The amount of postcrete needed per post depends on 5 key factors: the post’s intended use(s), post material, post height, soil conditions and environmental changes.
Our handy postcrete calculator (below) can help you to estimate the amount of postcrete needed, measured in bags and kilograms!
Postcrete is a fast-setting, ready-mixed cement blend for securing wooden (timber), metal and concrete posts into the ground. It can be used to install fence posts, gate posts, sports posts (e.g. football goal posts) and other garden structures like pergolas and gazebos.
Postcrete is a protected brand name, owned by the Tarmac company. Other brands use different terms to describe their powdered postcrete mixes. In the UK, you’re likely to encounter these alternative names for postcrete:
- Post Concrete
- Fencing Concrete
- Just-Add-Water Cement
- Blue Circle
- Post Fix
- Post Mix Concrete
- Portland Cement
- Fencing Concrete
Just-add-water postcrete is almost always used in domestic settings (i.e. DIY, garden projects, smaller fences) rather than industrial settings (i.e. a high-security prison fence).
This is because postcrete is more expensive compared to alternative concretes! Extra heating steps are needed to create a final powder that’s bone-dry and able to set in less than 15 minutes.
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How Much Postcrete Do I Need? (Factors)
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It can be hard to eyeball the amount needed for a specific situation, especially if you don’t have prior experience in using postcrete or other concretes.
Postcrete is considerably more expensive than standard cement mixes.
Depending on the brand and your location in the world, it may be up to 4 times more expensive than other concretes! For this reason, nobody wants to buy more postcrete than they need.
But on the other hand, nobody wants to waste money installing a fence post that’ll fall over, crack or sink! To make a sensible guess on the amount of postcrete you’ll need, you absolutely must first look at where the post is to be installed.
1. Ground Conditions
The intended location of the post could be stone, clay, chalk, silt, sand or soil; it’s more than likely that the ground will be a combination of these different materials.
You can view the UK-wide soil trends map from the UK Soil Observatory here for reference.
The ground surrounding the post’s foundations acts to stabilise the post, preventing tipping by pushing into the post’s postcrete foundation from all directions. The increased weight added to the post by the postcrete anchors the post downwards.
If the ground is soft (e.g. a very sandy, dry soil), increase the depth of the post submerged and use more postcrete accordingly.
You could also dig deeper and add a layer of firmer aggregates (e.g. smashed bricks, stones or hardcore) into the hole before introducing the post to reduce post sinking.
2. Post Material
Postcrete doesn’t actually “stick” to the ground it’s poured into. Instead, the postcrete binds tightly to the post. This makes the post far heavier towards the bottom than at the top, anchoring the post downwards.
Posts that have rough, irregular exteriors (e.g. rustic timber fence posts) form stronger bonds with postcrete than smooth posts (e.g. galvanised steel tube).
Postcrete will fill any nooks and crannies in the post’s material, increasing the end result’s durability.
3. Post Depth
How deep should a hole be for postcrete?
The “best” hole depth for postcrete depends on the post’s length and diameter. The stability of your post is primarily dictated by how much of the post is submerged below ground level!
A post that’s half-submerged will last years, perhaps decades, longer than a post that’s only 1/5th submerged. At a minimum, aim to submerge at least ¼ of a post into the ground.
According to official guidance from Quikrete, the hole you dig should be at least 3 times as wide as the post. Wider holes allow you to pour more ready-mixed concrete but will increase your project’s costs.
4. Intended Post Usage
Postcrete can effectively hold up posts of various materials and sizes, both freestanding and connected to fence panels.
You can use postcrete to install gate posts for metal and timber gates, but there are usually better options.
Generally, only small gates (less than 1 metre in length) can be hung on postcrete-fixed posts. This is because postcrete isn’t designed to handle the increased stresses exerted on it by large or heavy gates.
Consider using a stronger alternative such as Mastercrete for gate posts.
The absolute height limit for fence posts in the UK is 2 metres (measured from the ground) – fencing over this threshold generally requires planning permission.
5. Environmental Changes
Most postcrete mixes should not be used below around 3 degrees Celsius as they won’t harden effectively.
All postcrete blends are fast setting. Within approximately 5 to 15 minutes, it will set firm. However, it’s not yet at its maximum strength.
According to the British Ready-Made Concrete Association (BRMCA), ready-mixed concrete can take anywhere between a few hours to a few days to completely cure.
This length of time is decreased in hotter and less humid environments, as water molecules trapped inside of the postcrete can evaporate more rapidly.
