How the post hole calculation works
A post hole is close enough to a cylinder, so its volume is the circle area times the depth. The calculator works out the hole volume, takes away the volume of the post sitting in it, adds your waste allowance, and divides by what one bag makes. Bags are rounded up per hole, because you cannot mix two thirds of a bag for one hole and carry the rest to the next one without the first batch going off.
concrete per hole = (π × (diameter ÷ 2)² × depth) − (post width² × depth), then + waste
A 300 mm hole 600 mm deep holds 0.042 m³. A 90 × 90 mm post takes up 0.005 m³ of that, which leaves 0.037 m³. Add 10 percent and you are at 0.041 m³, or five 20 kg bags. That figure, four to five bags per fence post, is the one most fencers carry in their heads.
Hole size and depth
The hole needs to be wide enough that there is a proper collar of concrete around the post, and deep enough that the post cannot lever out under wind load. Three times the post width is the usual diameter, so 300 mm for a 90 or 100 mm post. For depth, the old rule of a third of the height above ground gives 600 mm for a 1.8 m fence, and that is a sensible minimum. Gate posts carry a swinging load and go deeper, 800 to 900 mm, often in a wider hole. Corner and end posts take the strain of the whole run and are treated the same way. In sand or soft clay everything goes deeper again.
| Post | Hole | Concrete | 20 kg bags |
|---|---|---|---|
| 90 × 90 fence post | 300 × 600 | 0.041 m³ | 5 |
| 100 × 100 fence post | 300 × 600 | 0.040 m³ | 5 |
| 125 × 125 gate post | 400 × 900 | 0.109 m³ | 12 |
| 90 × 90 deck post | 350 × 600 | 0.058 m³ | 7 |
| 65 × 65 steel post | 250 × 600 | 0.030 m³ | 4 |
Figures include 10 percent waste and are rounded up to whole bags.
Setting the post
Put 50 to 100 mm of gravel in the bottom of the hole so the end of a timber post is not sitting in water. Stand the post, plumb it in both directions and brace it with two stays before the concrete goes in. Mix to a stiff consistency, fill the hole and rod it to get the air out. Finish the concrete slightly above ground level and slope it away from the post so rain runs off instead of pooling against the timber. Leave the braces on until the next day for standard mix, or half an hour for rapid set.
Rapid set or standard?
Rapid set is worth the extra money for a few posts, a repair, or a gate post you need to hang the same day. For a full fence line, standard premix is cheaper and you can set the whole run and go home. Yield per bag is much the same for both, so the calculator does not distinguish.
Worked example
A 24 m paling fence with posts at 2.4 m centres needs 11 posts. Each 90 × 90 post goes in a 300 × 600 hole: 0.041 m³ with waste, so five bags each. Fifty-five bags for the run, plus two spare, is 57 bags, about 1.1 tonnes to carry. Water is around 2.3 litres a bag, so have a hose within reach.
Common questions
How many bags of concrete for a fence post?
Four to five 20 kg bags for a 90 or 100 mm post in a 300 × 600 mm hole. Gate posts in a 400 × 900 hole take about twelve.
How deep should the hole be?
A third of the post height above ground, with 600 mm as the practical minimum for a 1.8 m fence. Gate and corner posts 800 to 900 mm.
How wide should the hole be?
About three times the post width. A 90 or 100 mm post goes in a 300 mm hole.
Rapid set or standard concrete?
Rapid set for a few posts or same-day gates. Standard for a long run, it is cheaper and there is no hurry.
This calculator gives quantities from the dimensions you enter. Footing depth for structures such as decks, pergolas and retaining posts should follow the drawing or the engineer's advice for your soil.