Postmix Calculator
How many bags of postmix do I need?
Postmix, or fast-setting post concrete, is sold in 20 kg bags and you pour it dry into the hole around the post, then add water. As a rule of thumb a standard fence post in a normal-sized hole takes about two bags, and a heavier gate post or a deeper hole takes three. Enter the number of posts and bags each, and the calculator totals it.
Dig the hole roughly three times the width of the post and about a third of the post's height deep. Postmix goes off in minutes, so hold the post plumb and braced before you add water, and do not mix it in advance.
Frequently asked questions
Is the post mix calculator free?
Yes. It is completely free, with no sign-up and nothing to install. It runs entirely in your browser.
How accurate is it?
It gives a reliable estimate for ordering based on standard UK figures, but real quantities vary with ground conditions, wastage and how materials settle. Always add a margin and confirm with your supplier before buying.