Methodology & Data Sources
Every calculator on this site runs on published industry constants, not guesses. This page lists the exact figures each tool uses, where they come from, and where real-world conditions can push your actual needs higher.
How a calculation runs
Every tool follows the same three steps: (1) convert your inputs to a common unit (usually cubic feet or square feet), (2) apply a published coverage rate or density constant, (3) add a waste/safety margin appropriate to that material. Nothing is sent to a server — the arithmetic runs in your browser with the constants below.
Reference constants by material
| Material | Constant | Used in |
|---|---|---|
| Interior paint | 350 sq ft / gallon (1 coat, smooth primed drywall) | Paint Calculator |
| Standard interior door | 21 sq ft deduction | Paint Calculator |
| Standard window | 15 sq ft deduction | Paint Calculator |
| Concrete volume | 1 cubic yard = 27 cubic feet | Concrete, Slab, Driveway |
| 80 lb concrete bag | yields ≈ 0.60 cu ft mixed | Concrete, Slab Calculators |
| 60 lb concrete bag | yields ≈ 0.45 cu ft mixed | Concrete, Slab Calculators |
| 50 lb concrete bag | yields ≈ 0.375 cu ft mixed | Concrete Calculator |
| Crushed gravel / stone | ≈ 1.4 short tons / cubic yard (2,800 lbs/yd³) | Gravel, Driveway Calculators |
| Mulch / bagged bark | 2 cu ft and 3 cu ft standard retail bag sizes | Mulch Calculator |
| Standard modular brick | ≈ 7 bricks / sq ft (single wythe, 3/8" joint) | Brick Calculator |
| Masonry mortar (80 lb bag) | lays ≈ 30–35 standard bricks | Brick Calculator |
| Lumber board feet | (Thickness in. × Width in. × Length ft) ÷ 12 | Lumber Calculator |
| Flooring waste margin | +10% standard, +15% for diagonal/herringbone layouts | Flooring Calculator |
Material cost estimates
Every calculator with a price field lets you enter your own quote — always use that over our default. When a price field is left blank, we fall back to a rough 2026 US national average (e.g. $35/gallon paint, $6.50/80 lb concrete bag, $45/ton gravel, $40/yd³ bulk mulch, $0.65/brick, $150/yd³ ready-mix concrete). These defaults move with lumber, cement, and fuel costs and vary significantly by region — treat them as a starting order-of-magnitude, not a quote.
Waste & safety margins
Material estimators on this site default to a 5–10% waste margin (noted on each tool), covering irregular cuts, spillage, and subgrade compaction variance — a standard allowance recommended by concrete and masonry trade associations for DIY-scale pours. Complex layouts (arches, columns, diagonal cuts) warrant 15% or more; we expose that as an adjustable option wherever the geometry calls for it.
Review process
Each calculator's formulas and constants are reviewed by the Home Project Calculators team against current industry references whenever a tool is added or materially changed. The "Formulas reviewed [date]" line under every calculator's title reflects the last such review. We don't claim licensed-contractor sign-off on individual pages — these are planning estimates, and we say so; always confirm final quantities with your supplier or contractor before ordering material.
Sources
- Portland Cement Association — concrete volume, yield, and waste-margin guidance
- Brick Industry Association — Technical Note 10 (modular brick coursing & coverage)
- National Ready Mixed Concrete Association — bag yield and ready-mix ordering thresholds
- Paint manufacturer technical data sheets (Sherwin-Williams, Behr) — per-gallon coverage rates
- U.S. Forest Products Laboratory — board-foot lumber measurement standard