Post Concrete Calculator — Postcrete & Concrete

It depends on hole size, not post count. A 20kg bag of postcrete sets to about 10 litres (0.01 m³), so the number of bags is the hole volume minus the post, divided by 10 litres. A narrow 200mm-wide × 600mm-deep hole around a 100mm post needs roughly 1 bag; a wider 300mm hole at the same depth needs about 4 bags.

Firm in 5-10 minutes and ready to load (hang panels) after about 30 minutes. Full strength in 24 hours.

Postcrete (fast-set post mix) is convenient and needs no mixing — just add water in the hole. Regular concrete (a 1:2:4 ballast mix) is cheaper per litre, so it works out better for many posts on a large run.

Postcrete is sold by hole volume, not by post. Dig the hole roughly three times the post width — a 100mm post wants a ~300mm hole — and at least 600mm deep (750mm for fences over 1.5m). A 300mm × 600mm hole holds about 42 litres of concrete; subtract the post (~6 litres for a 100mm post) and divide by the 10-litre bag yield to get about 4 bags. A tighter 200mm hole needs only 1-2 bags.

Postcrete (fast-set) is firm in 5-10 minutes and fully set in 1-2 hours. Standard concrete mix takes 24-48 hours. Postcrete is ideal for fence repairs.