Number of posts and slabs for a precast (readymade) boundary wall.
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Precast Compound Wall Calculator: Free Online Tool
The Precast Compound Wall Calculator works out how many posts and slabs you need for a readymade precast boundary wall. You enter the total wall length, the wall height above ground, the post spacing (which equals the slab length), the slab height and the post depth in the ground. You can choose an open run or a closed boundary loop.
Contractors, plot owners, site engineers and suppliers use it to prepare an order before the wall is delivered. Precast walls are fast to install, but posts and slabs are made to fixed sizes, so a miscount means either a stopped job or unused stock. A clear list of posts, slab rows and post length also helps with transport planning and pricing.
When to use this calculator
Use this calculator when you are fencing a plot, a farm, a factory yard, a school ground or a housing site with a precast wall. It is helpful when you request a quotation from a precast supplier, because you can state the number of posts, slabs and the post length. It also helps when you compare different slab heights or post spacings, since each change alters the number of slab rows and spans. Students can use it to understand how counting rules differ for an open run and a closed loop.
How to use the Precast Compound Wall Calculator
- Open the calculator above on your phone, tablet or computer. Nothing needs to be installed and no sign-up is required.
- Choose the unit system first (feet / inches or metres / mm). The input labels change to match your choice.
- Pick the option you need from the drop-down list (Open run (posts at both ends); Closed boundary (loop)).
- Enter your values in the input boxes: Layout, Total wall length (ft), Wall height above ground (ft), Post spacing = slab length (ft), Slab height (inches), Post depth in ground (ft). The boxes already hold sample values, so overwrite them with your own.
- Press Calculate. The result appears straight away under the button.
- Change any value and press Calculate again to compare options. Press Reset to go back to the sample values.
- Check the result against your drawings, project specification and local code before you order material or approve work.
How the calculation works
The number of spans is the total wall length divided by the post spacing, rounded up. For an open run, posts equal spans + 1, because both ends need a post. For a closed loop, posts equal spans. Slab rows are wall height divided by slab height, rounded up. Slabs required equal spans multiplied by slab rows. Each post length is the wall height above ground plus the depth in the ground. Total post length is that length times the number of posts. With the defaults of 200 ft, 6 ft height, 8 ft spacing, 12 in slabs and 2 ft depth, you get 25 spans, 6 rows, 26 posts and 150 slabs.
Worked example
When you open the page the calculator is filled with sample values. Pressing Calculate with those values gives the result below, which you can use to check that you are reading the output correctly.
| Wall layout | |
|---|---|
| Number of spans | 25 |
| Slab rows (layers) | 6 |
| Posts and slabs | |
| Posts required | 26 nos. |
| Length of one post | 8.00 ft (2.44 m) |
| Total post length | 208.00 ft (63.40 m) |
| Slabs required | 150 nos. |
| Slab length | 8.00 ft (2.44 m) |
Tips and common mistakes
- Check that the slab length you buy matches the post spacing. Slabs span between grooves in the posts, so a mismatch makes the counts wrong.
- Corner posts, gate posts and gates are not included. Count them separately and add them to the order, as the calculator note states.
- Wastage is not included. Add a few extra slabs for breakage during transport and handling, especially on rough access roads.
- Select the layout carefully. A closed boundary needs one post fewer than an open run with the same number of spans.
- Confirm the post depth with your supplier or engineer. Soft soil, slopes or flood-prone land may need deeper embedment than the default of 2 ft.
Frequently asked questions
How are the number of posts calculated?
For an open run the calculator uses spans + 1, because both end posts are needed. For a closed boundary it uses spans, since the last span joins the first post again.
What is the post spacing used for?
The post spacing equals the slab length. The calculator divides the wall length by it, rounds up, and gets the number of spans. Each span holds one slab per row.
How are the slab rows found?
The wall height above ground is divided by the slab height and rounded up. For the defaults, 6 ft divided by 12 in gives 6 rows. Slabs required equal spans multiplied by rows.
Does the length of a post include the underground part?
Yes. Length of one post is wall height above ground plus the post depth in the ground. With the default 6 ft and 2 ft, each post is 8 ft long.
Are gates, corners and wastage included?
No. The calculator note says corner posts, gates and wastage are not included. Add them to the order separately, and confirm the final list with your supplier.
Important note
This tool gives estimates for planning, study and quick checks. It does not replace design by a qualified engineer, site-specific testing, or the requirements of your project specification and local building code. Always confirm final quantities, test results and acceptance limits with the responsible engineer before using them for construction or payment.
