About
Methodology and Sources
The conventions every calculator follows, and the sources behind the numbers it starts with.
Conventions
Your inputs come first. A calculator never substitutes its own figure for one you have entered. Defaults exist so the page shows a meaningful example; each is labelled and sourced in the assumption table.
Every component is a separate line. Allowances, losses, fees, taxes and rounding appear as their own rows in the breakdown, never folded into a single figure. If a line surprises you, you can see exactly what caused it.
Units are converted, not approximated. Conversions use exact factors, and the result of a calculation is the same whichever unit system you enter it in.
Rounding happens at the end. Intermediate values are kept at full precision; only the displayed figures are rounded, and quantities that are bought in whole units are rounded up.
Averages are averages. Where a default is a national or typical average, it is a starting point, not a claim about your situation.
Drywall conventions. Areas are measured from room dimensions with doors (20 sq ft) and windows (15 sq ft) taken out at their rough openings, both editable. Walls and ceilings are counted and rounded up separately. Sheets are laid out the way a hanger would: fewest sheets first, then the least joint to finish with each foot of butt joint counted twice. Fasteners, compound, tape, bead and insulation are rounded up to whole boxes, pails, bags, rolls, pieces and packs.
Sources for default figures
| Figure | Source | Used by |
|---|---|---|
| Fastener spacing, edge distance, screw penetration | Gypsum Association GA-216, Application and Finishing of Gypsum Panel Products | Screw, nail, adhesive and spacing calculators; sheet calculators’ screw counts |
| Board thickness by framing spacing | International Residential Code Table R702.3.5; GA-216 | Thickness selector, ceiling and sheet calculators |
| Levels of finish 0–5 | Gypsum Association GA-214 | Finish level planner and selector, compound, sanding, cost |
| Fire-rated and sound assemblies (WP 3660, 3661, 3810, 3910 and others) | Gypsum Association GA-600, Fire Resistance and Sound Control Design Manual; published lab test summaries, labelled | Soundproofing, fire-rated assembly and fire-rated drywall calculators |
| Garage separation; tile backers and water-resistant board | IRC R302.5, R302.6, R702.3.7, R702.4.2 | Garage, moisture-resistant board, fire-rated drywall |
| Board weights | Manufacturers’ data sheets (USG, National Gypsum, CertainTeed, Georgia-Pacific) | Sheet weight, delivery planner, ceiling, dumpster |
| Ready-mixed compound coverage, 9.4 gal per 1,000 sq ft at Level 4 | USG product data and FAQ | Joint compound and every calculator that counts compound |
| Setting compound coverage and water | USG product listings (Easy Sand, Durabond) | Joint compound, mixing calculator |
| Texture coverage | USG spray texture data; Homax popcorn texture | Texture calculator |
| Drying limits by temperature and humidity | USG technical bulletin J850 and FAQ; GA-236 | Drying time, schedule, time calculator |
| Damping compound, clips, channel | Green Glue and isolation clip makers’ installation guides | Green Glue and clip calculators |
| Insulation coverage | Rockwool Safe’n’Sound product data | Acoustic insulation, partition kit, soundproofing |
| Anchor ratings | Anchor makers’ published ultimate loads, with a stated safety factor | Anchor weight calculator |
| Rough openings | Door and window makers’ installation sheets | Rough opening, wall stud calculators |
| Wages | US Bureau of Labor Statistics, Occupational Outlook Handbook (May 2024 medians) | Cost calculators |
| Installed costs | Fixr, HomeGuide, HomeAdvisor, ThePricer, Inchcalculator, LendEDU, Fieldcamp, JourneymanHQ (2024–2025) | Cost and comparison calculators |
| Vehicle cost | IRS 2025 standard business mileage rate | Delivery and lift planner |
| Asbestos in pre-1980 materials | US EPA guidance (mention only) | Removal, texture, sanding, repairs |
Figures with no published source are the site’s own estimates and are labelled as such on every page that uses them: waste allowances, work rates per sheet, abrasives per 100 sq ft, the Level 5 skim and the pro-rating of other levels, coat and airflow factors for drying, plaster weight, loose debris bulking, labour shares by level, regional multipliers and the ceiling premium.
What the calculators do not model
- Structure. Framing is counted for non-load-bearing partitions only. Bearing walls, headers under load, spans and connections come from the span tables and an engineer.
- Ratings for untested combinations. Sound and fire figures are given only for published tested assemblies. Built walls usually test 3 to 5 STC points below the lab, and no calculator can turn a mix of parts into a fire rating.
- Your exact products. Board weights, coverage, set times and ratings differ by brand; the label or data sheet of what you buy governs.
- Local codes and prices. Code amendments, inspection rules, disposal rules and local prices vary. Cost ranges are national, adjusted only by a broad region.
- Hazardous materials. Asbestos and lead are mentioned so they can be tested for; the site gives no procedures for them.
Testing
Every calculator carries a set of test cases with hand-worked answers in both unit systems. The tests run automatically before each update to the site, and a calculator that fails any of them is not published. When an error is reported and confirmed, a test case for it is added so that it cannot come back.