To find how much concrete you need, multiply length by width by depth to get the volume in cubic metres, then add 5% to 10% for waste. A slab 3 m long, 2 m wide and 10 cm deep is 0.6 m³, or about 0.66 m³ once you allow 10% extra. The same method works for gravel, sand and mulch.
The basic volume sum
Concrete is bought by volume, so the first job is to convert everything to metres.
- Slab or rectangle: length × width × depth
- Circle: π × (diameter ÷ 2)² × depth
- Column or post hole: π × (diameter ÷ 2)² × height or depth
Depth is usually quoted in centimetres or millimetres, so divide by 100 or 1,000 to get metres. A common mistake is leaving the depth in centimetres, which gives an answer 100 times too big. If your measurements are in feet or inches, convert them first: 1 foot is 0.3048 m and 1 inch is 0.0254 m. One cubic metre is 1,000 litres, and one cubic yard is 0.7646 m³.
Why you need to add waste
You will not use exactly the volume you calculate. Ground is rarely perfectly level, formwork can bow slightly, some mix is spilt and the bottom of a trench is often a little deeper than planned. Most people add 5% to 10% for concrete and more for uneven or soft ground. It is cheaper to have a small surplus than to stop half way through a pour, because joins between batches can leave weak lines.
Worked example: a small patio slab
We used the Concrete, gravel and mulch calculator tool for a slab 3 m by 2 m and 10 cm thick, with a 10% allowance.
| Step | Result |
|---|---|
| Volume before waste | 0.6 m³ |
| Volume with 10% extra | 0.66 m³ (660 litres) |
| Cubic yards | 0.86 yd³ |
| Approximate weight at 2.4 t/m³ | about 1.58 tonnes |
| 20 kg bags at 9 litres each | 74 bags |
The weight uses an approximate density for concrete, so it is a guide. The bag count depends entirely on the yield of the bag you buy. Some mixes give a little more or less than 9 litres, so check the printed yield and enter it in the calculator.
Bags, mixer or ready-mix?
Bagged mix suits small jobs such as a path, a shed base or a few fence posts. At 74 bags, the patio above would be a lot of lifting, because 74 bags of 20 kg weigh nearly 1.5 tonnes. For pours of roughly a cubic metre or more, a ready-mix supplier or a hired mixer is usually easier, and a supplier will ask for the volume in cubic metres. Ask them about minimum loads, delivery and how quickly the concrete must be placed, and confirm the strength grade for the job. This guide gives general information only, so ask a builder or engineer about foundations, load-bearing work and drainage.
Gravel, sand and mulch
Loose materials are sold by weight, by the bulk bag or by the litre. For these, work out the volume first and then convert:
- Weight: volume × density. Loose gravel is typically around 1.6 tonnes per cubic metre. This is an approximate figure, and wet or compacted material is heavier.
- Bags: weight divided by the bag size, rounded up. Common sizes are 25 kg and 40 kg, and bulk bags are much larger, so check your supplier.
- Mulch: volume in litres divided by the litres in the bag.
For the same 3 m by 2 m area laid 10 cm deep in gravel, the calculator gives 0.66 m³ with waste, about 1.06 tonnes, 43 bags of 25 kg or two 850 kg bulk bags. A sub-base for a slab is usually compacted, so you may need more than the loose volume. Mulch is usually laid thinner, so enter the depth you actually want, such as 5 cm.
Post holes and columns
Fence posts and small columns add up quickly because each hole needs several bags. Six holes 30 cm wide and 90 cm deep come to 0.38 m³, or 0.42 m³ with 10% waste. At 9 litres a bag that is about 47 bags. Our How to measure for flooring and fencing guide covers how to count posts, and the Fencing calculator tool estimates post-mix bags directly.
Check before you order
- Measure the area twice and write down the units.
- Check the depth in several places.
- Decide on the waste allowance.
- Read the yield or coverage on the product.
- Round up, never down.
For other home projects, the How much paint, wallpaper or tile do you need? guide covers areas in the same way, and Area and square footage helps with irregular shapes.
Note: these are general estimates, not engineering advice. Densities and bag yields vary, so check with your supplier before ordering.
Try the calculator
Enter your shape, dimensions, depth and waste in the Concrete, gravel and mulch calculator calculator to get volume, weight and bag counts. If you are weighing up a patio against other home costs, our Area and square footage tool is useful too.
Open the free Concrete, gravel and mulch calculator
Worked example 2: a larger slab, and a circular base
Here are two more jobs, calculated with the House of Calculator concrete tool using 10% waste, a density of 2.4 tonnes per cubic metre and bags that yield 9 litres each.
| Job | Volume before waste | Volume with 10% | Weight | 20 kg bags |
|---|---|---|---|---|
| Shed base: 6 m × 3 m, 10 cm deep | 1.8 m³ | 1.98 m³ | about 4.75 tonnes | 220 |
| Driveway strip: 5 m × 2.5 m, 100 mm deep | 1.25 m³ | 1.375 m³ | about 3.3 tonnes | 153 |
| Circular base: 2.5 m diameter, 15 cm deep | 0.736 m³ | 0.81 m³ | about 1.94 tonnes | 90 |
The first job needs 220 bags weighing 4.4 tonnes, which is not realistic to mix by hand in a day. At nearly two cubic metres it is the sort of pour for which ready-mix delivery or a hired mixer with several helpers makes sense. The second row is deliberately entered in millimetres to show that the tool handles it; always check the depth units in your own sum.
