Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Hole diameter 10 in = 0.8333 ft; Hole depth 30 in = 2.5 ft; Post size (actual) 3.5 in = 0.2917 ft; Post depth in the hole 24 in = 2 ft
- Hole volume = π × 0.4167² × 2.5 = 1.3635 ft³
- Post volume = 0.1701 ft³
- Concrete per post = 1.3635 ft³ − 0.1701 ft³ = 1.1934 ft³
- Total = 1.1934 ft³ × 1 = 1.1934 ft³
- Net volume = 1.1934 ft³ ÷ 27 = 0.0442 yd³ (× 0.0283168 = 0.0338 m³)
- Order quantity = 0.0442 yd³ × (1 + 10%) = 0.0486 yd³ = 1.3127 ft³
- 80 lb bags = 1.3127 ft³ ÷ 0.6 ft³ per bag = 2.19 → 3 bags (always round up)
- Weight = 1.3127 ft³ × 150 lb/ft³ = 197 lb
How to measure
Measure the hole diameter at the top, which is the width of the auger. A fence-post hole is often about three times the width of the post. Measure the hole depth from the ground surface, and the buried part of the post (the embedment). The two are equal only if the post sits on the bottom of the hole.
A 4×4 post is 3.5 in wide and a 6×6 is 5.5 in wide, so use the actual size. If the post is round, switch the shape to “Round” and enter its diameter.
Worked example: a 4×4 fence post
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Hole diameter 10 in = 0.8333 ft; Hole depth 30 in = 2.5 ft; Post size (actual) 3.5 in = 0.2917 ft; Post depth in the hole 24 in = 2 ft
- Hole volume = π × 0.4167² × 2.5 = 1.3635 ft³
- Post volume = 0.1701 ft³
- Concrete per post = 1.3635 ft³ − 0.1701 ft³ = 1.1934 ft³
- Total = 1.1934 ft³ × 1 = 1.1934 ft³
- Net volume = 1.1934 ft³ ÷ 27 = 0.0442 yd³ (× 0.0283168 = 0.0338 m³)
- Order quantity = 0.0442 yd³ × (1 + 10%) = 0.0486 yd³ = 1.3127 ft³
- 80 lb bags = 1.3127 ft³ ÷ 0.6 ft³ per bag = 2.19 → 3 bags (always round up)
- Weight = 1.3127 ft³ × 150 lb/ft³ = 197 lb
One post in a 10 in × 30 in hole needs about 1.19 ft³ of concrete, which is 3 bags of 80 lb mix with 10% waste (2.19 bags exactly). The same post in a 12 in × 36 in hole with a 6×6 post takes 4 bags.
How many bags for a whole fence?
Twelve 4×4 posts, ten-inch holes, thirty inches deep, need 27 bags of 80 lb mix and weigh about 2,363 lb. Add a couple of extra bags: the first few holes always take more than planned, and the augers rarely cut perfect cylinders.
| Hole | Post | Concrete | 80 lb bags | 60 lb bags |
|---|---|---|---|---|
| 8 × 24 in | 3.5 in | 0.56 ft³ | 2 | 2 |
| 10 × 30 in | 3.5 in | 1.19 ft³ | 3 | 3 |
| 12 × 36 in | 5.5 in | 1.83 ft³ | 4 | 5 |
| 14 × 42 in | 5.5 in | 3.11 ft³ | 6 | 8 |
Hole size and depth
A common rule is to dig the hole about three times as wide as the post and bury about one third of its length, or more in poor soil. For a fence, the depth must reach below the frost line so that the post does not heave out of the ground, and for tall or heavy structures such as a basketball hoop or a flagpole the foundation should come from the manufacturer or an engineer.
In gravel, pea gravel or tamped crushed stone below the post gives drainage so that water does not pond at the bottom and rot a wood post. The concrete then sits around the lower portion of the post and is crowned above grade to shed water.
Fast-setting post mix
Some products are formulated to set in minutes after mixing with water in the hole. These have different yields per bag from standard mix, so read the bag. This calculator lets you edit the yield of each bag size in the Advanced assumptions so the bag count matches your product. Standard mix is mixed in a wheelbarrow first. Fast-setting mix is poured dry into a hole around the post and then watered according to the label.
Mistakes to avoid
- Using the nominal post size (4×4 or 6×6) instead of the actual width.
- Digging the hole at the same diameter as the post, leaving no room for concrete.
- Pouring concrete around a wood post without a gravel drainage layer.
- Not bracing the post plumb until the concrete sets.
- Forgetting to buy more bags than the arithmetic says. Holes are never perfect cylinders.
Frequently asked questions
How many bags of concrete for a 4×4 fence post?
A 10 in × 30 in hole with a 4×4 post buried 24 in takes about 2 bags of 80 lb mix with no waste and 3 with 10% waste. Fast-setting mixes can differ, so read the bag.
How wide should a post hole be?
A common rule is three times the width of the post. For a 4×4 (3.5 in) that is about 10 in.
How deep should a fence post hole be?
Commonly one third of the post length or more, and below the frost line in cold climates. Local code or your fence design decides.
Do I subtract the post volume?
Yes. The post takes up space in the hole. This calculator subtracts it for you.
Is it better to use bags or ready-mix for fence posts?
For fewer than about 20 posts, bags are easier. For a long fence with many posts, a small truck load is cheaper than hundreds of bags.
Should the concrete be crowned at the top?
Yes. Slope the top away from the post so water runs off instead of pooling around the wood.
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.