Concrete Slab Calculator
Estimate concrete volume and bag count for a rectangular slab or pad.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
Estimate concrete for a rectangular slab or pad from its length, width and thickness. This model does not calculate round columns or narrow footings. Bag yields vary by product; check the package.
Worked example
calculated with the default values below
Length 10 ft · Width 10 ft · Thickness 4 in · Waste allowance 10 % · Bag size 80 lb Volume 36.67 cu ft · Volume 1.358 cu yd · Volume 1.038 m³ · Bags needed 62 · Total weight 5,317 lb
General background references
Disclaimer
These calculators provide planning estimates only. Building codes, wiring regulations and safety requirements differ by country and change over time, and nothing here replaces a licensed professional or a permit. For structural, electrical or gas work, follow local code and have the installation inspected.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
V = L × W × t ÷ 12 × (1 + w ÷ 100)
- L
- length in ft
- W
- width in ft
- t
- thickness in inches
- w
- waste allowance in %
- V
- volume in ft³
Rectangular slab/pad: V = L × W × t ÷ 12 × (1 + w/100). This does not model columns or narrow footings. Bag counts assume yields of 0.30, 0.45 and 0.60 ft³ for 40, 60 and 80 lb bags; check the selected product. Weight is a rough 145 lb/ft³ estimate.
Frequently asked questions
Why does an 80-pound bag only yield six tenths of a cubic foot?
The calculator assumes an 80-pound bag of mix makes about 0.6 cubic feet once water is added, a 60-pound bag 0.45 and a 40-pound bag just 0.3, because the dry weight does not equal finished volume. A 10 by 10 foot slab at 4 inches with 10 percent waste needs roughly 37 cubic feet, so about 62 eighty-pound bags. Order by the cubic yard and the same slab is close to 1.4 yards.