How to Determine the Right Amount of Ready Mix Concrete for Your Project

Getting the Volume Right from the Start

Ordering ready mix concrete is not like buying bags of cement where you can pick up an extra one if you run short. Once the truck arrives, what you have is what you get. Order too little and the pour stops halfway, creating a weak cold joint. Order too much and you pay for material that hardens in the chute. The key is knowing how to calculate the volume you need before you call the supplier. This article walks through the simple maths, the common pitfalls, and the factors that affect your final order quantity.

The Basic Calculation

Concrete volume is measured in cubic metres. The formula is straightforward: length times width times depth. All three measurements must be in metres. For example, a slab that is 5 metres long, 4 metres wide, and 0.15 metres deep (150 mm) gives 5 x 4 x 0.15 = 3 cubic metres. That is the volume of solid concrete you need. But ordering exactly that figure is rarely wise because real-world conditions add waste, spillage, and slight variations in depth. Most contractors add a wastage allowance of 5 to 10 percent to the calculated volume. For the slab above, that means ordering 3.15 to 3.3 cubic metres.

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The same formula works for footings, strip foundations, and column bases. For a trench footing, measure the length of the trench, the average width at the base, and the depth. Multiply them together. If the trench has a stepped profile, break it into rectangular sections, calculate each one, and add them together. For a round column, use the formula pi times radius squared times height. For most domestic and small commercial jobs, the rectangular slab or trench is the common shape, so the simple length-width-depth method covers most needs.

Understanding Delivered Concrete Quantities

When you place an order, the supplier works in whole or half cubic metre increments. A typical truck carries around 6 cubic metres, though smaller loads are possible. This is where understanding delivered concrete quantities helps you avoid waste. If your calculation gives 1.2 cubic metres, you will likely order 1.5 cubic metres because the supplier rounds up to the next half metre. That extra 0.3 cubic metres is part of your wastage allowance. If the job is small, consider whether a delivered load makes sense or whether hand-mixing might be more practical. For larger pours, the ability to order exactly what you need improves with accurate measurement and a realistic waste factor.

Common Mistakes in Estimating Volume

The most frequent error is forgetting to convert inches or millimetres into metres. A plan drawn in millimetres shows a slab depth of 150, not 0.15. If you plug 150 into the formula as if it were metres, you get a volume that is a thousand times too large. Always convert every dimension to metres before multiplying. Another common mistake is measuring the inside of the formwork rather than the outer edge. If the formwork is 100 mm thick and you measure the cavity, that is correct. If you measure the overall footprint and subtract the formwork thickness, you need to do that subtraction carefully. It is easy to double-count or forget a side.

A third mistake is ignoring the slope. If the ground is not level and you intend to pour a slab of uniform thickness, the depth varies across the area. The correct approach is to take several depth measurements across the site and use the average. For a small patio, three or four depth readings are enough. For a large driveway, take readings every metre along the length and width. The average depth multiplied by the area gives a reliable volume.

Wastage and Over-Ordering

Wastage is not just about spillage. Some concrete stays in the truck drum and cannot be pumped out. Some is lost when you clean the chute between loads. Some is used to test the slump on site. A 5 to 10 percent waste factor covers these losses. For complex pours with many corners, steps, or tight access, lean toward 10 percent. For a simple rectangular slab with good access, 5 percent is usually enough. Never skip the waste factor unless you have a very good reason and are prepared to mix a small amount by hand at the end.

If you are ordering multiple loads for a large pour, check that the supplier can deliver them in sequence without long gaps. A delay of more than 30 minutes between loads can cause the first pour to begin setting before the second load arrives, creating a cold joint. The supplier will advise on the maximum time between loads based on the concrete grade and the weather. For a typical C30 mix on a warm day, the window is about one hour from batching to placement.

Concrete Grades and Their Effect on Volume

The grade of concrete affects the mix design but not the volume calculation. A cubic metre of C10 has the same volume as a cubic metre of C40. However, the grade does affect the cost and the strength. For a garden path or a non-structural slab, C10 or C20 is adequate. For a driveway that will take vehicle weight, C30 is common. For structural footings or heavy-duty industrial floors, C40 or higher may be specified. The supplier will ask for the grade when you order. If you are unsure, ask a structural engineer or check your building regulations. Ordering the wrong grade can lead to cracking or failure, and replacing it is expensive.

Delivered Concrete Quantities and Site Access

Once you know the volume you need, think about how the concrete gets to the pour area. A ready mix truck is large and heavy. It needs a firm, level access route and enough space to manoeuvre. If the truck cannot get within about 30 metres of the pour point, you may need a concrete pump. The pump adds cost and requires a separate pump truck or a trailer-mounted unit. If you are using a pump, tell the supplier when you order so they can adjust the mix for pumpability. A mix that is too stiff will block the pump hose.

For small pours where the truck can back up to the formwork, the driver can discharge directly into the forms. For larger pours, you will need a gang of helpers to spread and tamp the concrete. Plan your labour before the truck arrives. Concrete waits for no one, and a driver who has to sit idle while you find extra hands will charge for the delay.

Checking the Order Before It Arrives

When the supplier confirms your order, check the volume, the grade, and any additives such as plasticiser or retarder. A retarder slows the set time, which is useful in hot weather or for large pours. A plasticiser makes the concrete flow more easily without adding water. Both affect the workability but not the volume. Confirm the delivery time and the number of loads. If you are ordering 4.5 cubic metres, the supplier may send one load of 4.5 cubic metres or two loads of 2.25 cubic metres each. The latter costs more because of extra delivery charges. Ask for a single load if possible.

Final Checks on Site

Before the truck arrives, double-check your formwork. Make sure it is level, rigid, and sealed at the joints. A leak in the formwork lets concrete escape and reduces the volume in the slab. Have a wheelbarrow, a shovel, and a tamping tool ready. If you are using reinforcement, ensure it is positioned correctly and supported on chairs so it stays within the slab depth. The concrete must flow around the reinforcement without moving it.

Measure the formwork again. It is common to find that the depth or width is slightly different from the plan. If the formwork is 10 mm deeper than expected over a 20 square metre area, that adds 0.2 cubic metres to the volume. Adjust your order if possible. Most suppliers can increase the load size up to a few hours before dispatch, but they cannot reduce it once the concrete is batched.

A Note on the Supplier

Stacey Concrete Ltd, based at Ryder Point Works, Wirksworth, Matlock DE4 4HE, UK, supplies ready-mix concrete grades C10, C30, C40 and more with delivery across the East Midlands. You can contact them on +44 1629 825188 to discuss your project's volume and grade requirements.