Ask ten different tradies what mix ratio they use for a slab and you'll probably get ten slightly different answers, all delivered with total confidence. That's because concrete mixing sits in this odd space between exact science and trade instinct. The ratios themselves are simple maths, but knowing which ratio suits your job takes a bit of context. If you're about to mix your own for a footing, path or driveway, here's what those numbers actually mean and how to pick the right one.

What "1:2:4" Actually Means

Concrete mix ratios describe the proportion of three core ingredients by volume: cement, sand, and coarse aggregate (blue metal or crushed rock). So a 1:2:4 mix is one part cement, two parts sand, four parts aggregate, mixed with enough water to make it workable without drowning the strength out of it. Cement is the glue holding everything together; sand fills the smaller gaps and adds workability; aggregate is the bulk that gives concrete its compressive strength and stretches your cement further.

More cement generally means more strength and a smoother finish, but it also means more cost and more shrinkage as it cures, which shows up as cracking if you overdo it. The right ratio is the one that matches the load your slab or footing actually needs to carry, not the strongest mix you can make.

The Three Ratios Worth Knowing

1:2:4, general purpose. This is your default for garden paths, patios, shed slabs and other non-structural work around the property. It strikes a reasonable balance between strength, workability and cost, and it's forgiving enough for a first-time DIYer to get a decent result without a lot of drama.

1:3:6, economy mix. Lower in cement, this is the most budget-friendly option and suits lightweight, low-load jobs, think footings under a garden fence or a small non-structural foundation where you're not asking much of the concrete beyond holding its shape. It's not what you want under anything that needs to bear real weight.

1:1.5:3, high strength. This is the mix for load-bearing work: driveways, columns, structural footings, anything that's going to see vehicle weight or carry load from above. More cement relative to sand and aggregate gives you a denser, stronger result, at a higher material cost.

If you're not sure which category your job falls into, the safe default is to round up rather than down. A driveway that's slightly over-engineered will just outlast everything else on the property; a footing that's under-engineered can crack, settle or fail years down the track when it's expensive and disruptive to fix.

Water Is the Part People Get Wrong

The ratios above only cover the dry ingredients, but water is where most DIY pours go sideways. Too much water makes the mix easier to pour and finish, which is exactly why it's tempting to add a splash more when things start to stiffen up. But that also dilutes the cement paste, weakening the finished concrete and increasing shrinkage cracking. Too little water and you're fighting a stiff, unworkable mix that's nearly impossible to compact properly around reinforcement or into corners of formwork.

The mix should hold its shape without slumping flat, but still fold and work with a shovel or hoe without excessive effort. If you're using a bagged premix product, follow the water quantity on the bag rather than eyeballing it. Those ratios are calibrated for a reason, and it's the fastest way to get a consistent result without a batching scale.

Bagged Premix vs. Mixing Your Own

For smaller jobs (post holes, small footings, patch repairs), bagged premix concrete is hard to beat. You get a consistent, pre-proportioned mix every time, with no need to source and store separate piles of sand and aggregate. For anything larger, like a full driveway or shed slab, mixing your own from bulk cement, sand and aggregate (or ordering a wet mix delivered by truck) works out considerably cheaper per cubic metre.

Worth noting: if your job needs to be poured and finished fast, or you're working in a spot where a slow cure isn't practical, a standard mix might not be your best bet at all. We've covered when rapid set concrete makes more sense than a standard mix in an earlier post. It's a different product with a different working window, and it pays to know which one you actually need before you're standing at the yard with a trolley.

After the Pour: Curing Matters as Much as Mixing

A perfectly proportioned mix can still end up weak and cracked if it's cured badly. Concrete needs to stay damp for at least the first seven days to develop strength properly. Hose it down or cover it with plastic sheeting if you're pouring during a dry Melbourne stretch, especially in summer when surface moisture can evaporate faster than the cure can keep up. Full design strength typically isn't reached until around 28 days, so keep heavy loads and vehicles off a new driveway slab well beyond the point where it looks and feels solid underfoot.

A Few Practical Tips Before You Start

• Work in smaller batches than you think you need. A half-cured wheelbarrow load is easier to manage than a full one that's starting to go off before you've placed it.

• Mix on a hard, clean surface or in a proper mixer rather than directly on dirt, which will contaminate your mix with organic material and weaken it.

• Wear gloves and eye protection. Wet cement is caustic and can cause skin burns with prolonged contact, which catches a lot of first-timers off guard.

• Order slightly more material than your calculated volume; running short mid-pour with concrete already curing in the wheelbarrow is a genuinely bad afternoon.

If you're planning a pour and want to work out exactly how much cement, sand and aggregate to bring home, or you're weighing up bagged premix against a bulk order, come and see the team at A Class Building Materials in Ravenhall. We'll help you land on the right ratio for the job and make sure you've got enough on the trailer before you start mixing.