🔎 Methodology
This page explains how our calculators work: the formulas we use, the assumptions and default values, how we choose data sources, how we validate our tools, and the limits of what our outputs can be used for. Our goal is to deliver transparent, reproducible estimates for DIYers and pros.
Last updated: 26 Oct 2025
Scope & Purpose
Home Project Calculators provide planning-grade estimates for materials and costs across projects like asphalt, concrete, gravel, paint, roofing, fencing, decking, and more. These outputs help you budget, compare options, and communicate with suppliers.
- What these are Fast estimates based on geometry, standard densities/coverage rates, and adjustable assumptions.
- What these are not Engineering sign-off, permit documentation, or a substitute for local building codes or a licensed professional’s advice.
Units & Conversions
We support both imperial and metric. Inputs and outputs can be toggled per tool. The following conversions are used throughout the site:
| Quantity | Imperial | Metric | Conversion |
|---|---|---|---|
| Length | 1 in | 25.4 mm | in × 25.4 = mm |
| Area | 1 ft² | 0.092903 m² | ft² × 0.092903 = m² |
| Volume | 1 yd³ | 0.764555 m³ | yd³ × 0.764555 = m³ |
| Mass | 1 ton (US) | 907.185 kg | tons × 907.185 = kg |
| Density | lb/ft³ | kg/m³ | lb/ft³ × 16.01846 = kg/m³ |
Rounding is applied to practical purchase units (e.g., whole bags, boxes, bundles), explained per calculator.
Common Formulas
Geometry
- Area (rectangle):
A = L × W - Area (circle):
A = πr² - Volume (prism/slab):
V = A × thickness(ensure thickness unit matches area) - Roof pitch (rise over run):
tan(θ) = rise / run
Material Quantities
- Weight from volume:
Weight = Volume × Density - Bags (concrete, mortar):
Bags = (Volume × (1 + waste)) ÷ yield_per_bag - Tons (asphalt/aggregate):
Tons = (Volume × Density) ÷ 2000(US short tons) - Paint gallons:
Gallons = (Wall_Area − Openings) × Coats ÷ Coverage - Shingle squares:
Squares = Roof_Area ÷ 100 ft²(add waste) - Lumber board feet:
BF = (T × W × L) ÷ 144(inches)
Examples (with defaults)
Concrete slab → bags
Inputs: L, W, thickness; defaults: waste 10%, 60-lb bag yield ≈ 0.45 ft³.
Steps: Volume ft³ = L × W × (thickness_in / 12) → Adjust for waste → Bags = Volume / 0.45 → round up.
Asphalt tonnage
Inputs: Area, depth; defaults: compacted density 145 lb/ft³, compaction factor 5–8%.
Steps: Volume ft³ = Area × (depth_in / 12) → Pounds = Volume × 145 → Tons = ÷ 2000 → apply compaction → round to 0.25 ton.
Paint coverage
Inputs: Wall area, doors/windows, coats; defaults: 350–400 ft²/gal per coat on smooth walls.
Steps: Net area = Wall_Area − Openings → Gallons = Net × Coats ÷ Coverage → round up to whole cans.
Defaults & Assumptions
Each calculator ships with sensible defaults that you can override. We keep defaults conservative to reduce under-ordering.
Typical Defaults
- Waste allowance: 5–15% (material-dependent)
- Compaction (asphalt/base): +5–8%
- Paint coverage: 350–400 ft²/gal per coat
- Aggregate density: ~100–125 lb/ft³ (type-dependent)
- Asphalt density: ~145 lb/ft³
- Rounding: to whole bags/boxes/bundles
Regionalization
- Support for imperial & metric inputs/outputs
- Cost ranges are presented as national medians with regional notes when available
- Users can adjust labor & material rates to match local quotes
Per-calculator panels list the exact defaults and cite sources.
Data Sources & Citations
We prioritize primary and authoritative references for densities, coverage rates, and best practices:
- Standards bodies (e.g., ASTM, AASHTO), transportation agencies (DOT), and building codes
- Manufacturer technical data sheets (TDS) and installation guides
- Trade associations and textbooks for baseline practices
Each calculator includes an “Assumptions & Sources” panel with inline citations. We also maintain a sitewide Data Sources library that catalogs commonly used tables and references.
How we select sources
1) Prefer primary standards → 2) corroborate with ≥2 manufacturer TDS → 3) note ranges/variance → 4) set conservative defaults → 5) expose as editable inputs.
Validation & QA
- Unit tests: edge cases (zero/negative), extreme dimensions, rounding behaviors
- Cross-checks: compare outputs against hand calculations and at least one independent calculator or reference
- Sanity ranges: min/max guards (e.g., thickness, coverage)
- UI QA: input validation, error messages, accessibility (labels, keyboard, contrast)
Issues can be reported via the inline “Report a problem” link on each calculator or Contact.
Expert Review
Key calculators are peer-reviewed by subject-matter experts (e.g., paving contractor, concrete finisher, roofer, painter). Each reviewed tool displays a badge with the reviewer’s name and last reviewed date, linking to our Review Board.
- Cadence: at least annually, or sooner if standards change
- Change tracking: material defaults or formula tweaks are logged in the changelog
Versioning & Update Policy
- Semantic versioning per tool: vMAJOR.MINOR.PATCH (e.g., v1.4.2)
- MAJOR: formula change or breaking output behavior
- MINOR: new inputs/options, default tweaks
- PATCH: UI copy, rounding clarifications, bug fixes
Every calculator footer shows its version and last-reviewed date.
Limitations & Safety
- Estimates are for planning and budgeting, not stamped engineering calculations
- Always follow local building codes and manufacturer instructions
- For structural or load-bearing work, consult a licensed professional
- Site conditions (moisture, substrate, porosity, waste handling) can materially affect quantities