How much concrete you need in Rockhampton: 1m³ covers 10m² at 100mm

How to measure it properly with a tape and a phone

Ninety per cent of the ordering mistakes we see are measuring mistakes, not maths mistakes. Twenty minutes with a tape saves the argument at the chute.

1. Measure in metres to two decimal places. Write 8.65, not “about eight and a half”. On a 4 metre wide driveway, 150mm of sloppiness at each end is most of a cubic metre.
2. Measure both ends and the middle. Driveways and old slabs are almost never parallel. Use the average of the three widths, and if the difference is more than about 200mm, split the job into two rectangles instead.
3. Break odd shapes into rectangles and triangles. Rectangle is length times width. Triangle is base times height divided by two. Add them up. A curve gets treated as the rectangle that contains it, which is deliberately generous, and generous is the right direction to be wrong in.
4. Check the depth in three places, not one. Dig a test hole at each end and one in the middle, run a string line at the finished level, and measure down. The pad that looks flat is usually 20 to 40mm out somewhere.
5. Photograph the tape. Lay the tape out, photograph each measurement with the numbers readable, and put the photos in one album. When somebody asks how wide the driveway was, you will have it.
6. Allow for the fall. A driveway falling 1 in 100 across 4 metres is 40mm of extra depth at the high side if you are pouring to a level finish over a level base. Small, but it is real concrete.

The rules of thumb worth memorising

• 1m³ covers 10m² at 100mm. Also 8m² at 125mm, 6.7m² at 150mm, 5m² at 200mm.
• Every extra 25mm of depth on 50m² is 1.25m³, which is $375 to $500 of extra mix.
• Wastage: 5 per cent formed tight and level, 10 per cent normal, 15 per cent on soft or uneven ground.
• A 300mm by 300mm thickened edge beam adds about 0.06m³ per lineal metre over and above a 100mm slab, so 30 metres of perimeter is another 1.8m³.
• Mesh: one SL72 or SL82 sheet per 13m² after laps, plus about 4 bar chairs per m².
• Road base: order about 25 per cent more loose volume than the compacted depth you want.
• Bagged premix: about 100 bags of 20kg per cubic metre. Only sensible under about 0.3m³.

The sizing mistake that costs the most here

It is not area, it is depth. Area is easy: you can measure it, and if you get it wrong by a metre you can see the mistake. Depth is invisible once the base is in, and a pad that finished 30mm low across a 60m² slab quietly swallows an extra 1.8m³, which is $540 to $720 of concrete nobody ordered. You do not discover it until the last truck is empty and the slab is not full.

That moment is worse than the money. If the pour stops for an hour while another load is batched and driven out from town, the concrete already placed has started to set, and where the new load meets it you get a cold joint: a permanent line of weakness through the slab that no amount of finishing will fix. In February heat that window is shorter again, which is why a summer pour needs its order to be right the first time. Our guide to the best time of year to pour concrete in Rockhampton covers the temperature side of that.

So over order slightly, always. Leftover concrete is easy to place: a bin pad, a step, an air conditioner base, a path to the clothesline, or a mower ramp at the shed door. Have one of those pegged out and ready before pour day and the surplus becomes something useful instead of a wheelbarrow of waste going back on the truck.

What to say when you order the mix

If you are ordering direct from a batch plant, they will ask for six things. Have them ready and the delivery goes smoothly.

1. Volume in cubic metres, including your wastage allowance.
2. Strength in MPa. 25 MPa for standard residential here, 32 MPa near the coast, 40 MPa for heavy duty or where you need early strength.
3. Slump, which is how wet the mix is. Around 80mm is normal for a slab placed by hand, and a pumped mix is usually wetter. Do not order it wetter than you need.
4. Aggregate size, normally 20mm for a slab and 10mm for thin sections and around dense steel.
5. Additives, which in a Rockhampton summer usually means a retarder so the mix does not go off before it is finished.
6. Delivery time and spacing if you need more than one truck, so loads arrive as you are ready rather than all at once.

One rule above all of those: do not let anyone add water at the chute to make the mix easier to spread. Extra water increases shrinkage and reduces the final strength, and it is the reason a slab that looked perfect on the day dusts and crazes six months later. Any water added on site is recorded on the delivery docket, which is why you keep the docket. Our warranty page explains why that one piece of paper matters more than any other if there is ever a strength argument.

When you should not be ordering concrete at all

Three honest cases. First, if the job is under about 0.3m³, which is a clothesline footing, a small step or a couple of fence posts, bags from the hardware store beat a truck once you account for the short load fee. Second, if the slab is structural, on reactive black soil, or is going to carry a house, a shed or a vehicle, the quantity is the easy part and the design is the part that matters: get the soil test and the engineered detail first, because ordering the right amount of the wrong specification helps nobody. Our page on slab prep on reactive soil explains what changes.

Third, if you are pricing this because you are thinking about placing it yourself: a driveway or a shed slab is not a first concrete job. You have one window, it is measured in hours, and in a Central Queensland summer it is shorter than you think. If you are doing it anyway, get help, start at first light, and read the day by day breakdown of a driveway job so you know what you are taking on. If you would rather it was our problem, the driveway surface guide is the next thing to read, and the form below gets you a fixed price.