How Concrete Calculations Work: Cubic Yards, Bag Yields, and Waste
The math behind every concrete pour is deceptively simple — but ordering the wrong amount is expensive. This guide covers the formulas, the bag-yield conversion tables, and the field practices that prevent short-loads on pour day.
1. The Cubic Yard Formula
Always add a waste factor. The industry default is 10% for rectangular slabs and 15% for footings or irregular shapes.
2. Bag Size Yield Table
3. Concrete Strength: 3000 vs 4000 PSI
3000 PSI is the residential workhorse — patios, walkways, foundation walls in mild climates. 4000 PSI is required for driveways, garage slabs, and anywhere exposed to freeze-thaw cycles. Going above 4000 PSI adds $15-$30 per yard; below 3000 PSI risks cracking under load.
4. Ordering Tips
- Order 10% more than calculated — spillage and over-excavation are real.
- For pours above 1 yd³, ready-mix is cheaper per yard and saves 8+ hours of mixing.
- Schedule the truck for early morning to avoid mid-day heat that accelerates curing.
- Have your forms, rebar, and helpers staged before the truck arrives. Most companies charge $90/minute for wait time.
Pour the Right Amount, First Time
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