Concrete calculator
Calculator
The calculator needs JavaScript. The tables below give the same numbers for common cases.
How it is worked out
- Volume = length × width × thickness = 10 × 10 × 0.3333 ft = 33.33 cubic feet.
- Add 10% for waste: 36.67 cubic feet; ÷ 27 = 1.36 cubic yards.
- Bag yields differ by product; the default is a typical figure for an 80 lb bag, so check the label.
| Inputs | Answer |
|---|---|
| Units: Feet, thickness in inches; Length: 10; Width: 10; Thickness: 4; Extra for waste: 10; Yield of one bag: 0.6 | A 10 × 10 ft slab 4 in thick needs about 1.36 cubic yards (36.7 cubic feet) including 10% extra. |
| Units: Feet, thickness in inches; Length: 20; Width: 24; Thickness: 6; Extra for waste: 10; Yield of one bag: 0.6 | A 20 × 24 ft slab 6 in thick needs about 9.78 cubic yards (264 cubic feet) including 10% extra. |
| Units: Metres, thickness in cm; Length: 3; Width: 3; Thickness: 10; Extra for waste: 10; Yield of one bag: 0.6 | A 9.8 × 9.8 ft slab 3.9 in thick needs about 1.29 cubic yards (35 cubic feet) including 10% extra. |
How to use it
Concrete volume is length × width × thickness. A 10 × 10 foot slab 4 inches thick is 33.3 cubic feet, 1.23 cubic yards; with 10% extra for waste and spillage it is 1.36 cubic yards. Ready-mix is sold by the cubic yard (27 cubic feet).
The bag count uses the yield you enter: check the bag, because yields depend on the product (a typical 80 lb bag yields about 0.6 cubic feet). Order a little extra rather than run short.
Frequently asked questions
How much concrete do I need for a 10x10 slab?
At 4 inches thick, about 1.23 cubic yards, or 1.36 with 10% waste.
How many bags of concrete are in a cubic yard?
It depends on the bag yield; at 0.6 cu ft per bag, about 45.
Why add extra?
To cover uneven ground, spillage and over-excavation.
Method: standard arithmetic shown step by step; no outside data is used. Checked 2026-11-12.
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General information only. Results are plain arithmetic on the numbers you enter; check anything important (money, health, school work) against your own records or the official source.