Show the formula and working
This concrete calculator works out how much concrete you need for a slab, shed base, path, strip footing, round column or set of post holes, in cubic metres, tonnes and bags of ready mix. It is built for UK and European projects measured in metres and millimetres, with a feet and inches switch, and it adds a waste allowance so you do not run short halfway through a pour.
Quick answer: Multiply length x width x depth in metres to get cubic metres, then add about 5% for waste. A 3 m by 2 m base 100 mm deep needs 0.6 m³, or 0.63 m³ with waste, which weighs about 1.51 tonnes at 2,400 kg/m³. The concrete calculator also converts this into bags.
What Is a Concrete Calculator?
A concrete calculator is a tool that turns the size of a slab, footing or hole into the volume of concrete needed and the weight or number of bags that volume represents. For a rectangle, volume = length x width x depth, with the depth converted from millimetres to metres. For a round column or post hole, volume = 3.1416 x radius x radius x depth. Multiply by how many you are pouring and add the waste percentage.
Weight is volume x density. Normal concrete weighs around 2,400 kg per cubic metre, the figure given in the Wikipedia article on the properties of concrete, so each cubic metre is about 2.4 tonnes. Bags are the volume in litres divided by the litres of concrete each bag makes, which is printed on the bag.
How Do You Use the Concrete Calculator?
- Choose the Shape: a slab, path, base or strip footing, or a round column or post hole.
- For a slab or footing, enter Length and Width. For a round shape, enter Diameter.
- Enter Depth or height in millimetres: the slab thickness, the footing depth or the hole depth.
- Set How many, for example the number of post holes, and Extra for waste and uneven ground.
- Leave Concrete density at 2,400 kg/m³ unless your supplier gives another figure.
- Enter the Bag weight and Concrete made per bag (from the bag) for the ready mix you plan to buy.
- Read the cubic metres, tonnes and bags, then open “Show the formula and working” to see each step.
How Do You Calculate Concrete in Cubic Metres?
Multiply the length, width and depth together, all in metres. A depth of 100 mm is 0.1 m, so a 4 m by 3 m slab 100 mm thick is 4 x 3 x 0.1 = 1.2 m³. Measure the depth of the dug area rather than the finished slab if the ground is uneven, and take several depth readings and use the average.
For an L shaped area, split it into two rectangles, work each out and add them together. Remember that sub base, such as compacted hardcore, goes under the concrete and needs its own volume. If you are laying a concrete floor indoors and will level it later, our self levelling compound calculator covers the finishing layer.
How Much Does a Cubic Metre of Concrete Weigh?
A cubic metre of normal concrete weighs about 2,400 kg, or 2.4 tonnes. Lightweight concretes weigh less and heavyweight concretes more, so if your supplier quotes a different density, enter it in the calculator. This weight matters when you hire a skip for leftovers, plan barrow loads or check what a trailer can carry.
How Many Bags of Concrete Do I Need?
Divide the volume in litres by the litres of concrete one bag makes. One cubic metre is 1,000 litres, so if a 20 kg bag makes about 9 litres of concrete, 1 m³ needs about 112 bags. Yields differ between products, so always use the figure printed on your bag; the 9 litre default is only an example.
Bags make sense for post holes, small bases and repairs. For more than about 1 m³, mixing by hand becomes very hard work, and ready mixed concrete delivered by truck, or a volumetric mixer that mixes on site, is usually quicker and more consistent.
| Slab thickness | Concrete per 10 m² (no waste) | Weight at 2,400 kg/m³ | Bags at 9 litres per bag |
|---|---|---|---|
| 75 mm | 0.75 m³ | 1.8 tonnes | 84 |
| 100 mm | 1.0 m³ | 2.4 tonnes | 112 |
| 150 mm | 1.5 m³ | 3.6 tonnes | 167 |
| 200 mm | 2.0 m³ | 4.8 tonnes | 223 |
Do You Need Planning Permission for a Concrete Front Garden?
In England, covering more than 5 m² of a front garden with impermeable concrete needs planning permission unless the water drains to a lawn, border or soakaway on your own property. The government’s guidance on permeable surfacing of front gardens explains the options, and rules in Scotland, Wales, Northern Ireland and other countries differ, so check with your local planning authority.
Worked Example: A Shed Base
Example: a concrete shed base is 3 m long, 2 m wide and 100 mm thick. Add 5% for waste and uneven ground, use 2,400 kg/m³ for the density, and plan to buy 20 kg bags that each make 9 litres of concrete.
- Volume: 3 m x 2 m x 0.1 m = 0.6 m³.
- With waste: 0.6 m³ x 1.05 = 0.63 m³.
- Weight: 0.63 m³ x 2,400 kg/m³ = 1,512 kg, about 1.51 tonnes.
- Bags: 630 litres / 9 litres = 70 bags of 20 kg, 1,400 kg in total.
Seventy bags is a lot to mix by hand, so it is worth comparing the cost with a small ready mixed delivery. Once the shed is up, you can use the fence paint calculator for the timber around it.
Concrete Calculator FAQs
How much concrete do I need for a 10 m² slab?
A 10 m² slab 100 mm thick needs 1 m³ of concrete, or 1.05 m³ with 5% waste. At 150 mm thick it needs 1.5 m³. Multiply the area by the thickness in metres, then add your waste allowance.
How much extra concrete should I order?
Order about 5% to 10% more than the measured volume. Uneven ground, soft spots, formwork that bows and spillage all use extra concrete, and running short mid pour leaves a weak cold joint. Use the higher figure on rough or freshly dug ground.
How many 20 kg bags of concrete make a cubic metre?
It depends on the yield printed on the bag. If a 20 kg bag makes 9 litres, you need about 112 bags per cubic metre; if it makes 10 litres, you need 100. Check the bag and enter the yield in the calculator.
How do I work out concrete for post holes?
Choose the round shape, enter the hole diameter and depth, and set how many holes. Four holes 0.3 m wide and 600 mm deep need about 0.17 m³, or 0.18 m³ with 5% waste. The post itself takes up a little space, so this is slightly generous.
How many tonnes is a cubic metre of concrete?
A cubic metre of normal concrete is about 2.4 tonnes, based on a density of around 2,400 kg/m³. Lightweight mixes weigh less. Multiply your volume by 2.4 for a quick weight in tonnes.
What is the formula for concrete volume?
For a rectangle, volume = length x width x depth, all in metres. For a cylinder, volume = 3.1416 x radius squared x depth. Convert millimetres to metres by dividing by 1,000, so 150 mm becomes 0.15 m before you multiply.
Checked October 2026. Sources: Wikipedia: Properties of concrete (density around 2,400 kg/m³), GOV.UK: Permeable surfacing of front gardens guidance, Wikipedia: Cubic foot.