Methodology.
One page that explains what every calculator on this site assumes, where the numbers come from, and when to trust a default against when to override it.
Formulas, code constants, and material-spec values come from the following authorities, in descending order of priority. Where two sources conflict on the same value, the higher-priority authority governs.
- Adopted building codes. IRC 2021 for residential construction, IBC 2021 for commercial, IECC 2021 for energy and insulation. Section numbers cited inline in source where they drive a constant (for example, IRC R311.7 governs stair geometry).
- ANSI and ACI standards. ACI 318 for reinforced concrete, ACI 332 for residential concrete, ANSI A108 for tile installation, ANSI A118 for mortar and grout chemistry.
- ASTM material standards. ASTM C90 for concrete masonry units, ASTM C1063 for lath and plaster, ASTM C168 for thermal insulation terminology, ASTM F1667 for driven fasteners.
- Trade-association publications. BIA Technical Notes for brick, NRMCA CIP bulletins for ready-mix concrete, AWC NDS and span tables for wood framing, GAF and CertainTeed installation guides for asphalt shingles, RSMeans for regional cost factors.
- University extension services. State land-grant university horticultural and agronomic publications for landscape materials (mulch, gravel, soil amendments). Used because these are the only non-commercial sources that publish verified application rates.
- Manufacturer technical data sheets. Published TDS documents for products where a specific value (coverage rate, set time, R-value per inch) drives the calculator output. Linked by full PDF URL, not a brand homepage.
Every constant that mirrors a code or standard value carries an inline source comment that quotes the authoritative text. The file-level comment is the reader's verification path: a disagreement between the code and the calc can be resolved by reading the quoted source without leaving the file. When a new edition of a code changes a value, the constant is updated and the quote is refreshed with the new edition's language.
Every calculator has a default waste percentage that reflects what the trade typically adds above measured quantities. The conventions:
- Concrete, ready-mix: 10% per NRMCA residential default. Bag concrete for small jobs rounds up to full bags, which absorbs waste implicitly.
- Tile: 10% for straight-lay patterns, 15% for running bond or diagonal, 20% for herringbone. Large-format tile over 15 inches on any side triggers an ANSI A108 lippage check.
- Drywall: 15% for full rooms (sheets cut to fit corners, doors, windows, odd ceiling heights). Patches route through a different method entirely.
- Lumber framing: 5% for straight runs, 10% for trim and decking (more cuts per board). Board-foot calculators account for actual cut, not nominal.
- Roofing: 10% for gable roofs, 15% for hip roofs (valleys generate more waste). Starter and ridge bundles computed separately.
- Siding: 10% for lap siding, 12% for board-and-batten (batten cuts add waste). Inside corners are deducted; outside corners add trim.
- Fencing: 5% for pickets and rails, zero for posts (posts are cut from spec).
- Insulation: varies by type. Batts 5%, blown cellulose 8%, spray foam product-specific per TDS.
- Gravel and mulch: 5% for compaction loss (gravel) or settlement (mulch). Gravel also applies a separate compaction factor on top of waste to produce "ordered tons" vs "measured volume."
- Paint: 10% for one-coat, 15% for brush/roller, 35% for sprayed application (overspray plus line priming).
Every default is an override. When the job is tight (clean rectangular rooms, straight runs) reduce the waste; when it is messy (vaulted ceilings, odd cuts, inexperienced crew) increase it. The calculator recomputes on each keystroke so the order total shows in real time.
Every calculator rounds once, at the job total, never per section. A homeowner painting six bedrooms does not run the paint calculator six times and round each to the nearest gallon. Instead, the paint calculator accepts multiple rooms, accumulates paintable square footage across all of them, applies waste once, and rounds the final gallon count up once. Rounding per section produces a 15 to 40% over-order on a multi-room or multi-section job.
Every calculator that accepts length and width inputs assumes a rectangle. L-shapes, T-shapes, curves, and multi-section layouts break this assumption silently. For any non-rectangular project: decompose into rectangles, run the calc once per rectangle, sum the outputs at the end. This is called out on every calc page at the field-procedure note.
The same rule applies to non-flat ceilings (vaulted, cathedral, gambrel). A drywall calculator with a single ceiling-height input cannot model a vaulted space. Run the calc for the rectangular base and add the triangular peak separately.
A calculator that cheerfully returns a number for a project it was not designed for is worse than one that refuses the inputs. Every calc has a refusal case:
- Roofing calculator refuses slopes below 2:12 because those are low-slope membrane roofs under IRC R905, not shingle jobs.
- Stair calculator refuses rise configurations that violate IRC R311.7 (max riser 7.75 inches, min tread 10 inches, max total rise 151 inches per flight).
- Drywall calculator routes to the patch calculator when the damage area is under a threshold where the math for a full room is the wrong shape for the job.
- Tile calculator refuses large-format running-bond offsets above 33% per ANSI A108 lippage control.
Refusal cases carry a redline warning, not a silent compute. The goal is to avoid a confident wrong answer.
National-baseline cost ranges come from a rolling aggregate of retailer list prices (Home Depot, Lowe's, Menards, Ferguson) collected quarterly. ZIP-code adjustment applies a regional multiplier derived from BEA regional price parities for the nearest metro area. Cost ranges are material-only; labor is not priced because it varies too widely by crew and region to publish a defensible number. When a calculator is ZIP-aware, the regional multiplier is visible on the output.
Material changes to formulas, constants, or defaults are logged in the changelog with the date and the source. Every calc content file carries a last-reviewed date rendered on the page. When a code edition changes (IRC 2024 publication, ACI 318-25 ballot) the relevant calculators are reviewed and either updated or annotated with why the older edition still governs.
Disagreements with any of this are welcome. Email [email protected] with "Methodology" in the subject line and the specific paragraph or value in dispute. Include a source citation for the alternative. Methodology updates ship with a changelog entry.