Fence Post Hole Cement Calculator

Last updated: 12 May 2025 | Author: Brij | Review By: Irshad

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Easily estimate how much concrete and how many cement bags you'll need to fill your fence post holes. Just enter hole size, number of posts, and cement bag weight to get accurate results instantly!

Number of Posts:

Hole Diameter:

inches

Hole Depth:

inches

Cement Bag Weight:

kg/bag

Concrete Mix Ratio:

Cement Calculation

Total Concrete Volume:

cubic ft

Cement Bags Needed:

bags

Are you installing fence posts, sign posts, or deck supports? Accurately calculating how much concrete you’ll need to fill post holes is crucial for a strong and long-lasting foundation. Our Post Hole Concrete Calculator makes it simple! Just enter the hole dimensions, number of posts, and cement bag weight — and get an instant estimate of concrete volume, number of cement bags required, and total cost.

Formula Used in Post Hole Concrete Calculator

Volume of a single post hole (cylindrical hole)

$\text{Volume} = \pi \times r^2 \times h$

Where:

$r$ = radius of hole (in feet)
$h$ = depth of hole (in feet)

Total concrete volume for all posts

$\text{Total Volume} = \text{Volume per hole} \times \text{Number of Posts}$

Cement proportion in mix ratio

$\text{Cement Part} = \frac{\text{Cement Ratio}}{\text{Sum of all parts}}$

Example (1:3:5 mix):

Cement Part = $\frac{1}{1 + 3 + 5} = \frac{1}{9}$

Total weight of concrete

$\text{Concrete Weight} = \text{Total Volume} \times 145 \, \text{lbs/ft}^3$

Weight of cement required

$\text{Cement Weight} = \text{Concrete Weight} \times \text{Cement Part}$

Number of cement bags required

Since bag weight is in kg, we convert to pounds:

$1 \, \text{kg} = 2.20462 \, \text{lbs}$

$\text{Bags Needed} = \frac{\text{Cement Weight}}{\text{Bag Weight (kg)} \times 2.20462}$

Example Calculation

Given:

Hole Diameter = 8 inches
Hole Depth = 24 inches
Number of Posts = 10
Cement Bag Weight = 50 kg
Mix ratio = 1:3:5

Step 1: Convert dimensions to feet

$r = \frac{8}{2} = 4 \, \text{inches} = \frac{4}{12} = 0.333 \, \text{ft}$

$h = \frac{24}{12} = 2 \, \text{ft}$

Step 2: Calculate volume per hole

$\text{Volume} = \pi \times (0.333)^2 \times 2 = 0.697 \, \text{ft}^3$

Step 3: Total volume

$\text{Total Volume} = 0.697 \times 10 = 6.98 \, \text{ft}^3$

Step 4: Cement proportion

$\text{Cement Part} = \frac{1}{9} = 0.111$

Step 5: Concrete weight

$\text{Concrete Weight} = 6.98 \times 145 = 1012.1 \, \text{lbs}$

Step 6: Cement weight

$\text{Cement Weight} = 1012.1 \times 0.111 = 112.3 \, \text{lbs}$

Step 7: Number of cement bags

$\text{Bags Needed} = \frac{112.3}{50 \times 2.20462} = 1.02 \, \text{bags}$

Final Answer:

✅ You will need approximately 1 bag of 50 kg cement to fill 10 post holes of the given size.

FAQs

1. How deep should a fence post hole be?

A good rule of thumb is to dig the hole one-third to one-half the height of the post above ground.

For example, for a 6-foot tall fence, the hole should be 2 to 3 feet deep.

2. How wide should a fence post hole be?

The hole diameter should be 3 times the width of the post.

For a standard 4x4 post (which is 3.5 inches thick), the hole should be 10 to 12 inches wide.

3. Should I use concrete to set fence posts?

Yes, using concrete provides a strong and stable base, especially for wooden, metal, or vinyl posts. It helps resist wind, soil movement, and water damage.

4. How much concrete do I need per fence post?

Typically, a 50 lb (22.7 kg) bag of concrete will fill:

(1) 6-inch diameter × 2-foot deep hole
(½) of an 8-inch diameter × 2-foot deep hole

Our calculator will give you exact volume and cement bags needed based on your hole size.

5. Can I set a fence post without concrete?

Yes, but only in very stable, well-drained soil. Gravel tamped tightly around the post is an alternative, but concrete is more durable and long-lasting.

6. How long does concrete take to set for fence posts?

Concrete sets in 24 to 48 hours, but for best results, wait 3–7 days before applying full load (like attaching fence panels).

7. Do I need to put gravel at the bottom of the post hole?

Yes, adding 4–6 inches of gravel at the bottom improves drainage, prevents wood rot, and helps keep the post stable.

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