Postcrete vs concrete for fence posts (UK): which is best, how much you need, and common mistakes

If you are setting new fence posts, the mix you use matters just as much as the post itself. People often say “Postcrete” when they mean any fast-setting post mix. Others stick to traditional concrete. Both work well when used properly. The right choice depends on your ground, the job size, and how much time you have.

The quick answer

Fast-setting post mix is handy for smaller jobs and quick repairs because it sets quickly and is simple to use. Traditional concrete is often better value on bigger runs and can be more forgiving if you are working in awkward ground and need a bit more time to line everything up.

What is the difference between post mix and concrete?

Traditional concrete is usually mixed from sand, cement and aggregate (or bought as a ballast mix plus cement). You mix it with water before it goes in the hole. Fast-setting post mix is designed to be simpler. Many are added dry, then watered in. They start to firm up quickly, which is great when you need speed, but it also means you must get your post position right early.

When fast-setting post mix is the better choice

• One or two posts: ideal for quick replacements or repairs where you want the fence stable again fast.
• Tight access: less mixing and less mess, which helps in small gardens.
• Cold or damp days: you can often get a firm set sooner than with a wet concrete mix, but always follow the bag instructions.

If you are doing this route, keep your posts solid and straight. Start with good fence posts and a suitable mix from cement products.

When traditional concrete makes more sense

• Long runs of fencing: usually better value when you are doing lots of holes.
• Awkward holes: gives you a bit more working time to pack, level, and line up a full run.
• High wind areas: the key is the size of the footing and the depth, and traditional concrete works well when you build a proper base.

How much do you need per post? A practical rule of thumb

How much mix you need depends on the hole size and the ground. As a simple starting point for a typical 4x4 timber post, many installers aim for a hole around 300mm wide and 600mm deep for a 6ft fence, adjusting for exposure and soil. That amount of space can take roughly 40 to 60 litres of mix once you allow for the post itself and any packing. That often works out as around two to three 20kg bags of post mix per post, but it varies. If you go wider or deeper, you will use more. Always check the bag coverage as different mixes give different yields.

If your fence is on a windy corner, do not be shy with the footing size. A post that is set too shallow is one of the main reasons fences lean and rattle later.

Post depth and hole size: what matters most

The mix is only part of the story. A strong fence starts with a post that is deep enough and held firm while the footing sets. If you are fitting panels, a common approach is to set about a third of the post length below ground. Ground conditions can change that. Soft or sandy soil may need deeper posts or wider footings.

If you are installing timber panels, adding gravel boards helps keep the bottom edge off wet ground, which can reduce rot and save you replacing panels early.

Common mistakes (and how to avoid them)

• Not bracing the post: if it moves while setting, the whole run ends up out of line. Brace it and check it from two directions.
• Too much water with post mix: drowning the hole can weaken the set. Follow the bag instructions and add water steadily.
• Using a tiny footing: it might look fine on day one, then loosen over winter. If in doubt, go wider and deeper.
• No fall away from the post: finish the top of the footing so water sheds away from the timber post.
• Fixings not suited to outdoors: use exterior-rated brackets and screws from ironmongery so the fence stays tight over time.

Step-by-step: setting a post neatly

• Mark your line and spacing, then dig the hole to suit the post and exposure.
• Drop the post in and brace it so it cannot move.
• Add your mix (post mix or concrete), keeping the post plumb as you go.
• Finish the top of the footing with a slight slope away from the post.
• Let it set fully before hanging weight on it, especially if you are fitting heavy fence panels.

What you might need

For most fencing jobs you will need solid fence posts, the right setting mix from cement products, and exterior fixings from ironmongery. If you want the fence to last longer at the bottom edge, add gravel boards and keep timber off damp soil where you can.