How to calculate concrete
- Measure the hole. Diameter (auger) or width (dug by hand) and depth.
- Measure the post. 100 × 100 mm for most timber posts; set Post shape to round for round posts.
- Subtract the post. Hole volume − post volume = concrete for one hole.
- Multiply by the posts. Add 5–10% and divide by 0.0117 m³ for 25kg bags.
Worked example: 15 fence posts, 300 mm holes 600 mm deep
- One hole: π × 0.15² × 0.6 − 0.1 × 0.1 × 0.6 = 0.0364 m³.
- Fifteen holes plus 5%: 0.573 m³.
- 25kg bags: 49, about 1,225 kg.
Concrete per post hole
| Round hole (diameter × depth) | Post | m³ | 25kg bags |
|---|---|---|---|
| 250 × 600 mm | 100 × 100 mm | 0.023 | 3 |
| 300 × 600 mm | 100 × 100 mm | 0.036 | 4 |
| 300 × 900 mm | Ø 125 mm | 0.053 | 5 |
| 350 × 900 mm | 125 × 125 mm | 0.073 | 7 |
| 400 × 900 mm | 150 × 150 mm | 0.093 | 8 |
One hole, no waste included. 25kg bags at 0.0117 m³ each (Dricon CivilCrete states 0.012).
Concrete formulas by shape
All measurements in metres; divide millimetres by 1,000 first. Results are in cubic metres.
- Post hole
- (hole area − post area) × depth; round hole π × (diameter ÷ 2)², square hole width²
- Column / pier
- π × (diameter ÷ 2)² × height
1 m³ = 1,000 litres.
Frequently asked questions
How many 25kg bags of concrete per fence post?
A 300 mm hole 600 mm deep around a 100 × 100 mm post takes about 0.036 m³: 4 bags of 25kg.
How deep should a post hole be?
A common rule of thumb is about a third of the post's height above ground, with a hole about three times the post's width. For decks and anything load-bearing, NZS 3604 and your plans set the size.
How much concrete for a round post?
A 125 mm round post in a 300 mm hole 900 mm deep takes about 0.053 m³: 5 bags of 25kg.
About the prices
Costs are estimated ranges with source and date, never a quote. Enter your local prices under "Use your own prices", or read how we estimate prices.