Concrete & Cement Bags

Bagged concrete and cement for setting retaining wall posts, and for the smaller jobs around a build. All three are Easy Mix 20kg bags, so they handle the same way and stack the same way on a pallet.

Which bag

• Concrete is the standard mix for pier holes. Reach for it when you have the time to let the piers cure before the sleepers go in.
• Rapid set concrete is the one to use when you need to keep moving. Set the posts, get them plumb and braced, and carry on rather than waiting out a full cure. It costs a little more per bag and buys back a day on site.
• General purpose cement is cement on its own, not a concrete mix, so it needs sand and aggregate added. Use it for mortar and patching work rather than for piers.

How many bags

Bag counts come from your pier hole size and depth, which come from your wall design. The arithmetic is straightforward once you have those two numbers.

A 450mm diameter pier holds roughly 160 litres of concrete for every metre of depth. At about 10 litres of set concrete per 20kg bag, that is around 16 bags per post, per metre of pier depth. So six posts set 1 metre deep is about 96 bags, and the same six posts at 1.5 metres is about 144.

Two things that move the number: the post itself displaces concrete, which works slightly in your favour, and a hole dug in loose or collapsing ground will take more than its nominal volume, which works against you. Check the yield printed on the bags you actually buy, because it varies between products, and order a few spare rather than stopping the pour.

The wall calculator does this for you from your wall length and height, alongside sleepers, posts and drainage.

Before you dig

Pier depth and diameter are design decisions, not defaults. Our rule of thumb is a pier depth equal to the wall height in a 450mm hole, which is deliberately conservative. Where your wall has an engineered design, use the pier in that design instead. The steel post installation guide covers setting out, bracing and getting the lean back right before the concrete goes in.