How much postcrete per post?

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.

How long does postcrete take?

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

Postcrete vs regular concrete?

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.

How much postcrete per fence post?

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.

How long does postcrete take to set?

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.

GardenCalcs

Post Concrete Calculator — Postcrete & Concrete

Work out how much postcrete or concrete you need for fence posts.

Number of Posts

posts = ceil(run ÷ panel width) + 1, so a 30m run of 1.83m panels needs about 17 posts

Post SizeSquare timber post width. The hole should be about 3× the post width

How We Calculate This

Concrete needed = hole volume − post volume. The hole is an augered cylinder (π × radius² × depth) and the square timber post displaces side² × depth. A 20kg postcrete bag yields about 10 litres (0.01 m³), so bags per post = concrete volume ÷ 10 litres, rounded up. A 300mm-diameter × 600mm-deep hole around a 100mm post works out at roughly 36 litres → about 4 bags; a tighter 200mm hole needs only 1-2 bags.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.