Calculation Methodology
This page documents how every HomeBuild Atlas calculator produces its numbers: the formulas, the correction factors, where they come from, and - just as important - what they cannot tell you. If a result matters to your budget or safety, verify it here by hand before ordering.
Core principles
Geometric volume first. Every estimate starts from plain geometry: length × width × depth, with unit conversions shown (12 inches per foot, 27 cubic feet per cubic yard).
Real-world corrections second. Raw geometry always underestimates what you must buy. We apply the standard corrections used in construction estimating:
| Correction | Default | Basis |
|---|---|---|
| Concrete waste | 10% (adjustable 0–30%) | Common flatwork practice for form deflection, subgrade variation, spillage |
| Aggregate compaction | 15% (adjustable) | Typical loose-to-compacted reduction for crushed stone placed in lifts |
| Aggregate waste | 5% | Handling and spreading losses |
| Soil swell | 12–50% by soil type | Published earthwork swell ranges (sand 12%, mixed earth 25%, clay 30%, rock 50%) |
| Pipe/fabric overage | 5% / 50% overlap | Standard field allowances for cuts and laps |
Density-based weight conversion. Aggregates are sold by the ton, so volume converts through density: concrete ~4,050 lb/yd³, crushed stone ~2,700 lb/yd³, dense-graded base ~2,900 lb/yd³, pea gravel ~2,400 lb/yd³, sand ~2,600 lb/yd³. These are typical dry values; moisture adds 5–15%. Your supplier's figures govern.
Formulas by tool
Concrete: V(yd³) = L × W × (T/12) / 27, waste applied, rounded up to 0.5 yd³ for ready-mix ordering. Bag yields: 80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³ (manufacturer-published yields).
Gravel: compacted volume × (1 + compaction) × (1 + waste) = loose order volume; tons = yd³ × density / 2,000.
Excavation: bank volume geometrically (trapezoidal average width for sloped trenches); loose = bank × (1 + swell). Truckloads round up; weight limits may govern before volume.
French drain: gravel = trench volume minus pipe displacement (πr²L) plus 5% waste; fabric width = 2×width + 2×depth + 50% top overlap; drop = length × slope.
Slope: grade % = rise/run × 100; degrees = arctan(rise/run); slope length by Pythagoras. Grade recommendations follow common U.S. practice (1% pipe minimum, 2% surface drainage, 5% at foundations for the first 10 ft, ADA 1:12 ramps).
Mulch & topsoil: volume = area × (depth/12) / 27 with 5% default waste; bag counts at 2 and 3 ft³ per bag; typical densities: mulch ~500, compost ~1,100, soil blend ~1,800, topsoil ~2,050 lb/yd³.
Fence post concrete: hole volume = πr² × depth (cylinder), minus the embedded post's displacement, plus 10% default waste; bag yields as in the concrete tool.
Cubic yards: area (rectangle, circle, or triangle) × depth/12 / 27; m³ = ft³ × 0.0283168; tons via the selected material density. No waste factors; it reports raw geometric volume.
References we rely on
Where our guides cite standards, they refer to widely used U.S. references: ACI 302 (concrete floor and slab construction) and ACI 318 (structural concrete) for concrete practice; OSHA 29 CFR 1926 Subpart P for excavation safety; ASTM density and gradation designations for aggregates; and the International Residential Code (IRC) for grading and drainage provisions (e.g., R401.3). Local amendments override all of these - your building department has the final word.
Limitations - read this
Our calculators produce planning estimates for typical residential conditions. They do not know your soil bearing capacity, frost depth, groundwater level, slope stability, or local code. They are not a substitute for site-specific design by a licensed engineer, and results should be confirmed with suppliers and contractors before purchase. Structural elements - foundations, suspended slabs, retaining walls over locally permitted heights, deep trenches - require professional design. See the safety disclaimer.
Review process
Each tool and guide is written against the references above and reviewed by our editorial team (see About) before publishing, then re-reviewed when standards, typical prices, or best practices change. Every page shows its last-reviewed date. Corrections: contact us.