How Much Concrete per Post Hole?
We get asked this question a lot. It's a good thing to know when buying your concrete. There is nothing worse than getting to your last two posts and coming up a bag short. It's never good when the folks at the home center know you by first name by the end of the weekend.
How much concrete you need per hole revolves around knowing the following:
1. What size post you are using
2. The diameter of the hole you are digging
3. The depth of the hole you are digging
Item #1 you can get from your estimate or invoice. We break out the post sizes on those that we are giving you. They are typically 2in, 2.5in and 3in square for fence panels and walk gates. Then for the bigger driveway gates, you have a 4in or 6in post.
Item #2 has no hard and fast rule, but a common measurement is to go 3x the size of your post. So if you have a 2x2 post, you would dig a 6in wide hole. Again, this can vary and if you have any questions, let us know. Certain conditions like rock or sand can change that. But in 99% of situations, that 3x rule works.
Item #3 can also be determined pretty easily. Based on where you are located, you will typically be burying 24in to 36in in ground. How can you tell which you are? Well, if you have a 4ft tall panel and we gave you a 7ft long post, you have 36in roughly going in ground. One thing to keep in mind is to add in for space under the panel. On average, most folks want about 2in of open space under the fence panel so you can weed whip without harming the powder coat finish. In the case of our 7ft post, we know we have 36in going in ground. However, if we want the panel 2in above the grass, we want to make the post stick out of the ground another 2in. So we would be only burying 34in. Don't worry about being super exact when digging. You can always offset the hole by adding gravel back in or digging a little more out. Another little trick is not filling the hole up completely. In our situation above, the cement would be at the same level as your grass. Most folks want the grass to grow up to the post. In this case, leave the top 2-3 inches devoid of concrete and push fill dirt back in at the end. So while I am burying 34in of post below the surface, I am going to deduct 2 more inches for my concrete calculation since I am not filling the hole to the top. So the end number is going to be 36in in ground, minus 2in for the gap under the panel and minus 2in for being able to back fill with dirt. So my final number is going to be 32in
Ok, so now we know that we have a 2x2 post, that is going to have a 6in wide hole and we need cement for 32in in ground. Now you have all the info you need to go to the concrete calculator. The one we find easiest to use is on Sakrete's website. Here is a link to their calculator:
http://www.sakrete.com/products/calculators.cfm/selectedCalculator/Post%20Setting%20-%20Square%20Post#square
(You can also go to www.sakrete.com, then Products, then 'How much do I need?' and finally choose 'Post Setting - Square Post')
If you know the number of post holes, you can enter it there as well so it gives you the full amount of bags needed for the project. Or you can enter 1 hole to figure it out by the post and multiply it from there. In my example, I am going to use 1 so I know how many per hole. So I enter:
- 1 for number of holes
- 2 for post width in inches
- 6 for hole width in inches
- 32 for hole depth in inches
- click CALCULATE
The results then come back in 40lb, 50lb, 60lb, 80lb and 90lb bag sixes so you can tailor it to the bag size you intend to buy. Mine is a small project, so I am going to go with a pretty standard 80lb bag. According to Sakrete, I will need 0.75 bags per post hole. If I knew that I had 10 post holes, I could enter that and it would say 7.5 (or 8) of the 80lb bags. Or if I do it by post hole, I would take my 0.75 bags per hole and multiply it by 10 to come up with 7.5 (8) bags.
Math not your thing or have any questions about the info above? Give us a call or shoot us an email. We would be happy to help answer any questions!
