By Sam Alex, civil engineer · About the author
Running out of concrete halfway through a pour is one of the most stressful things that can happen on a small job. The wet edge starts to set, the truck is 40 minutes away, and you end up with a weak cold joint right through your slab. Over-ordering isn't free either: you pay for concrete you can't use and then have to get rid of it. Here is the method I use to get the quantity right.
Step 1: Measure the area properly
Measure the slab to the inside face of the formwork, because that's the concrete you'll actually pour. For a rectangle, that's simply length × width. Break L-shapes and irregular slabs into rectangles and add them together.
For a 6 m × 6 m garage slab, the area is 36 m².
Step 2: Choose the thickness, and convert it to metres
Typical thicknesses: - Paths and light shed slabs: 75–100 mm - Driveways for cars: 100–125 mm, usually reinforced with mesh - Garage floors: often 100 mm with thickened edges - House slabs: designed by an engineer to AS 2870. Don't guess these.
The most common mistake I see in online calculators is entering "100" for a 100 mm slab. Volume needs metres: 100 mm = 0.1 m. For our garage, 36 × 0.1 = 3.6 m³ for the main slab.
Step 3: Add the thickenings
Most slabs aren't a uniform thickness. Garage and house slabs usually have edge beams (thickenings around the perimeter), and sometimes internal beams under load-bearing walls. These add a lot of concrete and are easy to forget.
Say the garage has a 300 mm wide edge beam that goes 300 mm deeper than the slab: 1. Perimeter: 6 + 6 + 6 + 6 = 24 m 2. Extra volume: 24 × 0.3 × 0.3 = 2.16 m³
That's 60% more concrete than the flat slab alone. Our running total is now 3.6 + 2.16 = 5.76 m³.
Step 4: Allow for waste and sub-grade variation
Real ground is never perfectly flat. The sub-grade dips, formwork bows slightly, and some concrete stays in the barrow, the pump line or the truck chute. I allow: - 5% for a well-prepared, compacted and levelled base with good formwork - 8–10% for rougher ground or hand-dug edge beams
At 5%, the garage needs 5.76 × 1.05 = 6.05 m³. Ready-mix is usually ordered in 0.2 m³ steps, so I'd order 6.2 m³.
Step 5: Truck or bags?
A 20 kg bag of premix makes roughly 0.01 m³, so a cubic metre takes around 100–110 bags. That's over 2 tonnes of bags to lift and mix. As a rule of thumb: - Under about 0.3 m³: bags are fine (fence posts, small pads) - 0.3–1 m³: consider a mini-mix delivery or hire a mixer - Over 1 m³: ready-mix from a truck, often with a line pump if access is tight
When you book, ask about the minimum order and short-load fees. Many plants charge extra below 3 m³. Also ask about waiting time: trucks usually include a short unloading allowance, then charge by the minute.
Step 6: Order the right concrete
For residential work in Australia, you'll typically see normal-class concrete ordered by strength grade, such as N20, N25 or N32. The number is the characteristic 28-day strength in megapascals. Driveways and slabs are commonly N25 or N32, but your plans or engineer should specify the grade, slump and any additives. Tell the plant what the concrete is for and how it's being placed (chute, barrow or pump).
Step 7: Double-check with a second method
Before you ring the plant, check your number another way. I like to sketch the slab and write each part's volume on the sketch: main slab, edge beams, internal beams, pier holes. Then I add them up again. If the two totals don't match, find out why before you order.
You can run each part through the concrete calculator. For round piers, use the round column concrete calculator. If you're working in feet and inches, the square feet to cubic yards calculator converts in one step.
A checklist for pour day
- Sub-grade compacted, with a vapour barrier (plastic membrane) where required
- Formwork braced, levels and falls checked
- Reinforcement on bar chairs at the right cover, not lying on the ground
- Enough people and tools: at least three for a small slab, more for anything big
- A plan for curing: keep the slab moist or use a curing compound, especially in hot or windy weather
- A plan for any leftover concrete (a small extra pad or a few pier blocks)
Frequently asked questions
How much concrete do I need for a 3 m × 3 m slab?
At 100 mm thick, 3 × 3 × 0.1 = 0.9 m³, or about 0.95 m³ with a 5% allowance.
Should I round up my concrete order?
Yes, always. Running short causes a cold joint, while a small surplus can be used for a spare pad.
How long before I can walk or drive on new concrete?
Usually you can walk on it after about 24–48 hours. Wait about 7 days for cars and longer for heavy vehicles. Concrete keeps gaining strength for weeks.