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 1⁄3 as deep as the post height. See below for more information.
On this page:
- Concrete Calculator
- Using the Post Hole Calculator
- How to Find the Amount of Concrete Needed
- Post Hole Volume Formulas
- Calculate Concrete Volume
- Example Fence Post Concrete Calculation
- Pre-Mix Bag Concrete Yield
- Tips For Ordering Concrete
- Type of concrete and its proper installation
- Additional costs
- Additional concrete
- Allocate enough time
Using the Post Hole Calculator
Use the post hole estimator to quickly find out how much concrete you will need to set your fence posts. Enter your post and hole dimensions in any units.
The calculator will give you the results in cubic yards and the number of pre-mixed bags of concrete (40 lb, 60 lb, and 80 lb bags).
To ensure that your posts are sturdy for a long time to come, remember to make your holes three times the diameter of your post and 1⁄3 as deep as your fence. So, a 6’ high fence needs holes a minimum of 2’ deep.
Order additional material to account for settling and spillage. If you have many posts to set, it may pay you to mix the concrete yourself. Or, if you need a cubic yard or more, order mixed and delivered.
Concrete contractors can provide estimates for delivering and installing cement for your post project.
How to Find the Amount of Concrete Needed
Concrete is sold pre-made (and usually delivered) by the cubic yard or in pre-mixed bags by weight.
To calculate how much concrete you need, you need to know the post size you are going to use and how many posts you are installing. Your post diameter and height determine how big each hole needs to be.
As stated above, fence post holes should be three times the diameter of your post and 1⁄3 as deep as your post.
First, you need to calculate the volume for the post hole. Then calculate the volume for the part of the post that will be submerged in the hole. Make sure all your units are the same!
Post Hole Volume Formulas
Use these formulas to calculate volume or, for round holes, use our cylinder cubic yardage calculator.
Rectangle: volume = length x width x depth
Cylinder: volume = radius2 x π x depth (π = 3.14)
Calculate Concrete Volume
With the volume of the submerged post and the volume of the hole figured out, subtract the post volume from the hole volume, and you will have the volume of concrete needed for each hole. Simply multiply that amount times the number of posts to get a total quantity. Don’t forget to add in some extra for spillage and waste.
If you have multiple different size post holes to fill, calculate each set with matching dimensions separately, then add the totals together.
Example Fence Post Concrete Calculation
Let’s use an example to illustrate what we covered above. Let’s say we are installing a 6’ fence. Therefore, 2’ of the post will be in the ground. You need twenty (20) 4” nominal round posts. Note that 4” posts are actually 3 ½” in diameter. Use the formulas above for volume.
Calculate the post volume
Change post diameter to feet:
3.5” / 12 = 0.2917’
Find radius (½ of the diameter):
0.2917’ / 2 = 0.1458’
Volume of round post:
(0.1458’)2 x π x 2’ = 0.1336 cubic ft.
Calculate the hole volume
Hole radius (3x post radius):
3 x 0.1458’ = 0.4374’
Volume of post hole:
( 0.4374’)2 x π x 2’ = 1.2026 cubic ft.
Calculate concrete volume
Hole volume – post volume:
1.2026 – 0.1336 = 1.069 cubic ft. per post
TOTAL volume concrete:
1.069 cubic ft x 20 posts = 21.38 cubic ft.
Convert to cubic yards:
21.38 cubic ft. / 27 = 0.79 cubic yards
Hint: you can also use a concrete calculator for this step.
You might find it easier to convert the cubic footage to cubic yards; if so, divide the cubic footage by 27.
Most concrete suppliers will not sell fractions of a yard of concrete, though it is worth asking about. So, in this example, you would want to buy bags. Using the table below, you would need to buy at least eight 80 lb. bags and one 40 lb. bag (yielding a total of 5.1 cubic feet) of concrete to allow enough for extra.
Pre-Mix Bag Concrete Yield
The following table shows the approximate yield of mixed concrete for common bag weights.
|Bag Concrete Weight||Mixed Concrete Yield (approximate)|
|40 lb (18.1 kg)||0.30 cu ft (8.5 L)|
|60 lb (27.2 kg)||0.45 cu ft (12.7 L)|
|80 lb (36.3 kg)||0.60 cu ft (17 L)|
Tips For Ordering Concrete
Consider these tips to ensure your concrete installation goes smoothly.
Type of concrete and its proper installation
This depends on the application and the climate where you live. Colder temperatures need longer curing times; sandy soils need different shaped holes; more extreme freeze/thaw cycles need higher strength concrete than more temperate regions.
Your concrete supplier can advise you on the proper mix to use for your project and region.
For ready-mix concrete delivery, there are typically additional fees for delivery, weekend service, small loads, and overtime for long unloading times. Discuss these issues with your concrete supplier ahead of time so that you have a clear understanding of costs and can plan accordingly.
Order extra concrete (overage) for your project. The amount of overage you plan for will vary based on your quantity, but plan for at least 10% overage to accommodate spillage, settling, and grading inconsistencies.
This will save you from a bad pour (concrete installed in parts resulting in a separated, weak mass) in the event that your estimates are even slightly off.
Allocate enough time
Finally, digging can be hard, time-consuming work. Allow plenty of time to dig all the post holes for the project before the concrete is delivered or mixed.
Mixed concrete that must sit before being poured into place can partially or even fully set. This can result in significant waste from hardened, unused concrete or create air pockets and compromised strength from pouring concrete that is too thick.