For the circle, the arithmetic is area first, then depth: radius 1.25 m, so area = 3.14159 × 1.25 × 1.25 = 4.909 m², multiplied by 0.15 m depth = 0.736 m³. Add 10% and you reach 0.81 m³.
Step-by-step: working it out by hand
- Sketch the shape and note every dimension in metres. Split an L-shaped area into two rectangles.
- Convert the depth. 10 cm is 0.1 m, 75 mm is 0.075 m and 4 inches is 0.1016 m.
- Multiply length × width × depth for each rectangle, then add them together.
- Add waste. Multiply by 1.05 for 5% or 1.10 for 10%.
- Convert to bags or loads. Divide the litres (volume in m³ × 1,000) by the yield per bag, and round up.
- Check the weight that you will have to move, especially for bags. A 20 kg bag is manageable one at a time, but a hundred of them is a serious day’s work.
As a quick check on any answer, one cubic metre of ready-mixed concrete weighs about 2.4 tonnes, so a result of, say, 12 tonnes for a small patio should ring an alarm bell.
Planning the whole job, not just the concrete
Concrete is only one line in the budget. Depending on the job you may also need:
- A sub-base. Slabs are normally laid on compacted hardcore or crushed stone, which adds depth to the excavation. The mulch and gravel method in the original guide applies. For example, 4 m × 3 m at 5 cm deep in bark mulch comes to 0.66 m³ with waste, about 198 kg at an approximate density of 0.3 t/m³, or 14 bags of 50 litres (calculator figures; mulch density varies widely).
- Formwork to hold the edges, and stakes to keep it straight.
- Reinforcement, such as mesh, if the use requires it.
- Damp-proof membrane where moisture could rise through the slab.
- Expansion joints or a fall for drainage on outdoor slabs.
Allow for the extra excavation too, because the hole you dig must be as deep as the sub-base and slab combined. If the project is a fence or deck as well, our guide on How to measure for flooring and fencing covers the measuring side.
Common mistakes to avoid
- Mixing units. Length in metres and depth in centimetres is the classic error. Convert before multiplying.
- Forgetting the sub-base. The concrete depth is only one layer of the excavation.
- Not allowing for waste. Running short mid-pour produces cold joints, which are weaker.
- Pouring in the wrong weather. Frost can ruin fresh concrete, and hot, windy conditions can dry it too quickly. Check the manufacturer’s guidance and the forecast.
- Mixing too wet. Extra water makes the mix easier to spread but weakens it.
- Ignoring access. A ready-mix lorry needs space and a firm approach. If it cannot reach the site, you will need barrows.
- Skipping the curing. Fresh concrete should be kept from drying out too fast and from heavy loads for the period the supplier recommends.
Comparing the options
| Method | Good for | Watch out for |
|---|---|---|
| Bagged mix, mixed by hand | Post holes, small bases | Lots of lifting; consistency varies |
| Hired mixer | Around 0.5 to 1.5 m³ | Needs a team and quick working |
| Ready-mix delivery | Larger pours, driveways | Minimum loads, time limits, access |
| Pump | Difficult access | Extra cost; ask the supplier |
If you are still weighing up the overall cost of a home project, the same area-and-coverage thinking applies to How much paint, wallpaper or tile do you need?.
When to check with a professional
For foundations, retaining walls, structural slabs, driveways carrying vehicles or anything near drains and boundaries, ask a builder or structural engineer. Building regulations or planning rules may apply, and in some cases you may need approval. Check with your local council or the GOV.UK planning pages before starting, and ask the supplier about the strength grade, additives and the working time of the mix.
Frequently asked questions
How many bags of concrete are in a cubic metre?
It depends on the yield of the bag. With a 20 kg bag giving 9 litres, a cubic metre of 1,000 litres needs about 111 bags. Check the yield printed on the bag.
How thick should a concrete slab be?
It depends on the use. Paths and light-duty bases are often thinner than driveways or garage floors, which need more depth and sometimes reinforcement. Ask a builder or check the supplier’s guidance for your job.
How much extra concrete should I order?
Many people add 5% to 10%. Add more if the ground is uneven or the formwork is not tight, and round up to the next whole bag or load.
Can I use the same method for gravel?
Yes. Work out the volume, multiply by an approximate density of about 1.6 tonnes per cubic metre for loose gravel, and divide by the bag or bulk bag size.
How many cubic metres of concrete do I need for a 10 cm slab?
Multiply the area in square metres by 0.1. A 20 m² slab 10 cm deep is 2 m³, or 2.2 m³ with a 10% allowance.
How much does a cubic metre of concrete weigh?
About 2.4 tonnes, though the exact figure depends on the mix. A 20 kg bag of dry mix produces roughly 9 to 10 litres of wet concrete, so check the printed yield.
How long does concrete take to set?
It usually becomes hard enough to walk on within a day or two, but reaches most of its strength over about four weeks. Times vary with weather and mix, so follow the supplier’s advice.
Is it cheaper to mix my own or order ready-mix?
For small jobs, bags are simple. For larger pours, ready-mix is often better value once you count your time, equipment hire and the risk of an inconsistent mix.