Concrete Yardage & Bag Estimator

Master Guide to Concrete Volume Calculations & Bag Conversions

Pouring concrete requires exact volumetric estimation. Whether ordering ready-mix trucks or purchasing pre-mixed bags for DIY patio slabs, under-estimating causes cold joints and structural slab defects, while over-ordering results in wasted money and difficult cleanup. This guide details the mathematical formulas behind concrete cubic yardage calculations, bag conversions, slab thickness guidelines, and rebar reinforcement spacing.

1. The Cubic Yardage Formula

Concrete in North America is measured in Cubic Yards (1 cubic yard = 27 cubic feet). The basic formula to determine concrete volume for a rectangular slab or footing is:

Because slab thickness is typically specified in inches (e.g., 4", 6", or 8"), you must convert inches to feet by dividing by 12. A 4-inch slab converts to 0.333 feet ($4 \div 12 = 0.333$).

2. Pre-Mixed Concrete Bag Yield Conversions

For smaller DIY residential projects (sidewalks, post footings, small patio slabs), pre-mixed concrete bags (such as Quikrete or Sakrete) are used instead of ready-mix trucks. Use these standard bag yield multipliers:

• 80 lb Concrete Bags: 1 bag yields approximately 0.60 cubic feet. It takes 45 bags (80 lb) to make 1 cubic yard.
• 60 lb Concrete Bags: 1 bag yields approximately 0.45 cubic feet. It takes 60 bags (60 lb) to make 1 cubic yard.
• 50 lb Fast-Setting Bags: 1 bag yields approximately 0.375 cubic feet. It takes 72 bags (50 lb) to make 1 cubic yard.

3. Recommended Slab Thickness Guidelines