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Calculation methodology

All calculations use metres, square metres and litres internally. The interface converts to feet, inches, US liquid gallons, metres or centimetres without intentionally rounding intermediate results. Displayed input values have six decimal places; conversions can therefore introduce negligible input-display rounding.

Area

Room walls = 2 × (length + width) × height. Individual wall = length × height. Ceiling = length × width. Net area = gross area − door area − window area − other exclusions. An opening total greater than gross area produces an error instead of a silently clamped result.

Paint and primer

Each paint group inherits global coverage, coats and unit price unless a field is overridden in Advanced: paint groups. Blank overrides follow future changes to defaults. Group names are matched after trimming and ignoring case; changing a name selects that group’s settings. Paint = group net area × its finish coats ÷ its coverage. Extra allowance multiplies that quantity by (1 + percentage/100). Primer uses its own coverage and coat count. A zero primer count disables primer. Primer applies to all included area and assumes one compatible product.

Purchasing

Paint is pooled only across entries with matching paint-group names (case-insensitive). Each group is rounded up independently. Available sizes are configurable. Starting enabled sizes remain 1/5 US gallons and 2.5/10 litres; optional sizes can be enabled, disabled, added or removed. Minimum quantity minimizes excess volume, then container count. Lowest entered cost requires a price for every enabled size for that product; it minimizes entered cost, then volume, then container count. No retailer savings are inferred. Sizes support three decimals and container prices two decimals. An exact integer-size search is capped at 200,000 states; larger combinations ask you to split the project or use larger sizes.

Defaults and limits

Example room: 12 × 10 × 8 ft, one 3 × 7 ft door, two 3 × 4 ft windows. Paint coverage: 350 sq ft/US gal; primer: 250 sq ft/US gal; two paint coats; zero primer coats; 10% extra. These are editable assumptions. Surface dimensions are limited to 1,000 m, custom area to 1,000,000 m², coats to six and extra allowance to 50%. Projects allow 50 surfaces.

Cost

DIY materials = selected paint purchase costs + selected primer purchase cost + supplies budget. An entered container price replaces the unit-price estimate for that size. Missing container prices use the product’s unit price only for budgeting and cannot support a lowest-entered-cost claim. Professional scenarios add painted area × entered labor-only rate. No tax, delivery, repair labor, equipment rental or price research is implicit. Changing currency changes its label, not an exchange rate.

Limitations

Flat rectangular surfaces only unless manually measured. This is not a specification for coating safety, substrate preparation, access or product compatibility. Doors and trim can be entered as measured custom areas in a separate paint group. Irregular exterior features require separate measured areas.