Concrete Slab Calculator
Concrete volume for a slab or pad in cubic yards or cubic metres, plus the number of premixed bags.
Your slab
Filled in with example values. Replace them with your own.
Optional
Adds a percentage on top of the measured volume.
Each bag makes a set volume of concrete, called its yield. It is printed on the bag and varies by product.
Results update as you type.
Result
Concrete needed: 1.23 yd³
You need about
1.23cubic yards
- Volume in cubic feet
- 33.33 ft³
- Volume in cubic metres
- 0.944 m³
- Slab area
- 100 ft²
- Premixed bags
- Turn on “Count premixed bags”
How it’s calculated
Area
10 ft × 10 ft = 100 ft²
Volume
100 ft² × 0.333 ft thick = 33.33 ft³
In cubic yards
33.33 ft³ ÷ 27 = 1.23 yd³
How the concrete slab calculator works
A slab is a rectangular block, so its volume is length × width × thickness. The calculator converts every measurement into the same unit first, then shows the volume in cubic yards (how ready-mix concrete is ordered in the US) or cubic metres.
You can add an allowance for spillage, an uneven base or formwork that bows outwards. If you are using premixed bags, enter the yield printed on the bag and the calculator rounds the number of bags up to the next whole bag.
The formula
Volume = Length × Width × Thickness
Volume with extra = Volume × (1 + Extra % ÷ 100)
Bags = Volume with extra ÷ Yield per bag, rounded up
Slab thickness is usually given in inches or centimetres, so convert it before multiplying: 4 inches is 4 ÷ 12 = 0.333 feet, and 10 centimetres is 0.1 metres.
Worked example
A shed base is 10 feet by 10 feet and 4 inches thick, and you want 5% extra.
- Thickness in feet: 4 ÷ 12 = 0.333 ft
- Area: 10 ft × 10 ft = 100 ft²
- Volume: 100 ft² × 0.333 ft = 33.33 ft³
- In cubic yards: 33.33 ÷ 27 = 1.235 yd³
- With 5% extra: 1.235 × 1.05 = 1.30 yd³ (35 ft³)
Suppose the bag you plan to buy says it yields 0.6 cubic feet. This is an example only; use the figure on your bag. 35 ÷ 0.6 = 58.33, so you would need 59 bags.
In metric: a slab 4 m × 3 m and 10 cm thick needs 4 × 3 × 0.1 = 1.2 m³ of concrete before any extra.
Units, assumptions and limitations
- Conversions use exact definitions: 1 inch = 2.54 cm, 1 foot = 12 inches, 1 yard = 3 feet, 1 litre = 0.001 m³.
- The slab is treated as a rectangle with an even thickness. A base that dips or slopes needs more concrete.
- Reinforcement, a gravel sub-base and formwork are not included.
- No bag yield is assumed. Products differ, so enter the yield from the bag you will buy.
- The right thickness depends on what the slab is for and on local building rules. This calculator does not recommend one.
Frequently asked questions
What is the yield of a bag of concrete?
It is the volume of finished concrete one bag makes once mixed with water. It depends on the product and the bag weight, so it is printed on the bag, usually in cubic feet or litres. Enter that figure to count bags.
Does this include footings, thickened edges or steps?
No. It calculates one flat rectangular slab of even thickness. Work out footings, thickened edges or steps separately and add their volumes to the total.
Does it include the gravel base under the slab?
No, only the concrete. Use the Gravel Calculator for a sub-base: enter the same length and width, and the depth of the base layer.
Should I use bags or order ready-mix?
Bags are practical for small jobs you can mix and place in one go. For larger slabs, compare the bag count with a ready-mix delivery, which is ordered by the cubic yard or cubic metre. Suppliers may have minimum order sizes, so ask them.
How do I calculate a round slab?
The area of a circle is 3.1416 × radius × radius. Multiply that area by the thickness to get the volume, or enter a square with the same area into the calculator.