Postcrete Calculator
Estimate how many bags of fast-setting post mix — often called Postcrete, post mix or fence-post concrete — you need for a run of fence posts, using the yield printed on your bag.
Results
Enter your project measurements to see your results.
What this Postcrete calculator does
Fast-setting post mix is poured dry or wet straight into a post hole, so the usual question is simply how many bags to buy. This page works out the space around each post, multiplies it across every post in the run, adds the waste allowance you choose and divides by the yield of the bag you are using. It uses the same hole and displacement maths as our post-hole concrete calculator.
Measuring holes and posts
- Measure the hole as dug — its diameter or width and length, and its depth from ground level.
- Measure the actual post size, not the nominal size on the label.
- Enter how far the post goes into the hole, and any concrete or gravel you place beneath it.
- Count every post, including corner and gate posts if they share the same hole size.
How the calculation works
Each hole's volume is found first (π × radius² × depth for a round hole, width × length × depth for a square one). The post's cross-section multiplied by its embedded depth is taken away. The result is multiplied by the number of posts, the waste percentage is added once to that total, and the final figure is divided by the bag yield. Partial bags are always rounded up.
Worked metric example
Six 300 mm round holes, 600 mm deep, each holding a 100 mm square post set the full depth, need about 36.4 litres per hole, or 218.5 litres in total. With 10% waste that becomes about 240 litres. If your bag states a yield of 10 litres, that is 24.0 bags, so you would buy 25 — but use the yield on your actual product, not this example figure.
Worked imperial example
Ten 12 in round holes, 24 in deep, with 4 × 4 in posts need about 1.35 ft³ each, 13.5 ft³ in total, or 14.8 ft³ with 10% waste. Divide by the yield printed on your bag to get the count.
Product names and yields
Postcrete is a Blue Circle product name owned by Tarmac; other brands sell similar fast-setting post mixes. Material Measures is independent and not affiliated with any manufacturer. We have not added product presets because we could not confirm reliable manufacturer wet-yield figures for these specialist products, so the calculator always asks for the stated yield.
Quantities, not installation advice
The right hole size, post size, embedment depth, water quantity and setting time depend on the product, soil, exposure and what the post supports. Follow the manufacturer's instructions and local guidance, and ask a qualified professional for gates, retaining or structural posts. For bigger pours, see the concrete bag calculator.
Frequently asked questions
- How many bags of Postcrete do I need per post?
- It depends on the hole size, the post size and the yield printed on the specific bag. Enter your hole and post dimensions with the manufacturer-stated yield and the calculator divides the concrete needed by that yield, then rounds up to whole bags.
- Why do I have to enter the bag yield myself?
- Fast-setting post mixes are specialist products and their yields are not the same as general-purpose concrete. We have not found manufacturer technical data we can rely on for a universal figure, so the calculator uses the yield stated on the packaging of the product you are buying.
- Is Postcrete the same as post mix or fence-post concrete?
- Postcrete is a brand name of Tarmac's Blue Circle range. People often use it, along with post mix and fence-post concrete, to describe any fast-setting concrete sold for setting posts. Products differ, so follow the instructions on the bag you buy. Material Measures has no affiliation with any manufacturer.
- Does the calculator tell me how much water to add or how long it takes to set?
- No. Water quantities, setting times and installation methods vary by product and conditions, so follow the manufacturer's instructions. This page only estimates quantities.
- Why is the post volume subtracted?
- The post fills part of the hole, so less mix is needed. The calculator subtracts the post's cross-section multiplied by how far it sits in the hole, allowing for any concrete you place beneath the post.