Fence post concrete calculator

How It Works

The calculator:

Multiplies by the number of posts to get the total amount needed

Calculates the volume of the hole (cylindrical shape)

Subtracts the volume of the post that will occupy part of the hole

Converts the difference to your preferred unit (cubic feet, cubic yards, or bags)

Calculation Options:

1. Concrete Type Selection:
   • Pre-Mixed Concrete: Calculates the number of 80 lb pre-mixed concrete bags
   • Make Your Own: Breaks down the materials needed for mixing your own concrete
2. Material Breakdown Section:
   • For pre-mixed concrete: Shows total 80 lb bags needed
   • For mix-your-own option: Calculates and displays:
     • Portland Cement (94 lb bags)
     • Sand (cubic feet)
     • Gravel (cubic feet)
3. Material Calculation Logic:
   • Uses standard 1:2:3 ratio (cement:sand:gravel) for the mix-your-own option
   • Automatically rounds up cement bags to whole numbers
   • Provides precise measurements for sand and gravel

The different bag weights (80 lb and 94 lb) represent industry standard packaging for different types of concrete products:

80 lb Bags (Pre-Mixed Concrete)

• These are the standard size for pre-mixed concrete bags commonly sold at hardware stores and home improvement centers
• Pre-mixed concrete contains cement, sand, and gravel already blended together in the proper ratio
• You just add water and mix
• 80 lb is the most common size for DIY and residential projects (though 60 lb bags are also available)
• One 80 lb bag typically yields about 0.6 cubic feet of concrete when mixed

94 lb Bags (Portland Cement)

• This is the standard weight for bags of Portland cement (not pre-mixed concrete)
• Portland cement is just the binding agent used in concrete, not the complete mix
• When making your own concrete mix, you combine Portland cement with sand and gravel
• 94 lb (or sometimes 92.6 lb) has been the industry standard weight for cement bags for decades
• One 94 lb bag of Portland cement, when mixed with the appropriate amounts of sand and gravel, will yield approximately 6 cubic feet of concrete

The calculator uses these different weights because they represent what you’ll actually find when purchasing materials. If you buy pre-mixed concrete, you’ll be counting 80 lb bags. If you’re mixing your own concrete, you’ll be buying 94 lb bags of cement plus separate sand and gravel.

I’ll walk you through the calculation logic used in the calculator, clarify what’s included in the results, and explain about the fence posts.

How the Calculator Works (Step-by-Step)

Let’s use an example calculation with typical inputs:

• Post shape: Circular
• Post diameter: 4 inches
• Hole diameter: 10 inches
• Hole depth: 24 inches
• Number of posts: 10

1. Calculate Post Volume (what the post occupies in the hole)

For a circular post:

Post volume = π × (diameter/2)² × depth
Post volume = π × (4/2)² × 24
Post volume = π × 4 × 24
Post volume = 301.59 cubic inches

2. Calculate Hole Volume

Hole volume = π × (diameter/2)² × depth
Hole volume = π × (10/2)² × 24
Hole volume = π × 25 × 24
Hole volume = 1,884.96 cubic inches

3. Calculate Concrete Volume Needed

Concrete volume = Hole volume - Post volume
Concrete volume = 1,884.96 - 301.59
Concrete volume = 1,583.37 cubic inches

4. Convert to Cubic Feet

Concrete volume in cubic feet = 1,583.37 ÷ 1,728 (cubic inches in a cubic foot)
Concrete volume in cubic feet = 0.92 cubic feet per post

5. Calculate Total Concrete for All Posts

Total concrete = 0.92 × 10 posts = 9.2 cubic feet

6. Calculate Materials

For pre-mixed concrete bags:

Number of 80 lb bags = 9.2 ÷ 0.6 (cubic feet per bag) = 15.33 bags
Rounded up = 16 bags

For mix-your-own (using 1:2:3 ratio of cement:sand:gravel):

Total parts = 1 + 2 + 3 = 6 parts
Cement = 9.2 ÷ 6 = 1.53 cubic feet (about 2 bags)
Sand = 1.53 × 2 = 3.06 cubic feet
Gravel = 1.53 × 3 = 4.59 cubic feet

About the Fence Posts

1. Posts Not Included: The calculator only determines the concrete needed for securing fence posts. The actual fence posts themselves are not included in the calculation – you would purchase these separately.
2. Post Material: Fence posts typically come from hardware stores, lumber yards, or home improvement centers. Common materials include:
   • Pressure-treated wood
   • Cedar or redwood
   • Metal (galvanized steel or aluminum)
   • Vinyl/PVC
3. Post Installation: The calculation assumes you’re:
   1. Digging holes of your specified diameter and depth
   2. Placing the posts in these holes
   3. Filling the space between the posts and hole walls with concrete
4. Concrete Purpose: The concrete around the posts provides:
   • Stability and strength
   • Protection against rotting (for wooden posts)
   • Resistance to fence movement from wind and other forces

The calculator subtracts the post volume from the hole volume because that space will be occupied by the post itself, not concrete. You only need concrete for the space between the post and the walls of the hole.