Use this post hole concrete calculator for fence, deck and mailbox posts: enter your hole size and how many posts you are setting to see how many bags of concrete mix to buy.
Quick answer: a 4 × 4 fence post in a 10 in hole 30 in deep takes about 1.15 cubic feet of concrete. That is 2 bags of 80 lb concrete mix (0.60 ft³ each) per post, or about 3 bags of 50 lb fast-setting mix (0.375 ft³ each) when you stop 3–4 in below the top.
How to use the post hole concrete calculator
- Enter the hole diameter and depth in inches. A common rule is three times the post width, a third of its height.
- Pick the post that goes in the hole — the calculator subtracts the space it takes up.
- Enter how many holes you are filling and keep some extra for spillage.
- Choose your bag size to see the bag count, and add a price for a cost estimate.
Formulas
Hole volume (ft³) = π × (diameter ÷ 2)² × depth, in feet Post volume (ft³) = post cross-section (ft²) × depth Concrete per hole = hole volume − post volume Bags = total volume ÷ bag yield, rounded up
Bag yields come from QUIKRETE’s Concrete Mix (No. 1101) data sheet: 0.60 ft³ for an 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb and 0.30 for 40 lb. A 4 × 4 timber is really 3.5 in and a 6 × 6 is 5.5 in, which is what the calculator uses.
Worked example: four 4 × 4 posts in 10 in holes, 30 in deep
- Hole volume: π × (0.417 ft ÷ 2)² × 2.5 ft = 1.36 ft³.
- Post volume: (3.5 ÷ 12)² × 2.5 = 0.21 ft³.
- Concrete per hole: 1.36 − 0.21 = 1.15 ft³.
- Four holes: 1.15 × 4 = 4.60 ft³. At 0.60 ft³ per 80 lb bag that is 8 bags.
How deep and how wide
The usual guidance is a hole three times the width of the post and deep enough to bury a third of the post’s above-ground height, with the bottom below your local frost line. A 6 ft fence post therefore wants roughly a 2 ft hole, and a 4 × 4 wants a 10 to 12 in diameter. Put 4 to 6 in of gravel in the bottom first so water drains away from the end grain.
Quick reference
| Hole size | Concrete per hole | 80 lb bags per hole | Bags for 10 posts |
|---|---|---|---|
| 8 in × 24 in | 0.55 ft³ | 1 | 10 |
| 10 in × 24 in | 0.92 ft³ | 2 | 16 |
| 10 in × 30 in | 1.15 ft³ | 2 | 20 |
| 12 in × 30 in | 1.75 ft³ | 3 | 30 |
| 12 in × 36 in | 2.10 ft³ | 4 | 35 |
Assumes a 4 × 4 post in the hole and no extra allowance.
Questions
How many bags of concrete per fence post?
For a 4 × 4 post in a 10 in hole 30 in deep, about 2 bags of 80 lb mix per post. Wider or deeper holes go up quickly — a 12 in hole 36 in deep takes nearly 4.
How deep should a fence post hole be?
About a third of the post’s above-ground height, and always below the frost line in your area. A 6 ft fence usually means a hole around 2 ft deep.
How wide should a post hole be?
Roughly three times the width of the post, so 10 to 12 in for a 4 × 4 and about 18 in for a 6 × 6.
Do I need gravel under the post?
4 to 6 inches of gravel in the bottom of the hole lets water drain away from the end of the post instead of sitting against it, which is the usual reason wooden posts rot at the base.
How many 50 lb bags of concrete do I need per post?
Enter your hole and post size above and pick the 50 lb bag size; the calculator subtracts the space the post takes up. Bigger bags mean fewer trips, but the volume needed per hole stays the same.
How many bags of fast-setting concrete per fence post?
QUIKRETE Fast-Setting Concrete Mix yields about 0.375 cu ft per 50 lb bag (0.45 cu ft per 60 lb). QUIKRETE’s own rule of thumb is two 50 lb bags for a 4 in post in a 10 in hole 2 ft deep. For a 4 × 4 in a 10 in hole 30 in deep, filled to 3–4 in below the top, plan on 3 bags. Pick your bag size in the concrete bag calculator to compare.
Should a post be buried a third or half of its height?
Both rules are in use. The common fence guideline is a third of the post’s above-ground height, while QUIKRETE’s Fast-Setting data sheet says one-half. Either way, go below your local frost line, and follow the fence or deck maker’s instructions and any local code.
How much concrete for a 6 × 6 post?
A 6 × 6 (5.5 in actual) in an 18 in hole 36 in deep needs about 4.67 cubic feet of concrete, which is 8 bags of 80 lb mix. For deck footings poured as round columns, the concrete calculator handles the cylinder volume in cubic yards. Put 4–6 in of drainage gravel in first; the gravel calculator covers bulk orders.
Sources
- Concrete Mix (No. 1101) data sheet, QUIKRETE.
Used for: Cubic feet of mixed concrete per 40, 50, 60 and 80 lb bag. - NIST Guide to the SI, Appendix B.8: Factors for units listed alphabetically, National Institute of Standards and Technology.
Used for: Exact inch, foot and gallon definitions and the metric conversions. - Fast-Setting Concrete Mix (No. 1004-50) data sheet, QUIKRETE.
Used for: Fast-setting bag yields, the two-bag post example and post-setting method.