About the Post Hole Concrete Calculator
This post hole concrete calculator works out how much concrete it takes to set posts, and how many pre-mixed bags to buy. Enter the hole diameter and depth, the depth of any gravel at the bottom, the post size and the number of posts. It subtracts the volume the post itself occupies, so you do not over-buy.
Use it for wood fence posts, chain-link line and terminal posts, mailbox and sign posts, pergola and deck posts. Bag yields follow the figures printed on common US bags: about 0.30 cu ft for a 40 lb bag, 0.375 cu ft for 50 lb, 0.45 cu ft for 60 lb and 0.60 cu ft for 80 lb; fast-setting post mixes usually yield the same per pound.
A common rule of thumb is a hole about three times the width of the post and deep enough to bury one-third to one-half of the post’s above-ground height, and always below your local frost line. Holes dug by hand are rarely perfect cylinders, so a 10% allowance is included by default.
With the default inputs, the bags of concrete is 34. Change any value above to recalculate instantly.
How to use the post hole concrete calculator
- 1Enter the hole diameter and total depth.
- 2Enter the gravel base depth (or 0).
- 3Choose the post type and how many posts you are setting.
- 4Pick the bag size you plan to buy.
- 5Read the total bags and bags per post.
Formula and method
Each hole is treated as a cylinder of diameter D. The concrete only fills the part above the gravel base (H − G), and the post occupies part of that column, so its cross-section (3.5 × 3.5 in for a 4×4, πr² for round posts) is subtracted before multiplying by the fill depth. Dividing cubic inches by 1,728 gives cubic feet.
The per-post volume is multiplied by the number of posts and the allowance for irregular holes, then divided by the bag yield and rounded up to whole bags. Bags-per-post is shown unrounded so you can plan whether to mix full bags per hole or batch several holes at once.
- D
- Hole diameter in inches
- H
- Hole depth in inches
- G
- Gravel base depth in inches
- A_post
- Post cross-section area in square inches
- N
- Number of posts
- w
- Extra allowance as a decimal
Worked examples
10 posts, 4×4 in 10 in × 36 in holes, 6 in gravel, 50 lb bags
The hole is π × 5² = 78.54 sq in, minus 12.25 sq in for the post = 66.29 sq in, times 30 in of fill = 1,988.7 cu in = 1.151 cu ft per post. Ten posts with 10% extra need 12.66 cu ft, or 12.66 ÷ 0.375 = 33.8 → 34 bags of 50 lb.
6×6 deck posts, 12 in × 48 in holes, 80 lb bags
Each hole is 113.10 sq in minus 30.25 sq in for the post = 82.85 sq in, times 42 in of fill = 3,479.6 cu in = 2.014 cu ft. Six posts need 12.08 cu ft, and 12.08 ÷ 0.6 = 20.1, so 21 bags of 80 lb.
Chain-link line posts, 8 in × 30 in holes, no gravel
An 8 in hole is 50.27 sq in; a 1⅞ in pipe takes 2.76 sq in, leaving 47.50 sq in × 30 in = 1,425.1 cu in = 0.825 cu ft per post. Twelve posts plus 10% = 10.89 cu ft ÷ 0.375 = 29.03, so 30 bags.
Frequently asked questions
How many bags of concrete do I need per fence post?+
For a 4×4 post in a 10 in wide, 3 ft deep hole with 6 in of gravel, plan on about 3 to 3½ bags of 50 lb concrete, or roughly 2 bags of 80 lb, per post.
How deep should a fence post hole be?+
A common guide is one-third to one-half of the post’s above-ground height — about 24 to 36 in for a 6 ft fence — and always below the local frost line to prevent heaving.
How wide should a post hole be?+
Aim for about three times the post width: roughly 10–12 in for a 4×4 and 16–18 in for a 6×6. Wider holes use more concrete but resist leaning better in soft soil.
Should I put gravel at the bottom of a post hole?+
Yes, 4–6 in of compacted gravel under the post helps water drain away from the end grain of wood posts and gives a firm base to stand the post on.
What is the yield of an 80 lb bag of concrete?+
An 80 lb bag of standard pre-mixed concrete yields about 0.60 cu ft; a 60 lb bag about 0.45 cu ft, a 50 lb bag about 0.375 cu ft and a 40 lb bag about 0.30 cu ft.