Postcrete: Bags Per Fence Post, Setting Time and How to Use It

Postcrete is the bagged fast-setting post mix that most British fences get built with. The name is a brand - Blue Circle Postcrete, made by Tarmac - and like Hoover and Biro it has drifted into being the word for the whole product category, so merchants and tradespeople will use it for any bagged post-fixing mix.

It is worth knowing what it actually is, because it behaves nothing like the concrete next to it on the pallet: it goes into the hole dry, it sets in minutes, and the number of bags a post swallows is decided by something most people never measure.

The thing that decides your bag count

Not the depth. The width of the hole.

A post hole is a cylinder, and the volume of a cylinder goes up with the square of its diameter. Dig a hole half as wide again and you have not bought yourself half as much concrete again - you have bought nearly three times as much. Depth, by contrast, is linear: twice as deep is exactly twice the concrete.

Here is what that looks like for a standard 100 mm timber post at 600 mm deep, with the volume the post itself takes up already subtracted:

The often-repeated “one or two bags a post” is not wrong, but it describes a narrow hole - the sort an auger or a post-hole spade leaves. The moment you are digging 300 mm across, which people routinely do when the ground is stony or the spade will not go in cleanly, the same post takes four.

So measure the first hole you dig before you buy for the whole run. On a twenty-post fence, guessing two when the answer is four is forty bags short.

Working it out for your own hole

hole volume  = π × (hole diameter ÷ 2)² × depth
post volume  = post width × post width × depth
concrete     = hole volume - post volume
bags         = ceil( concrete ÷ bag yield )

A 20 kg bag makes roughly 9 to 10 litres of set concrete. That is an approximation - check the yield printed on your own bags, since it varies a little between brands - but it is close enough to buy from, and rounding up is the right direction to be wrong in.

Two adjustments worth making before you order:

• A hand-dug hole is not a neat cylinder. It comes out squarer and wider at the top than you intended. Add a bag.
• A gravel or hardcore base of 100 mm to 150 mm at the bottom of the hole helps the post drain, and it takes that much off the concrete. If you are using one, work the volume off the remaining depth.

The calculator above does the post count for the whole fence and multiplies it by your bags per post, so once you have settled on a figure for one hole you get the number for the pallet.

How much post to bury

The standard is to put about a third of the above-ground height into the ground, with roughly 600 mm as the floor for a garden fence. So:

That is why 2.4 m posts and 1.8 m panels are the pairing you see stacked outside every builders’ merchant in the country. For component sizes and post spacings to spec, BS 1722 is the British Standard that covers fences.

Gate posts, corner posts and end posts are the exception. They carry loads the line posts never see, so give them more depth, a wider hole and a heavier section.

How to actually use it

This is the part that catches people who have used ordinary concrete. You do not mix Postcrete. There is no barrow, no mixer and no shovelling it about.

1. Dig the hole and put in the gravel base if you are using one.
2. Stand the post in and brace it. Two timber battens screwed to the post and pegged to the ground will hold it. Get it plumb on a level in both directions now, and get the height right against your string line - once the mix is in, the argument is over.
3. Pour water into the hole, roughly a third to half full. Yes, water first.
4. Tip the dry mix in around the post, straight from the bag, distributing it evenly around all four faces rather than dumping it down one side.
5. Top up with a little more water if the mix looks dry on top, and give it a poke with a batten to release trapped air.
6. Leave it alone. It goes off in around ten minutes - check your own bag, and expect it to be quicker on a warm day.
7. Finish the top above ground level and slope it away from the timber so rain runs off rather than sitting against the post. Water standing on a flat concrete collar is what rots a post at ground level.

Work one post at a time. The fast set is a gift when you are on your own and a trap if you have four holes open and are trying to be clever.

Common mistakes

• Mixing it in a barrow first. It sets in the barrow. This is the one everybody has done once.
• Plumbing the post after the pour. Ten minutes is not the leisurely window it sounds like, and nudging a post once the mix has started to go off breaks the bond without giving you a straight fence.
• Buying for the rule of thumb instead of the hole. See the table above - a 300 mm hole is double a 200 mm one.
• Finishing the collar flat or dished. It holds water against the post. Slope it away.
• Using it where ordinary concrete would do. For a slab, a footing or anything you can leave overnight, bagged concrete is far cheaper by the bag and there is no reason to pay for a fast set you do not need.
• Hanging a heavy gate the same afternoon. The mix is walkable quickly; a gate is a different kind of load. Give a gate post proper time.