Postcrete Calculator
Last updated: 12 August 2026
Work out how many bags of postcrete or post mix you need to set fence posts. Calculates the hole volume, subtracts the post itself, and converts to 20kg bags.
How the calculation works
Hole volume = π × radius² × depth. We subtract the volume the post itself occupies, then divide by the yield of a 20kg bag of postcrete, which is about 0.011 m³ (11 litres) once mixed.
Postcrete goes into the hole dry, then water is added on top — roughly 3 litres per bag. It sets in about 10 minutes, so have the post plumb and braced before you pour.
Hole size and bags per post
| Fence height | Hole | Post | Bags per post |
|---|---|---|---|
| 1.2m (4ft) | 200 × 450mm | 75mm | ~1 |
| 1.8m (6ft) | 250 × 600mm | 100mm | ~2 |
| 1.8m exposed | 300 × 750mm | 100mm | ~4 |
| Gate post | 450 × 750mm | 150mm | ~9 |
Two useful rules of thumb: the hole diameter should be about three times the post width, and the post should go into the ground by a third of its above-ground height.
Getting posts right
Put 75–100mm of compacted hardcore in the bottom of the hole for drainage — water sitting on the post end is what rots it. Set posts slightly proud of the concrete and dome the top so water runs off. For gate posts and exposed sites, go deeper rather than wider; depth resists leverage, width does not.
Frequently asked questions
How many bags of postcrete per fence post?
About two 20kg bags for a standard 6ft fence post in a 250mm wide, 600mm deep hole. A wider 300mm hole takes three to four bags, and a heavy gate post in a 450mm hole can need nine.
How deep should a fence post hole be?
600mm for a 1.8m (6ft) fence, which means a 2.4m post. The traditional rule is to bury a third of the above-ground height. Go deeper on exposed sites and in soft or sandy ground.
How wide should a fence post hole be?
About two and a half to three times the post width, so 250 to 300mm for a 100mm post. Wider means more concrete and more bags, but depth matters more than width for resisting wind leverage — dig deeper before you dig wider.
How long does postcrete take to set?
It goes off in around 10 minutes and is firm enough to work around in 30. Have the post plumb and braced before you add water, because there is no time to adjust afterwards.
Do you add water before or after postcrete?
Pour about half the water into the hole first, then add the postcrete, then the rest of the water on top — roughly 3 litres per 20kg bag. Do not mix it in a barrow; it is designed to be used dry in the hole.
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Estimates are for guidance only. Material dimensions, densities and coverage vary by supplier — always confirm quantities before ordering, and consult a qualified professional for structural work.