Postcrete Bag Calculator
Estimate whole bags of post-mix concrete from the post count and the quantity evidence that applies to the product and holes you are actually using.
Build the bag quantity from the product information
Count the posts first, then choose the method that matches the information you genuinely have. Do not use a generic “bags per post” figure just because another product or fence uses it.
How the Postcrete bag quantity is calculated
The calculator deliberately separates what the job needs from how many whole bags you buy. Use one of two methods depending on the evidence you have.
Use the final post count after gates, corners, ends and layout changes are known.
Enter either a verified bags-per-post rate or both the known fill volume and actual bag yield.
The base and allowance stay visible before the final result rounds up to whole bags.
Volume mode: theoretical bags = (posts × required litres per post) ÷ actual litres yielded per bag. Final bags = ceil(theoretical bags × (1 + chosen allowance ÷ 100)).
Worked quantity example
Blue Circle’s current fence guide shows a method using one 20 kg bag of Postcrete per post. If that exact product guidance and your installation support that rate, 12 posts × 1 bag/post = 12 theoretical bags. With the calculator’s default 0% extra allowance, the order remains 12 whole bags.
That is a product example, not a universal rule. Other post-mix products and fencing suppliers publish different guidance, and larger holes, posts or gate foundations can change the quantity materially.
Why bags per post is not hard-coded
Current UK guidance varies. Blue Circle’s own fence guide uses one bag per post in its worked method. Tarmac’s Postmix describes one 20 kg bag per standard fence post, while other fencing suppliers publish higher rates for different posts, holes and products. The safe calculator behaviour is therefore to expose the rate rather than pretend one number fits every installation.
Blue Circle — How to build a fence · Blue Circle Postcrete product page · Travis Perkins — Blue Circle Postcrete 20 kg · Tarmac Postmix Concrete
Supplier data can change. Check the current packaging/data sheet for the exact product before ordering or installing.
Use volume/yield mode only when you genuinely know both figures
Travis Perkins currently lists Blue Circle Postcrete coverage as 0.018 m³ per 20 kg bag when applied as instructed, equivalent to 18 litres. That can be useful as a product-specific yield reference, but the calculator still does not invent the required hole volume. Enter volume mode only when the real fill requirement has been established independently.
Keep the fence workflow in the right order
1. Count panels and posts
Use the Fence Panel Calculator to establish the fence bays and post positions first.
2. Quantify post mix
Use this page only after the post count and product quantity basis are known.
3. Check the whole material list
Use the Fence Materials Estimator to keep panels, posts and foundation material separate.
Material estimate only. This calculator does not design post holes, determine post embedment, assess soil, size gate foundations or verify structural/fencing safety. Follow the fencing specification and the exact product manufacturer’s instructions.
Frequently asked questions
How many bags of Postcrete do I need per fence post?
There is no single reliable figure for every product and installation. Use the rate stated for the exact post-mix product and the real post/hole arrangement, or use volume/yield mode if you genuinely know both values.
Why does the calculator show theoretical and whole bags separately?
A decimal calculation is useful for checking the quantity basis, but bags are normally bought whole. The calculator therefore shows the theoretical base before rounding, the whole-bag base minimum, any chosen allowance and the final whole-bag order.
Should I always add 10% spare?
No. The allowance starts at 0%. Add an amount only when you have a reason for the actual project, such as known variation or a deliberate spare-bag policy.
Can this calculate the hole size or post depth?
No. Those are installation and potentially structural decisions that depend on the fencing system, post, ground and loading. This page converts known requirements into a material quantity only.