cement concrete mixes

There are two ways this usually goes wrong. Either you buy too little and end up standing in the yard at 3pm on a Sunday with four posts set and three still leaning, or you buy far too much and stack the leftovers in the shed where they slowly go hard. Neither is necessary. The maths is genuinely simple, and once you've done it for one hole you've done it for all of them. Here's the method we give customers over the counter at Ravenhall. The idea behind it You're filling a hole. So you need the volume of the hole, minus the bit of it the post takes up, multiplied by the number of holes. That's the whole calculation. Everything else is just making sure you measure the hole honestly. Step one: settle your hole size Before you can measure anything you need to know how big each hole is going to be. The rule of thumb most fencers work to: Diameter — roughly three times the width of the post. A 125mm post wants a hole around 300mm across. It feels generous when you're digging it. It isn't. Depth — around a third of the fence's above-ground height. A 1.8m fence lands you at about 600mm. Corner posts, straining posts and gate posts should go deeper and wider than the line posts, because they carry a concentrated load that never lets up. Factor those in separately rather than averaging them into the rest. If you're in reactive clay common enough across Melbourne's western suburbs err towards more depth rather than more width. Depth is what gets you below the zone where the soil moves with the seasons. Step two: work out the volume of one hole For a round hole: Volume = π × radius² × depth Work in metres and you'll get cubic metres, which is the unit everything is sold in. Take a 300mm hole at 600mm deep. Radius is 0.15m, depth is 0.6m. 0.15 × 0.15 × 3.14 × 0.6 = 0.042 m³ So roughly 42 litres of hole. For a square hole, it's even easier — width × width × depth. Step three: take the post out of it The post is occupying part of that hole, so you're not filling all 42 litres with concrete. A 125×125 post sitting 600mm into the ground displaces: 0.125 × 0.125 × 0.6 = 0.009 m³ Subtract it: 0.042 − 0.009 = 0.033 m³ per hole. Call it 33 litres of concrete per post. For a 125×75 post the displacement is smaller, so you'll need a touch more concrete per hole around 0.036 m³. Step four: multiply out, then add a margin Count your posts. If you've got 15 line posts at 0.033 m³ each, that's 0.495 m³ — near enough to half a cubic metre. Then add 10%. Always. Holes are never as neat as they are on paper, some will be wider than you intended, and running out mid-job is a far worse problem than having one bag spare. That takes you to roughly 0.55 m³. Step five: turn cubic metres into bags This is the step people get stuck on, and it's worth committing to memory. A standard 20kg bag of pre-mixed concrete yields roughly 0.01 m³ — about 10 litres of wet concrete. So the conversion is about as friendly as it gets: cubic metres × 100 = number of 20kg bags. Our 0.55 m³ job needs about 55 bags. Per hole, that's between three and four bags for a typical 300mm × 600mm footing. That single figure three to four bags a post is the one most fencers carry in their head, and now you know where it comes from. Always check the yield printed on the bag you're actually buying, as it varies a little between products. When bags stop making sense Somewhere around the one cubic metre mark, the calculation changes character. That's 100 bags, which is two tonnes of material to lift, carry and mix by hand, and a long day of it. At that point pre-mixed delivered concrete is usually the better call less handling, consistent mix, done in a fraction of the time. If you're planning a long boundary run, a commercial job or a fence with a lot of gate and corner posts, talk to us before you order bags. We'd rather point you at the right approach than sell you a pallet you'll regret. What we stock for it We carry Boral Concrete Mix and Boral Cement along with the Cement Australia range, so whether you want a straightforward bagged mix for post holes or cement to batch your own, it's on the shelf. While you're ordering, the things people forget: a wheelbarrow if you're mixing more than a handful of bags, a decent shovel, a spirit level, and something to brace the posts while the concrete goes off. All of it sits in our tools and garden ranges. We also stock cement and concrete sealers if you're finishing exposed footings. A few practical notes from the yard Let it cure before you load it. Fresh concrete looks set long before it's strong. Give the footings time before hanging rails, palings or especially a gate. Rushing this is the most common reason a new fence starts moving in its first year. Brace everything. Posts drift while concrete sets, and a post that's 10mm out at ground level is a lot more than 10mm out at the top. Crown the top of the footing. Finish the concrete slightly proud and sloped away from the post so water runs off rather than sitting against the timber. Small detail, meaningful difference to how long the post lasts. Don't mix more than you can place. Work in batches you can actually get into the hole and vibrate down before it starts stiffening. Do the counting once If you'd rather not run the numbers by hand, our Fencing Calculator will size up your run and tell you what you need. And if the job is big enough that you want a second opinion on quantities, ring us on (03) 9989 1247 or email sales@aclassbuildingmaterials.com.au. Pick up from Ravenhall or Corio, or we'll ship Australia-wide. Trade credit is available if you'd prefer to keep the cash flow tidy.