Offcut WorksFence post concrete calculator
Work out how many bags of post mix you need from the size of your holes, with a range shown when bag yields differ.
Why a fixed number of bags per post is unreliable
Online guides give anything from one bag to two or more per post. The reason is that the amount of concrete depends mostly on the width of the hole. Doubling the hole width roughly quadruples the volume, so a 300 mm hole can need three or four times as much as a 200 mm hole of the same depth. This calculator works from the size of your hole instead.
How it is worked out
Concrete per post = volume of the hole − volume of the post. For a round hole, the volume is π × (diameter ÷ 2)² × depth. Bags = concrete per post ÷ yield of one bag.
For a 300 mm round hole 600 mm deep with a 100 mm square post: 0.0424 m³ − 0.006 m³ = 0.0364 m³, or about 36 litres of concrete.
The yield of a bag depends on the product
Two Tarmac figures explain most of the disagreement. Tarmac Postcrete is a sand and cement mix, and its product data sheet table works out at about 13 litres per 20 kg bag if the holes are round. Tarmac Postmix concrete contains coarse aggregate, and its stated coverage of 0.12 m² at 75 mm deep works out at about 9 litres per 20 kg bag. An Australian data sheet we found gives 9.3 litres, which agrees with the Postmix figure.
If you enter nothing, the calculator shows the range from 9 to 13 litres a bag. If you enter the yield for your product, it shows one number. Ordering too few is more annoying than ordering too many, so use the higher bag count unless you know your product.
Tarmac's Postcrete table, for comparison
This is the manufacturer's table of approximate 20 kg bags of Postcrete for square posts. It does not state the hole shape. The figures fit round holes, and a square hole of the same width needs about a quarter more.
| Hole (width × depth) | 75 mm post | 100 mm post |
|---|---|---|
| 200 × 450 mm | ¾ | ⅔ |
| 200 × 600 mm | 1 | ¾ |
| 200 × 750 mm | 1⅓ | 1 |
| 300 × 450 mm | 2¼ | 2 |
| 300 × 600 mm | 3 | 2¾ |
| 300 × 750 mm | 3¾ | 3½ |
Some merchants publish the same numbers, so they are not a second source. Tarmac says Postcrete can fix decking posts, rotary washing lines and gateposts for gates under 1.0 m wide. For wider or heavier gates, ask your supplier. Follow the method and the quantities on the bag you buy.
Hole size and depth
Guides we checked commonly use about 600 mm in the ground for a 1.8 m fence. A wider hole uses more concrete but does not necessarily make the post more secure, and the right depth depends on the post length and the ground. Ask your supplier if you are unsure.
Questions
How many bags of postcrete do I need per post?
It depends on the hole size. A small 200 mm hole with a 100 mm post may need around one bag, and a 300 mm hole may need around three. Use the calculator with your hole size.
Is one bag per post enough?
Hanson and Tarmac Postmix say one 20 kg bag suits a standard post, and that holds for a small hole such as 200 mm wide and 750 mm deep around a 100 mm post. Tarmac's own Postcrete table shows a 100 mm post in a 250 mm round hole 600 mm deep needs about 1¾ bags, and a 300 mm hole needs about 2¾.
What is postcrete?
A fast-setting concrete mix made for fixing fence posts. It is sometimes sold as post mix.
Why do online guides disagree?
They assume different hole sizes and bag yields. Check the yield on your bag and measure your hole.
Does the calculator include concrete gravel boards or posts?
No. It counts only the concrete needed to set the posts. Use the fence panel calculator for panels, posts and gravel boards.
Where the defaults come from
Both Tarmac documents are UK manufacturer information.
- Tarmac Postcrete product data sheet (PDF). A UK manufacturer's table of 20 kg bags by post size and hole size. It also says Postcrete suits decking posts, washing lines and gateposts for gates under 1.0 m wide.
- Tarmac Postmix concrete. One 20 kg bag covers about 0.12 m2 at 75 mm deep and 0.09 m2 at 100 mm deep, which is about 9 litres.
- Boral post hole concrete data sheet (PDF). 108 twenty-kilogram bags per cubic metre, about 9.3 litres a bag. An Australian product, but it agrees with Tarmac Postmix.
- Tarmac does not print a yield or a hole shape. The 13 litre figure is worked back from its Postcrete table, which fits about 13 litres per 20 kg bag for round holes (several rows give exactly that). The data sheet is dated 2014 to 2016, so read the bag you buy.
- The 9 litre figure is worked out from Tarmac's stated coverage for Postmix concrete. It applies to that kind of mix, not to every product sold as post mix.