Every concrete job ends with the same question: do we mix on site, or order ready-mix? The answer depends on the volume, your local prices, how many hands you have and what strength the plans ask for. Prices vary a lot from town to town, so this guide doesn’t quote any. Instead it shows how to make the comparison with your own supplier quotes, which BouWise can hold in its price list.
Step 1: get the volume right
Start with the volume including waste. Use the concrete slab calculator or the floor slab calculator. Both work in cubic metres and show the site-mix materials and the ready-mix volume side by side.
Step 2: cost a site mix
For a site mix, the materials per cubic metre come from the Concrete Society of Southern Africa’s figures: 290 kg of cement for a low-strength foundation mix, 385 kg for a medium mix for floors and driveways, and 460 kg for a high-strength mix, each with about 0.6 m³ of concrete sand and the same of stone. To cost it:
- Bags of cement = volume × cement per m³ ÷ 50, rounded up, × your price per bag.
- Sand and stone = volume × 0.6 m³ (or the exact figure for the mix), rounded up to the next half cube, × your price per cube. Don’t forget that half cubes are the smallest you can usually order, so small jobs pay for some sand and stone they won’t use.
- Delivery of the cement, sand and stone, if it isn’t included.
- Labour to mix and place. Mixing by hand or with a small mixer is slow; work out how many people and how many hours the pour needs, and cost it at your day rate.
- Mixer hire and fuel, if you don’t own one.
Step 3: cost ready-mix
- Volume rounded up to what the supplier delivers. BouWise rounds to the next 0.1 m³; ask your supplier about their minimum load.
- The price per cubic metre for the strength on your plans.
- Any small-load, waiting-time or pump charges the supplier quotes.
- Labour to place and finish, which is far less than for a site mix because the concrete arrives ready.
Step 4: compare per cubic metre
Divide each total by the volume. Then try the sum again at half and double the volume. You will usually find that a site mix wins on very small pours, where a truck would carry mostly empty space, and that ready-mix wins as the volume grows, because labour and the time it takes to mix by hand rise with every cubic metre while the truck’s fixed charges are spread thinner. The volume where the two lines cross is your break-even for that supplier and that crew. Write it down; it changes only when prices change.
Formula: site mix per m³ = (cement + sand + stone + delivery + mixing labour + mixer) ÷ m³ and ready-mix per m³ = (m³ ordered × rate + extra charges + placing labour) ÷ m³.
It isn’t only about price
- Strength. The Concrete Society says its site batches don’t guarantee a particular strength. If the plans or an engineer specify a strength in MPa, ready-mix in that strength (or a tested mix) is the safe choice, whatever the price.
- Consistency. A large slab mixed in dozens of batches will vary from batch to batch. Ready-mix arrives as one consistent mix.
- Time. A slab must be placed and finished before the first concrete stiffens. If your crew can’t mix fast enough, joints form where you didn’t want them.
- Access. A truck needs a route to the pour, or a pump. On tight sites, a site mix moved by barrow may be the only option.
- Weather. Hot, windy days shorten the time you have. That favours ready-mix and an early start.
Using BouWise to keep the numbers
Put your cement, sand, stone and ready-mix prices into the BouWise price list once. Each calculator then prices its own material list, and when you add a result to a job, the quote picks up the same prices. Run the same slab once as a site mix and once as ready-mix and compare the two job totals: that is your break-even, worked out for your town and your suppliers.
A rule of thumb to check against
If the pour is more than a few cubic metres, has to be finished in one go, or the plans give a strength, price ready-mix first. If it is a small footing, a few post holes or steps, mixing on site is usually simpler. Either way, do the sum with real quotes before you decide.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check SANS 10400 (the National Building Regulations) and your local municipality’s building control officer for your project.