How Much Postcrete Per Post Calculator
The postcrete calculator assumes you’re intending to use postcrete outdoors under “typically British” conditions: large seasonal fluctuations in temperature, occasionally very high winds and variable rainfall.
The number of postcrete bags needed is measured in 20 kg sacks and all data contained in the table below specifies the minimum amount of postcrete you’ll need per post.
| Submerged Depth* | Metal Post (UK outdoor washing line size / 45 mm diameter pole) | Wooden Post (75 mm x 75 mm / 3” x 3”) | Wooden Post (100 mm x 100 mm / 4” x 4”) | Concrete Post (100 mm x 100 mm / 4” x 4”) |
|---|---|---|---|---|
| 150 mm / 6” | 2 kg or 1/10 bag | 5 kg or 1/4 bag | 8 kg or 3/8 bag | 8 kg or 3/8 bag |
| 300 mm / 12” | 3 kg or 3/20 bag | 10 kg or ½ bag | 14 kg or ¾ bag | 14 kg or ¾ bag |
| 450 mm / 18” | 5 kg or ¼ bag | 13 kg or ⅔ bag | 23 kg or 1 ¼ bags | 23 kg or 1 ¼ bags |
| 600 mm / 24” | 6 kg or ⅓ bag | 17 kg or 1 bag | 30 kg or 1 ½ bags | 30 kg or 1 ½ bags |
| 750 mm / 30” | 8 kg or ⅜ bag | 21 kg or 1 1/10 bag | 38 kg or 2 bags | 38 kg or 2 bags |
*The section of the post submerged below the ground.
Calculations based on information from Tarmac BlueCircle, Cemex, Carlton and Maxrete data sheets.
Always follow the manufacturer’s instructions, take all necessary safety precautions and adhere to local laws and bylaws, especially surrounding the usage and disposal of cement.
Postcrete: How Much Water Do I Need?
As a rule of thumb, postcrete and postcrete-like products need around 200 millilitres of water per kilogram of postcrete.
This works out at around 2 litres of water per 20kg bag of postcrete; this is the most common postcrete bag size in the UK and internationally.
Overly-watery postcrete is considerably less strong once cured than slightly-too-thick postcrete. It’s a good idea to measure the required volume of water before erecting any posts.
The manufacturer’s instructions, printed in brief on the postcrete’s outer packaging, should specify the “ideal” volume of water to mix in.
If no instructions can be found, check the postcrete manufacturer’s official website for more reliable information.
Can I Mix Postcrete In A Bucket?
Yes, but don’t accidentally kill your grass and garden plants!
When mixing up a batch of postcrete, most people prefer to pour the dry postcrete directly into the hole, add the required water on top, quickly mix it with a stick and then wait for the postcrete to cure into a hard, durable post foundation.
This is undoubtedly the easiest way to prepare postcrete, but it’s not an ideal technique.
Pouring the dry postcrete powder into the hole un-mixed could cause harm to you, your pets and your garden. Postcrete is highly toxic, more so than regular concrete.
As a powder, it can float through the air on weak breezes or become airborne when the water is poured from above, unintentionally spilling onto any surrounding grass.
Instead, some prefer to mix the required water and postcrete in a cheap bucket or similar mixing container first, whilst wearing breathing apparatus and enclosed eye protection. You could do this over a tarpaulin to prevent the postcrete from damaging the post’s surroundings.
Then quickly introduce the postcrete into the hole, within at most 5 minutes from adding the water.
Neither method is inherently better, but you’ll have more control over the mixture if you use a mixing bucket.
What Is Postcrete Made Of?
All concretes, including postcrete, contain sand and cement as their two major ingredients.
Other common postcrete ingredients include silicates, lime, aluminates, chlorides, miscellaneous aggregates and other chemical additives. These can harm animals and wildlife if used incorrectly.
Postcretes typically contain one or more additional “soluble chromium (VI)” compounds – these are respiratory irritants which can increase lung and nose cancer risk, even when minuscule volumes <0.0005% (<5 ppm) are included.
Buying And Storing Postcrete Correctly
In the UK, you should exclusively look for ready-mixed cements that are certified by the Quality Scheme for Ready Mixed Concrete (QSRMC); all major post-fixing cement brands are certified as safe by the QSRMC and carry the red-kite mark of quality assurance.
Good-quality postcrete will be packaged in rip-resistant, waterproof packaging. Normal powdered cement can tolerate far more pre-mix moisture than postcrete; a small increase in moisture can prematurely set postcrete, rendering it useless.
Store in a dry, covered area that isn’t prone to extreme temperature fluctuations. Always use postcrete before its use-by date – raw cement can become more hazardous to health over time.