Concrete Calculator for Post Hole: How Much Mix?

This concrete calculator works out how much ready-mix concrete you need to fill a post hole. You enter the hole diameter, hole depth, and post diameter (or width), and it returns the volume of concrete in cubic feet and cubic yards (with liters and cubic meters as a secondary output). The calculation subtracts the volume of the post itself from the volume of the hole, giving you the true amount of concrete required.

Formula

The calculator uses the standard volume formula for a cylinder: V = π × r² × h, where r is the radius (half the diameter) and h is the height (depth). For a post hole, you subtract the volume of the post that sits inside the hole.

• Hole volume = Ï€ × (hole diameter / 2)² × hole depth
• Post volume = Ï€ × (post width / 2)² × hole depth (for round posts; for square posts, use width × width × depth)
• Concrete volume = hole volume − post volume

All inputs are in inches. The result is converted to cubic feet by dividing by 1728 (12³), then to cubic yards by dividing by 27, and to liters by multiplying cubic feet by 28.3168.

Worked Example

Using the default values: hole diameter = 12 in, hole depth = 36 in, post width = 4 in.

1. Hole radius = 12 / 2 = 6 in
2. Hole volume = π × 6² × 36 = π × 36 × 36 = 3.14159 × 1296 = 4071.5 in³
3. Post radius = 4 / 2 = 2 in
4. Post volume = π × 2² × 36 = π × 4 × 36 = 3.14159 × 144 = 452.39 in³
5. Concrete volume = 4071.5 − 452.39 = 3619.11 in³
6. Convert to ft³: 3619.11 / 1728 = 2.09 ft³
7. Convert to yd³: 2.09 / 27 = 0.077 yd³
8. Convert to liters: 2.09 × 28.3168 = 59.2 L

The calculator shows 2.09 ft³ (0.077 yd³, 59.2 L). This matches the default output.

How to Use the Calculator

Measure the hole diameter at the top (most holes are slightly wider at the top; use the average if it varies). Measure the depth from the ground surface to the bottom. For the post, measure its actual width or diameter—not the nominal size (e.g., a “4×4” is usually 3.5 in wide). Enter these in inches and the calculator does the rest.

If you are setting a square post in a round hole, the calculator assumes a round post. For a square post, you can approximate by using the post width as the diameter, but that slightly overestimates the post volume (and thus underestimates concrete). For accuracy with square posts, compute the post volume as width × width × depth and subtract manually.

Choosing the Right Hole Size

For most fences and decks, a common rule of thumb is: the hole diameter should be about 3 times the post width, and the depth should be about 1/3 to 1/2 of the above-ground post height. For a 4×4 post (3.5 in actual), that means a 10–12 in diameter hole. For a 6×6 post (5.5 in), use a 12–18 in hole.

Deeper holes are needed in colder climates where frost heave is a concern—the bottom of the concrete must sit below the frost line. Check local building codes for minimum depths in your area.

Common Mistakes and Misconceptions

• Using nominal post size: A “4×4” is actually 3.5 in wide. Using 4 in overestimates the post volume, leading to too little concrete.
• Forgetting the post volume: Some calculators only compute the hole volume. That can overestimate concrete by up to 15% for a typical 4×4 in a 12-in hole.
• Mixing units: If you measure the hole in inches and the depth in feet, the result is off by a factor of 12. Always use the same unit.
• Not accounting for gravel: Many builders put 4–6 inches of gravel at the bottom of the hole for drainage. Subtract that from the depth before calculating.
• Assuming the hole is a perfect cylinder: Hand-dug holes are often wider at the top. Use an average diameter or measure at mid-depth.

How Much Concrete Do I Buy?

Concrete is sold in two ways: by the cubic yard (ready-mix delivered by truck) or by the bag (dry mix that you add water to). For most post holes, bags are more practical.

A standard 40 lb bag of concrete mix yields about 0.3 cubic feet of concrete when mixed. A 60 lb bag yields about 0.45 cubic feet, and an 80 lb bag yields about 0.6 cubic feet. To find the number of bags, divide the total cubic feet by the yield per bag.

For the default example (2.09 ft³), you would need about 7 bags of 40 lb mix, 5 bags of 60 lb, or 4 bags of 80 lb. Always round up—it’s better to have a little extra than to run short.

Cost Considerations

Bagged concrete typically costs more per cubic foot than ready-mix, but for small projects, the difference is negligible compared to the delivery fee for a ready-mix truck. Ready-mix has a minimum order (often 1 cubic yard), so if you need less than that, bags are the economical choice.

For a single post hole, you will almost always use bags. For a long fence with many holes, calculate the total volume and compare the cost of bags versus a ready-mix delivery. Remember to include the cost of mixing tools and water.

Safety and Practical Tips

• Wear gloves and eye protection when handling dry concrete mix—it is caustic and can cause burns.
• Mix concrete in a wheelbarrow or mixing tub on a flat surface. Add water gradually, mixing to a uniform, workable consistency.
• Set the post plumb (vertically) before the concrete hardens. Use a level and brace the post with stakes.
• Allow concrete to cure properly—keep it moist for at least 24–48 hours. Do not put heavy loads on the post for at least 3–7 days.
• Check for underground utilities before digging. Call your local utility locator service.
• Dispose of excess concrete properly—never pour it down a drain or onto the ground where it can contaminate soil.

Frequently Asked Questions

How deep should a post hole be?

Typically, the hole depth should be about 1/3 to 1/2 of the above-ground post height. For a 6-foot fence post, dig a hole 2–3 feet deep. In cold climates, the hole must extend below the frost line to prevent frost heave—check local building codes for the frost depth in your area.

How many bags of concrete do I need for a 4×4 post?

For a standard 4×4 post (3.5 in actual) in a 12-inch diameter hole that is 36 inches deep, you need about 2.09 cubic feet of concrete. That translates to roughly 5 bags of 60 lb mix or 4 bags of 80 lb mix. Adjust the numbers based on your actual hole size.

Can I use this calculator for square posts?

Yes, but with a caveat. The calculator assumes a round post. For a square post, the volume is width × width × depth, which is slightly larger than a round post of the same width. To be accurate, calculate the post volume separately and subtract it from the hole volume manually.

What if my hole is not a perfect cylinder?

Hand-dug holes are often irregular. Measure the diameter at the top, bottom, and mid-depth, then use the average. For a more conservative estimate, use the largest diameter—this gives you extra concrete, which is rarely a problem.

Should I include gravel in the depth?

If you plan to put gravel at the bottom of the hole for drainage, subtract that depth from the hole depth before calculating. For example, if the hole is 36 inches deep and you add 4 inches of gravel, use 32 inches as the depth.

How do I convert cubic feet to bags?

Divide the total cubic feet by the yield per bag. Common yields: 40 lb bag = 0.3 ft³, 60 lb = 0.45 ft³, 80 lb = 0.6 ft³. Always round up to the nearest whole bag.

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in d | Hole diameter | in | 12 | 0
in h | Hole depth | in | 36 | 0
in p | Post width/diameter | in | 4 | 0
out concrete_ft3 | Concrete volume | ft³ | (pi * ((d/2)^2) * h - pi * ((p/2)^2) * h) / 1728 | 2
out concrete_yd3 | Concrete volume | yd³ | ((pi * ((d/2)^2) * h - pi * ((p/2)^2) * h) / 1728) / 27 | 2
out concrete_liters | Concrete volume | L | ((pi * ((d/2)^2) * h - pi * ((p/2)^2) * h) / 1728) * 28.3168 | 2