Working out how much concrete to order might seem tricky, but it’s simple once you know the method. Getting this calculation right saves you money and prevents delays that can mess up your project.
Why Getting the Amount Right Matters
Concrete isn’t like other building materials, where you can store leftovers. Once mixed, you have about 90 minutes before it becomes unusable. Order too little and you’ll face expensive emergency deliveries plus weak joints where different pours meet. Order too much and you’re wasting money on concrete you can’t use.
Having to make a second order mid-project creates problems. The new concrete won’t bond properly with the old, leaving visible lines and weak spots that can crack later.
The Basic Formula You Need
Concrete is measured in cubic metres (m³). To find how much you need:
Length × Width × Thickness = Volume in cubic metres
Here’s a simple example. You’re laying a patio that’s 4 metres long, 3 metres wide, and 10cm thick:
- 4m × 3m × 0.1m = 1.2 cubic metres
Always convert your thickness to metres first. A 10cm thick slab becomes 0.1m, and a 15cm thick driveway becomes 0.15m.
Different Shapes Need Different Methods
For Rectangular Areas
This is the easiest calculation. Simply multiply length × width × thickness.
For Circular Areas
For circular slabs or foundations, use this formula: π × radius² × thickness = volume
If you have a circular patio with a 2-metre radius and it’s 10cm thick:
- 3.14 × 2² × 0.1 = 1.26 cubic metres
For Complex Shapes
Break complex areas into simple rectangles and circles. Calculate each section separately, then add them together.
Common Project Types and Their Needs
Driveways need to be 15cm thick to handle a car’s weight. A standard single-car driveway (5m × 3m) needs about 2.25 cubic metres.
Patios can be thinner at 10cm for foot traffic only. A typical garden patio (4m × 4m) needs 1.6 cubic metres.
Garden paths usually need a 7.5cm thickness. A 20-metre path that’s 1 metre wide needs 1.5 cubic metres.
Foundation strips are often 60cm deep and 20cm wide, but always check building regulations for your specific project.
Always Add Extra for Waste
Even with perfect calculations, add 5-10% extra to cover:
- Spillage during pouring
- Small variations in ground level
- Compaction of the base
For our 1.2 cubic metre patio example, adding 10% gives you 1.32 cubic metres to order.
Choosing the Right Concrete Mix
Different projects need different concrete strengths:
- C20 concrete: Perfect for garden paths and light use
- C25 concrete: Good for patios and workshop floors
- C30/C35 concrete: Best for driveways and heavier loads
- C40/C45 concrete: For commercial work and heavy-duty areas
At Cardinalis, our expert concrete delivery in Cambridgeshire includes advice on choosing the right mix for your project.
Professional Tips to Get It Right
Measure twice, calculate once. Walk the area with a tape measure and write down each measurement. It’s easy to make mistakes when you’re rushing.
Check your access routes. Our volumetric mixers need clear access to pour concrete. If we can’t get close enough, you might need to factor in wheelbarrowing time and potential spillage.
Consider the weather. According to the Concrete Society, hot weather makes concrete set faster, whilst cold weather slows it down. This doesn’t change how much you need, but it affects your timing.
Plan your pour sequence. For large areas, concrete should be poured in one go to avoid cold joints. This means getting your calculations right so the entire area can be completed without breaks.
Account for reinforcement. If you’re using steel mesh or rebar, this takes up space within your concrete thickness. Don’t forget to include this in your planning.
Step-by-Step Calculation Process
Step 1: Measure your area carefully. Use a proper tape measure and get someone to help with longer distances.
Step 2: Convert all measurements to metres. Write them down to avoid confusion.
Step 3: Multiply length × width × thickness to get your basic volume.
Step 4: Add 5-10% for waste allowance.
Step 5: Round up to the nearest 0.1 cubic metres for ordering.
Let’s work through a real example. You want a rectangular drive measuring 6 metres long, 3.5 metres wide, and 15cm thick:
- 6m × 3.5m × 0.15m = 3.15 cubic metres
- Add 10% waste: 3.15 + 0.315 = 3.465 cubic metres
- Round up for ordering: 3.5 cubic metres
Understanding Volumetric Mixing Benefits
Traditional concrete trucks arrive with pre-mixed loads in fixed quantities. This often means ordering more than you actually need. Volumetric mixing works differently – we mix concrete on-site using precise measurements.
This method offers several advantages. You get exactly what you’ve calculated without paying for excess. The concrete is fresh because mixing happens at your location, not at a distant plant. You can also adjust the mix if ground conditions aren’t quite what you expected.
Common Mistakes That Cost Money
Using centimetres in calculations. Always convert to metres before multiplying, or your answer will be 100 times too small.
Forgetting thickness variations. Sloping areas need measuring at the thickest point to ensure adequate coverage.
Ignoring compaction. Loose ground and hardcore bases settle under concrete weight, creating depressions that need extra material.
Underestimating access challenges. If concrete needs wheelbarrowing from the mixer truck, spillage increases and timing becomes more critical.
Mixing measurement units. Stick to metres throughout your calculations to avoid confusion.
When Professional Help Makes Sense
Some projects benefit from expert advice. Complex shapes, unusual load requirements, or areas with drainage considerations are worth discussing with experienced suppliers.
Structural work like foundations, retaining walls, or commercial applications often needs engineering input. These projects might require specific concrete grades, reinforcement details, or special additives that affect quantity calculations.
Our team has been helping customers across Cambridgeshire, Suffolk, Hertfordshire, Essex, and Norfolk for over ten years. We can verify your calculations and suggest the most suitable concrete grade for your specific application.
Getting Ready for Delivery Day
Prepare your base properly. A well-prepared sub-base ensures consistent concrete thickness across the entire area.
Have your tools ready. You’ll need rakes, screeds, floats, and any edge forms prepared before delivery arrives.
Check weather conditions. Extreme hot or cold weather affects concrete performance and may require adjusted timing.
Clear access routes. Make sure our delivery vehicle can reach your location safely and efficiently.
Confirm your calculations. Double-check measurements and quantities one final time.
Making the Most of Expert Advice
Professional concrete suppliers offer more than just delivery. We can help verify your calculations, recommend appropriate concrete grades, and suggest timing strategies that ensure successful results.
Our volumetric mixing service means you pay for exactly what you use. We can adjust quantities slightly on-site if ground conditions reveal small measurement variations, giving you the flexibility that traditional pre-mix deliveries can’t match.
Take time to measure carefully, choose the right concrete grade, and add appropriate waste margins. With these steps, you’ll order exactly what you need for a concrete pour that delivers excellent results and lasts for years.