Post Hole Concrete Calculator
Calculate how many yards or how many bags of concrete you’ll need for a
set of post holes by entering the post diameter and hole size. If you’re
not sure what size hole to dig, a rule of thumb is to use a hole
diameter three times the post diameter and ⅓ as deep as the post height.
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On this page
- Calculation Steps
- Step 1: Determine the post diameter and hole size
- Step 2: Calculate the volume of each hole
- Step 3: Calculate the total cost of concrete
- Example
- Step 1: Post diameter and hole size
- Step 2: Volume of each hole
- Step 3: Total cost of concrete
- Calculation Summary
- What is the best type of concrete to use for post holes?
- How much concrete is typically needed for a post hole?
- How long does it take for the concrete in a post hole to dry or cure?
- What are the steps to properly mix and pour concrete into a post hole?
- Can I use post hole concrete in wet or cold weather conditions?
Calculation Steps
To calculate the amount of concrete needed for a set of post holes, follow these steps:
Step 1: Determine the post diameter and hole size
Measure the diameter of your post. A rule of thumb for a sturdy footing is a hole three times the post diameter, dug to a depth of one third of the post height (at least 2 feet for a 6 foot fence).Step 2: Calculate the volume of each hole
Concrete fills the ring between the hole and the post, so subtract the post from the hole:Volume = π/4 x (Hole Diameter² - Post Diameter²) x Depth
Work in feet (divide inches by 12) so the result comes out in cubic feet, then multiply by the number of holes.Step 3: Calculate the total cost of concrete
Add about 10% for waste, divide the cubic feet by 27 to get cubic yards, and multiply by your price per cubic yard. For pre-mixed dry bags instead, divide the cubic feet by the bag yield: about 0.45 cubic feet per 60 lb bag or 0.60 cubic feet per 80 lb bag, rounding up.Example
Suppose you are setting 4 fence posts. Each post is 6 inches in diameter, each hole is 18 inches in diameter (three times the post diameter), each hole is 4 feet deep, and ready-mix concrete costs $125 per cubic yard in your area.Step 1: Post diameter and hole size
The post diameter is 6 inches (0.5 feet). Using the rule of thumb, the hole diameter is three times the post diameter: 3 x 6 = 18 inches (1.5 feet). The hole depth is 4 feet.Step 2: Volume of each hole
Concrete fills the ring between the hole and the post. Convert the diameters to feet, then:Volume = π/4 x (1.5² - 0.5²) x 4 = 6.28 cubic feet per hole
For 4 holes, the subtotal is 6.28 x 4 = 25.13 cubic feet.Step 3: Total cost of concrete
Add 10% for waste: 25.13 x 1.10 = 27.65 cubic feet, which is 27.65 / 27 = 1.02 cubic yards. At $125 per cubic yard the concrete costs about $128. If you use pre-mixed bags instead, an 80 lb bag yields about 0.60 cubic feet, so you would need 27.65 / 0.60 = 47 bags.Calculation Summary
In summary, setting 4 posts in 18 inch x 4 foot holes takes about 1.02 cubic yards of concrete (or 47 80-lb bags), costing roughly $128 in ready-mix concrete. Enter your own post size, hole size, and local price in the calculator above for an exact figure.
What is the best type of concrete to use for post holes?
The best type of concrete to use for post holes is a high-strength concrete mix. This type of concrete is specifically designed to withstand the pressure and weight of posts. It typically has a compressive strength of 3,000 to 4,000 pounds per square inch (psi).
When choosing a concrete mix, look for one that is labeled as 'post-setting' or 'post-hole' concrete. These mixes usually contain a blend of cement, sand, and gravel, along with additives that improve workability and durability.
It's important to follow the manufacturer's instructions for mixing and setting the concrete. Generally, you'll need to dig a hole that is wider at the bottom than at the top, place the post in the hole, and then pour the concrete mix around the post. Make sure to properly brace the post until the concrete sets.
How much concrete is typically needed for a post hole?
The amount of concrete needed for a post hole depends on the size and depth of the hole. As a general guideline, a standard post hole with a diameter of 8 inches and a depth of 2 feet typically requires around 0.6 cubic feet of concrete. However, it is recommended to consult with a professional or refer to specific building codes for accurate calculations based on your project requirements.
How long does it take for the concrete in a post hole to dry or cure?
The drying or curing time for concrete in a post hole can vary depending on several factors such as weather conditions, type of concrete used, and the size of the post hole. Generally, it takes about 24 to 48 hours for the concrete to dry enough to support light loads. However, it is recommended to wait at least 72 hours before applying heavy loads or putting excessive pressure on the post.
What are the steps to properly mix and pour concrete into a post hole?
- Prepare the post hole by digging it to the desired depth and width.
- Place a layer of gravel or crushed stone at the bottom of the hole for drainage.
- Insert a post into the hole and use a level to ensure it is straight.
- Secure the post in place using braces or stakes.
- In a separate container, mix the concrete according to the instructions on the bag.
- Pour the mixed concrete into the hole, filling it up to a few inches below the surface.
- Use a shovel or rod to agitate the concrete and remove any air bubbles.
- Smooth the surface of the concrete with a trowel.
- Allow the concrete to cure according to the instructions on the bag before removing the braces or stakes.
Can I use post hole concrete in wet or cold weather conditions?
It is generally not recommended to use post hole concrete in wet or cold weather conditions. Wet weather can affect the curing process of the concrete, leading to weaker and less durable results. Cold weather can also slow down the curing process and may cause the concrete to freeze, which can result in cracking and other structural issues.
It is best to wait for dry and mild weather conditions before using post hole concrete to ensure optimal results. If you must work in wet or cold weather, consider using specialized concrete mixes or additives designed for such conditions.
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