Concrete calculator

Pick the shape you are pouring, punch in your measurements, and the volume and bag counts update as you type. Three modes cover almost every home job: a flat slab for paths, driveways and floors, round post holes for a fence or deck, and a footing or trench for a wall or a set of piles.

Measure in the units the fields ask for. Length and width in metres, depth in millimetres, because that is how depth gets quoted on site. A 100mm slab is 0.1 of a metre deep. Get the depth wrong and the whole number is wrong, so measure it twice.

Thickness is about load, not looks. Here is the rough NZ convention for domestic work:

• Garden path, 75 to 100mm. Foot traffic only. 75mm is fine, 100mm if a wheelbarrow or mower will cross it often.
• Driveway, 100mm. Standard for a car. Push it to 125mm if a boat, trailer or the odd truck uses it, and lay it over a compacted hardfill base.
• Shed or garage floor, 100mm. Over proper hardfill, with mesh if anything heavy sits on it.
• Anything structural, ask first. A slab tied to a house, a retaining footing, or anything holding a building up is not a DIY guess. That needs an engineer.

This is the decision that catches people out. Bags feel cheaper because the sticker price is small, but the maths turns fast. Once you are mixing more than about half a cubic metre by hand, you are into a serious afternoon of lugging, tipping and mixing, and the per-bag cost has quietly overtaken what a truck would charge.

The rough rule: under 0.5m³, bags are usually fine. A few post holes, a small path, a repair. Over 0.5m³ and heading toward a full metre, a ready-mix truck is almost always cheaper per cubic metre, gives you a consistent mix, and lands the whole pour in one go before it starts going off. Our calculator flags the moment your job crosses that line.

One catch with a truck: most suppliers have a minimum load and a part-load fee, so for a genuinely tiny pour bags can still win. If you are near the line, ring the yard and ask for the delivered price both ways.

Nobody pours the exact calculated volume. Some spills, the base is never perfectly level, the trench is always dug a touch wide, and a skim always clings to the mixer. Running short mid-pour is far worse than having a little over, because a cold joint where fresh concrete meets set concrete is a weak point. So we add a margin by default. Seven percent suits most jobs. If your base is rough or your formwork is loose, tick it up toward 10 in your head and order accordingly.

Concrete does not dry, it cures, and it needs water and time to reach full strength. The first few days matter most. Keep a fresh slab damp, cover it with plastic or hessian in hot or windy weather, and keep weight off it. It is walkable in a day or two but nowhere near full strength for weeks. Rushing the cure is the cheapest way to crack an otherwise perfect pour.

Plenty of concrete work in New Zealand needs no paperwork: a garden path, a patio, fence post footings, a standalone slab under a small structure. But the line arrives sooner than people expect. Structural slabs, most retaining over a certain height, and anything forming part of a building generally need a building consent and often an engineer's design. If in doubt, a quick call to your local council saves an expensive undo later. We are planning a simple "does my job need a consent" tool to sit next to this one.