Concrete calculator
Estimate concrete volume, waste allowance and the number of bags for a slab or cylinder.
Enter slab length and width, or cylinder diameter, then depth, quantity and waste. Mix feet and inches or metric units. Results show cubic metres, cubic yards, cubic feet, litres and whole bags rounded up from your product’s stated volume yield. This is a material estimate, not a structural design. Change the input or settings to update your result. Results update as you type. Up to 1,000,000 characters.
Your result
How to use Concrete calculator
- Choose Slab for a rectangular pour or Cylinder for a solid round post. Enter the dimensions, units and number of identical pieces.
- Choose a waste allowance and enter the volume yield printed on your concrete bag. Bag weight alone does not determine yield.
- A 3 m × 2 m slab, 100 mm deep, needs 0.6 m³ net. With 10% waste, that becomes 0.66 m³, or 33 bags yielding 20 litres each. Copy the result or download the estimate.
Useful next steps
How Concrete calculator works
The concrete calculator estimates how much concrete a rectangular slab or a solid cylinder needs. It multiplies the area by the depth and the number of identical pieces, adds your waste allowance, and divides by the yield of one bag, rounding bags up to whole numbers. Results appear in cubic metres, cubic yards, cubic feet and litres, with a downloadable estimate, all calculated in your browser. Use the volume yield printed on your bag, since bags vary. It estimates material only, not strength or structural design.
Before you start
Measure length, width or diameter and the depth, and choose the matching units, since you can mix feet and inches or metric units. Read the volume yield per bag from your product, not its weight.
A worked example
A 10 ft by 10 ft slab, 4 in deep, with 10 percent waste and bags yielding 0.6 ft³ needs 36.67 ft³, or about 1.36 yd³, which is 62 bags.
What to expect
Estimate concrete volume, waste allowance and the number of bags for a slab or cylinder. Enter slab length and width, or cylinder diameter, then depth, quantity and waste. Mix feet and inches or metric units. Results show cubic metres, cubic yards, cubic feet, litres and whole bags rounded up from your product’s stated volume yield. This is a material estimate, not a structural design.
Common questions
How do I calculate concrete for a slab?
Multiply length by width by thickness in the same unit. A 10 ft by 10 ft slab 4 in thick is 10 × 10 × 0.333 ft, about 33.3 ft³. Divide cubic feet by 27 for cubic yards, about 1.23 yd³ here, then add extra for waste and uneven ground.
How many bags of concrete do I need?
Divide the total volume, including waste, by the yield of one bag and round up. With 36.67 ft³ and bags yielding 0.6 ft³, that is 61.1, so the calculator shows 62 bags. Always use the yield printed on your product, because similar bag weights can yield different volumes.
How much extra concrete should I add for waste?
The calculator defaults to 10 percent, and you can set anything from 0 to 100. Spillage, an uneven subgrade, form bulges and slightly deeper spots all use extra material. Running short in the middle of a pour is usually worse than having a little left over.
How do I calculate concrete for a round post hole?
Choose Cylinder and enter the full diameter, not the radius, plus the depth and the number of holes. Four holes 12 in wide and 4 ft deep with 5 percent waste need about 13.19 ft³, or 22 bags yielding 0.6 ft³ each. Subtract the volume of any post yourself.
Why is my depth result so large or so small?
Depth has its own unit selector, separate from length and width. A common slip is entering 100 for a 100 mm slab while the depth unit is set to m, which makes the slab 1,000 times too deep. Changing a unit keeps the number you typed, so recheck both selectors.