Fence post depth calculator
Hole depth = 1/3 of the post’s overall length, and its width 3 times the post’s: a 4x4 standing 6 ft above the ground goes 3 ft into a hole 10 1/2 in wide, a 10 ft post to buy, about 4 bags of 50 lb Fast-Setting concrete a hole.
Example: replace these measurements
Your post holes
40 bags of 50 lb Fast-Setting concrete
40 bags of 50 lb Fast-Setting concrete
Your shopping list
- 40 bags of 50 lb Fast-Setting concrete
Quantities and package details
- Hole depth
- 36 in: 1/3 of the post’s overall length
- Hole width
- 10.5 in across, 3 × the post
- Post in the ground
- 36 in
- Post length to buy
- 10 ft, for 9 ft of post
- Concrete per hole
- 1.47 cu ft, 3.9 bags
- Bags of 50 lb Fast-Setting
- 40 for 10 posts
The calculation
- In the ground: 72 in above ÷ 2 = 36 in, so 36 in is 1/3 of a 108 in post
- Hole width: 3 × 3.5 in = 10.5 in
- Concrete: (3.1416 × 10.5² ÷ 4 − 3.1416 × 3.5² ÷ 4) × (36 − 3 in) = 2,540 cu in = 1.47 cu ft
- 1.47 cu ft ÷ 0.375 cu ft a bag = 3.92 a hole; × 10 posts, rounded up = 40 bags
Common questions
How deep should a fence post be?
QUIKRETE’s rule is 1/3 of the post’s overall length in the ground: a post standing 6 ft above the ground goes 3 ft deep, so you buy a post of at least 9 ft. That is the deeper end of the range in QUIKRETE’s project instructions, 1/3 to 1/2 of the height above the ground: 2 to 3 ft for a 6 ft fence. Where the ground freezes, go at least as deep as the frost depth your building department gives.
Source: QUIKRETE fence post methods: Concrete Mix No. 1101 and Fast-Setting Concrete Mix No. 1004 data sheets. Listed on the Sources page.
How wide should a fence post hole be?
About 3 times the post’s width: 10 1/2 in for a 4x4, which is 3 1/2 in square, and 16 1/2 in for a 6x6.
Source: QUIKRETE fence post methods: Concrete Mix No. 1101 and Fast-Setting Concrete Mix No. 1004 data sheets. Listed on the Sources page.
How many bags of concrete for a fence post?
For a 4x4 standing 6 ft high, in a hole 10 1/2 in wide and 3 ft deep, about 1.5 cubic feet: 4 bags of 50 lb Fast-Setting, filled to 3 in below the top. QUIKRETE says two 50 lb bags set a 3 in post in a 9 in hole 30 in deep.
Source: QUIKRETE fence post methods: Concrete Mix No. 1101 and Fast-Setting Concrete Mix No. 1004 data sheets. Listed on the Sources page.
How long before a fence post can take weight?
Fast-Setting concrete sets in 20 to 40 minutes and QUIKRETE says to wait 4 hours before the post takes any strain. With its standard Concrete Mix, wait 24 hours.
Source: QUIKRETE fence post methods: Concrete Mix No. 1101 and Fast-Setting Concrete Mix No. 1004 data sheets. Listed on the Sources page.
How to plan the quantity
QUIKRETE’s fence post method digs a hole about 3 times as wide as the post and 1/3 of the post’s overall length deep. So a post standing 6 ft above the ground is a 9 ft post with 3 ft in the ground, in a hole about 10 1/2 in wide for a 4x4. QUIKRETE’s project instructions give a range, 1/3 to 1/2 of the height above the ground: 2 to 3 ft for a 6 ft fence. The data sheets’ rule, used here, is the deeper end; at the shallow end a 6 ft fence takes 8 ft posts.
With Fast-Setting concrete, stand the post in the hole, pour the dry mix around it to 3 to 4 in below the top and soak it with about a gallon of water a bag; finish the hole with soil. The post can take strain after 4 hours. With Concrete Mix, put 6 in of dry mix in the bottom and stand the post on it, so the hole is 6 in deeper; fill with mixed concrete, slope the top away from the post, and wait 24 hours.
The concrete is the hole less the post, counted as round so the bags never fall short. QUIKRETE says two 50 lb bags of Fast-Setting set a 3 in post in a 9 in hole 30 in deep; the arithmetic on its own yield gives about 2.4, so the list rounds up to a spare.
Before you order
- This is QUIKRETE’s rule for fence posts. Gate posts, a tall fence in strong wind, soft or wet soil and your local rules can call for deeper or wider holes: ask your building department, and call 811 before you dig.
- It does not size a footing for a deck, a pergola or anything that carries a structure: those follow the plan and the code.
Before ordering, read the guide: How to measure for a fence: posts, gates, corners and slopes. Or choose another calculator.