is 3.5 in actual, three times the width is about 10.5 in. Many people use a 10 in or 12 in auger. For a 6×6 (5.5 in actual), it’s 16–18 in.
The formula
Fill depth = hole depth − gravel layer − unfilled top Concrete = π × (hole diameter ÷ 2)² × fill depth − post width² × fill depth Bags = concrete ÷ bag yield
The post’s own volume comes off because the post takes up space in the hole. Bag yields come from QUIKRETE’s data sheets: 0.375 ft³ for a 50 lb Fast-Setting bag and 0.60 ft³ for an 80 lb bag of standard Concrete Mix.
Chart: bags per post (6 in gravel, 3 in left at top)
| Post | Hole Ø | Hole depth | Concrete | 50 lb fast-set bags | 80 lb mix bags |
|---|---|---|---|---|---|
| 4×4 (3.5 in) | 10 in | 24 in | 0.58 ft³ | 1.53 | 0.96 |
| 4×4 (3.5 in) | 10 in | 30 in | 0.81 ft³ | 2.15 | 1.34 |
| 4×4 (3.5 in) | 10 in | 36 in | 1.04 ft³ | 2.76 | 1.73 |
| 4×4 (3.5 in) | 12 in | 30 in | 1.23 ft³ | 3.27 | 2.04 |
| 4×4 (3.5 in) | 12 in | 36 in | 1.58 ft³ | 4.20 | 2.63 |
| 6×6 (5.5 in) | 16 in | 30 in | 2.08 ft³ | 5.54 | 3.46 |
| 6×6 (5.5 in) | 16 in | 36 in | 2.67 ft³ | 7.12 | 4.45 |
| 6×6 (5.5 in) | 18 in | 36 in | 3.50 ft³ | 9.34 | 5.84 |
Round up when you buy. Partial bags across several posts add up: for ten 4×4 posts in 10 in × 30 in holes, 10 × 2.15 = 21.5, so buy 22 bags of 50 lb.
Metric chart (150 mm gravel, 75 mm left at top)
| Post | Hole Ø | Depth | Concrete | 25 kg bags (11.6 L) | 30 kg bags (14 L) |
|---|---|---|---|---|---|
| 75 mm | 225 mm | 600 mm | 12.8 L | 1.10 | 0.91 |
| 100 mm | 300 mm | 600 mm | 22.8 L | 1.96 | 1.63 |
| 100 mm | 300 mm | 750 mm | 31.9 L | 2.75 | 2.28 |
| 125 mm | 375 mm | 900 mm | 64.0 L | 5.52 | 4.57 |
A reality check against the manufacturer
QUIKRETE’s Fast-Setting data sheet says two 50 lb bags set a 3 in square post in a 9 in hole 30 in deep. Filling that hole to the brim, by geometry, takes about 0.95 ft³, or 2.5 bags. With 6 in of gravel and 3 in left at the top, it takes 1.8 bags. So the manufacturer’s figure sits inside the range the formula predicts, and the result depends on how much of the hole you actually fill. Real holes are rarely perfect cylinders either. They flare at the top and wander at the sides. Buy one spare bag for every 8 to 10 posts.
Fast-setting vs regular mix
Fast-setting mixes are designed to be poured dry into the hole and then soaked with water. QUIKRETE says they set in 20–40 minutes and can take strain after 4 hours. Standard concrete mix is mixed with water first and placed in the hole; it takes longer to set but makes more concrete per pound (0.60 ft³ per 80 lb bag, against 0.375 ft³ per 50 lb). For a long fence, the bag count and price per post can tip the choice either way, so compare both.
Before you dig
- In the US, contact 811 before digging so buried utility lines are marked.
- Check frost depth, fence height limits and any permit or HOA rules with your local authority.
- Brace each post plumb on two sides before adding concrete.
- Slope the top of the concrete away from the post, or finish with soil, so water doesn’t pool against the wood. QUIKRETE suggests both the gravel base and a sloped finish to help protect posts from rot.
To count posts, rails, pickets and concrete for a whole fence in one go, use the fence calculator.
Frequently asked questions
How many bags of concrete per 4×4 post?
Typically 2–3 bags of 50 lb fast-setting concrete. A 10 in hole 30 in deep, with 6 in of gravel and 3 in left at the top, takes about 2.15 bags.
How big should the hole be for a 4×4 post?
QUIKRETE’s guidance is about three times the post width (around 10–12 in for a 4×4). Make it 1/3 to 1/2 of the above-ground height deep plus 6 in, with 6 in of gravel at the bottom.
How much concrete for a 6×6 post?
In a 16 in hole 30 in deep (6 in gravel, 3 in top), about 2.1 ft³, which is 5.5 bags of 50 lb fast-setting or 3.5 bags of 80 lb mix.
Do I need gravel under a fence post?
QUIKRETE recommends 6 in of gravel at the bottom of the hole for drainage, which helps protect wooden posts from rot.
Sources
- QUIKRETE: Setting posts in concrete (hole size and depth guidance)
- QUIKRETE Fast-Setting Concrete Mix (No. 1004) data sheet: yields incl. 25 kg / 30 kg bags, post-setting method
- QUIKRETE Concrete Mix (No. 1101) product data sheet: bag sizes and yields
- Tarmac Blue Circle: Postcrete product page (UK, 20 kg bag)
- Call 811 (US): contact before you dig