Concrete Slab Calculator
Calculates concrete volume for rectangular slabs based on length, width, and thickness in inches, converting dimensions directly into cubic yards for ready-mix transit delivery trucks or premixed bags.
Formula
Cubic Yards = (Length ft × Width ft × (Thickness in ÷ 12)) ÷ 27 × (1 + Waste %)
1. Slab Volume (Cubic Feet) = Length ft × Width ft × (Thickness in ÷ 12) 2. Base Cubic Yards = Slab Volume ÷ 27 3. Total Cubic Yards with Waste = Base Cubic Yards × (1 + Waste %) 4. 80 lb Bags = Total Cubic Yards × 45 bags/yard 5. 60 lb Bags = Total Cubic Yards × 60 bags/yard
Inputs
- Slab Length
- Slab Width
- Slab Thickness
- Include 4" Gravel Sub-base (Crushed Stone)?
Worked example
Pouring a 240 sq ft patio slab at 4" standard thickness with 10% subgrade over-dig allowance.
Outcome: Volume is 80 cu ft (2.96 cubic yards). Adding 10% waste equals 3.26 cubic yards. If using bags: 147 bags of 80 lb (or 196 bags of 60 lb). If ordering ready-mix: 3.5 cubic yards.
When should I order a ready-mix truck instead of mixing bags?
As a rule of thumb, anything over 1 to 1.5 cubic yards (45 to 65 eighty-pound bags) is best ordered via a ready-mix truck. Hand mixing over 50 bags is physically exhausting and risks cold joints between batches.