This example shows arithmetic, not a rule for every room or paint. Four walls are each 12 ft wide and 8 ft high. One measured door is 3 × 7 ft and two windows are each 3 × 4 ft. The user chooses two coats and reads 350 ft²/US gal from the exact finish-paint label. A container holds 1 US gal and costs $42. Ceiling, trim, painted doors, primer, texture, absorption, waste, labour, tax, and delivery are excluded. Changing any source value requires a new result.
From 384 ft² gross to 1.94 US gal
Each wall is 12 × 8 = 96 ft²; four total 384 ft². The door is 21 ft² and two 12 ft² windows total 24 ft². Openings total 45 ft², leaving 384 − 45 = 339 ft² net. These are measured rectangles, not standard deductions. Two coats make 339 × 2 = 678 coated ft². Dividing by the supplied 350 ft²/US gal gives 678 ÷ 350 = 1.937142… US gal, displayed as about 1.94. PaintCalc adds no other coat, waste, or coverage adjustment.
Round packaging to two cans and $84
Dividing 1.937142… US gal by the entered 1 US gal container yields 1.937142… containers, so purchase quantity rounds upward to 2 whole cans. They provide 2 US gal. The difference from about 1.94 is package rounding, not a secret reserve. At $42 per can, optional cost is 2 × $42 = $84. PaintCalc does not include tax, delivery, discounts, stock, tools, primer, labour, or returns. Before ordering, verify coverage, net contents, colour, finish, batch guidance, price, and final quantity with the supplier or painter.