Data Source and Methodology
Authoritative Data Source:
- QUIKRETE Company. Product Data Sheets and Project Calculator (2023–2024). Concrete yield per bag (40/60/80 lb) and mix guidance. https://www.quikrete.com/
- ASTM C138/C138M-17a (or latest): Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete.
- Portland Cement Association (PCA). Design and Control of Concrete Mixtures (16th ed., 2016) — Typical normal-weight concrete density ~ 150 lb/ft³ (≈ 2400 kg/m³).
All calculations strictly follow the formulas and data provided by these sources.
The Formula Explained
Strip footing volume:
V = L \times B \times T
Rectangular pad footing volume:
V = N \times (L \times W \times T)
Cylindrical pier footing volume:
V = N \times \pi \left(\frac{D}{2}\right)^2 \times H
Waste factor:
V_{\text{total}} = V \times \left(1 + \frac{w}{100}\right)
Conversions:
1~\text{yd}^3 = 27~\text{ft}^3,\quad 1~\text{m}^3 = 35.3147~\text{ft}^3
Estimated weight (normal-weight concrete):
W_{\text{US}} = V_{\text{ft}^3}\times 150~\frac{\text{lb}}{\text{ft}^3}, \quad
W_{\text{Metric}} = V_{\text{m}^3}\times 2400~\frac{\text{kg}}{\text{m}^3}
Bag counts (US examples):
\text{Bags}_{80} = \left\lceil \frac{V_{\text{ft}^3}}{0.60} \right\rceil,\quad
\text{Bags}_{60} = \left\lceil \frac{V_{\text{ft}^3}}{0.45} \right\rceil,\quad
\text{Bags}_{40} = \left\lceil \frac{V_{\text{ft}^3}}{0.30} \right\rceil
Bag counts (Metric example):
\text{Bags}_{25\text{kg}} = \left\lceil \frac{V_{\text{m}^3}}{0.012} \right\rceil
Glossary of Variables
- L: Length of footing (strip/pad).
- B: Width of strip footing.
- W: Width of pad footing.
- T: Thickness (depth) of footing.
- D: Diameter of pier footing.
- H: Height (depth) of pier footing.
- N: Number of discrete footings (pads or piers).
- w: Waste allowance in percent.
- V: Calculated volume before waste.
- V_total: Volume after applying waste.
- W_US / W_Metric: Estimated weight in lb or kg.
How It Works: A Step-by-Step Example
Suppose you need a strip footing 80 ft long, 2.0 ft wide, and 0.75 ft thick, with 10% waste.
- Base volume: V = 80 × 2.0 × 0.75 = 120 ft³.
- Apply waste: V_total = 120 × (1 + 10/100) = 132 ft³.
- Convert to cubic yards: 132 ÷ 27 ≈ 4.889 yd³.
- Bags (80 lb): ceil(132 / 0.60) = ceil(220) = 220 bags.
- Weight: 132 × 150 = 19,800 lb.
If a supplier quotes $150/yd³ delivered, cost ≈ 4.889 × 150 ≈ $733.35.
Frequently Asked Questions (FAQ)
Do I need to include the footing key or pedestal in the volume?
Yes. If you plan a key or pedestal that adds volume, include it by increasing thickness or adding additional volumes to your estimate.
What if the trench is irregular?
Use average dimensions: measure the smallest and largest width/depth and average them, or break the footing into segments and sum the volumes.
How do I switch between cubic yards and cubic meters?
Select the unit system. US shows yd³ as primary; Metric shows m³. Both systems display helpful secondary conversions.
Are the bag yields guaranteed?
No. Yields vary with compaction, water content, and product line. Always verify with the product’s technical data sheet and purchase extras to avoid shortages.
Can I use this to size my footings?
No. This tool only estimates concrete quantity. Sizing must meet code requirements (e.g., ACI 318) and soil capacity. Consult a licensed structural engineer.
What waste percentage should I use?
Small projects: 5–10%. Complex forms, uneven trenches, or remote pours: up to 15%. Your contractor’s experience is the best guide.