Concrete Calculator
Work out how much concrete you need for a slab, footing, post holes or steps in m³ or cubic yards, and how many pre-mixed bags to buy.
Reviewed by Dany, RightSums team · Updated
Runs in your browser. Free, no sign-up.
home-energy · Daily life · UK & US
Choose a slab or base, strip footing, round post hole or column, or a flight of steps, and enter the sizes in metres and millimetres or feet and inches. The calculator works out the volume in cubic metres, cubic yards and cubic feet, then adds 5%, 7.5% or 10% for spillage and uneven ground. It also counts the bags you need, using yields from the makers' own data sheets: about 0.01 m³ per 20 kg bag of Blue Circle Multi-Purpose Concrete, and 0.30 to 0.60 cubic feet per bag of Quikrete Concrete Mix. For any other bag, enter the yield printed on it. A 3 m by 3 m base 100 mm thick needs 0.99 m³ with 10% extra.
Accepted inputs
- Shape
- Length, width and depth
- Diameter and depth for post holes
- Step width, rise, going and count
- Wastage allowance
- Your bag's yield
Outputs
- Volume in m³
- Cubic yards and cubic feet
- Number of 20 kg bags
- Number of 40 to 80 lb bags
- Bags of your own size
How to use the Concrete Calculator
- Pick the shape you are filling and choose metric or imperial units.
- Enter the sizes, with lengths in metres or feet and depths in millimetres or inches, plus how many of them you need.
- The calculator multiplies out the volume, adds the wastage you chose and converts the total to cubic yards and cubic feet.
- It divides the total by each bag's published yield and rounds up, so you see how many bags to buy.
Frequently asked questions
How much concrete do I need for a 3m x 3m base?
A 3 m by 3 m base 100 mm thick holds 0.9 m³ of concrete. With 10% extra for spillage and uneven ground, plan on 0.99 m³. That is about 99 bags of 20 kg Blue Circle Multi-Purpose Concrete, based on Tarmac's figure of 0.01 m³ per bag.
How many bags of concrete make a cubic metre?
Tarmac says one 20 kg bag of Blue Circle Multi-Purpose Concrete makes about 0.01 m³, so a cubic metre takes about 100 bags. Quikrete's 80 lb bag makes about 0.60 cubic feet, so a cubic yard takes 45 bags. Yields vary by product, so check your bag.
How much concrete do I need for a fence post?
Work out the volume of the hole: pi times the radius squared times the depth. Four holes 300 mm wide and 600 mm deep hold about 0.17 m³, or 0.187 m³ with 10% extra. That is 19 bags of 20 kg concrete.
How many 80 lb bags of concrete do I need for a 10x10 slab?
A 10 ft by 10 ft slab 4 inches thick is 33.3 cubic feet. Adding 10% gives 36.7 cubic feet, or 1.36 cubic yards. At Quikrete's stated 0.60 cubic feet per 80 lb bag, you need 62 bags.
How much extra concrete should I order?
The calculator lets you add 5%, 7.5% or 10% to cover spillage, uneven ground and concrete left in the mixer, and uses 10% unless you choose less. Running short partway through a pour is worse than a little spare, as Quikrete warns that part-filled areas can lead to cold joints.
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