Calculate concrete and gravel needed for setting fence or deck posts in the ground.
Enter the number of holes, the hole diameter and depth, and the post width (which is subtracted from the volume). Optionally add a gravel base depth. The tool returns the concrete (or “postcrete”) needed to set the posts firmly.
| Guide | Value |
|---|---|
| Hole diameter | ≈ 3 × the post width |
| Hole depth | 1/3 of post height above ground |
| Gravel base | 50–100 mm for drainage |
A 4 in (100 mm) post in a 12 in (300 mm) diameter hole, 2 ft (600 mm) deep: the hole holds ≈ 1.2 ft³, the post takes up ≈ 0.15 ft³, leaving ≈ 1.05 ft³ of concrete per post — about one and a half 50 lb / 20 kg bags of postcrete.
Tip: slope the top of the concrete away from the post so rainwater runs off, which helps timber posts last longer.
A common rule is to bury 1/3 of the total post length. For a 6 ft (1.8 m) fence with 8 ft (2.4 m) posts, that is about 24–30 inches (600–750 mm) deep.
For a standard 4×4 post in a 10-inch diameter, 24-inch deep hole, you will need about 1–2 bags of 60 lb concrete mix.
Yes, 3–4 inches (75–100 mm) of gravel improves drainage and prevents the post base from sitting in water.
As a rule, make the hole about three times the post width — roughly 250–300 mm across for a 100 mm post. A wider hole gives the concrete collar more grip.
Postcrete (fast-set concrete) sets in minutes and is ideal for single posts, while mixed concrete is cheaper for many posts but needs propping while it cures. Both work well.
