Measure the walls and compare the same job
Measure the inside length and width of a rectangular room and the height of the walls. The calculator uses 2 × (length + width) × height for the four walls. Add the areas of doors and windows you will leave unpainted and enter that sum as openings. A room with sloping walls, alcoves or different wall heights needs separate measurements. This form does not model those shapes, a ceiling or the floor.
Enter the number of coats you plan to apply to both products. The comparison only answers the same-job question: which entered product quote costs less for this area and this number of coats? It cannot determine whether two products will give the same finish. Check each exact product's label and technical data sheet for the surface you will paint. Coverage is entered per coat in square feet per US gallon. A UK imperial gallon is a different unit and must not be entered as a US gallon.
How whole cans are calculated
Net wall area = 2 × (length + width) × height − openings. Required paint = net wall area × coats × (1 + extra allowance ÷ 100) ÷ coverage. Cans to buy = required paint ÷ can size, rounded upward to a whole can. Purchase cost = cans × the entered price of one can. The can count uses the full input precision, not the rounded gallon estimate shown in the table. Decimal gallon values in the result are estimates rounded for display.
For example, a room 12 ft long, 10 ft wide and 8 ft high has 352 ft² of wall area. With 32 ft² of openings, that leaves 320 ft². Two coats with no extra allowance require 640 ft² of coverage. A product covering 350 ft² per US gallon needs about 1.829 gallons, so two 1-gallon cans at $35 each cost $70. A product covering 400 ft² per US gallon needs 1.6 gallons, so two 1-gallon cans at $45 cost $90. These are arithmetic examples, not current prices or product recommendations.
Use the actual product and purchase unit
A label may give a coverage range. Enter the figure appropriate to your surface and application, then compare another input if you want to see how the purchase changes. Absorption, texture and application can change actual use. The extra allowance is your own planning assumption, and leaving it blank means zero. It does not replace the manufacturer's instructions. Leftover paint is the purchased amount minus the entered estimate, including any extra allowance.
This version compares one can size for A with one can size for B. It does not search combinations of quart, gallon and larger containers. Use a separate comparison for another size and enter its real price. A zero price is allowed for supplies you already have or a free product, but a blank price is incomplete. If openings equal the full wall area, the net paint area and required cans are zero. Openings larger than the walls are rejected.
Dimensions accept up to four decimal places and 10,000 ft per side. Coverage accepts up to four decimals and 10,000 ft²/US gal; can size accepts up to four decimals and 100 US gal. Price accepts cents up to $1,000,000 per can. Coats must be whole numbers from 1 to 10. Extra allowance accepts up to four decimals from 0% to 100%. Very large purchases beyond the calculator's safe range are rejected. Taxes, shipping, primer, tools, labor and future price changes are excluded.
Frequently asked questions
Where do I get the coverage rate?
Use the coverage for the exact product and surface from its current label or technical data sheet. Enter square feet per US gallon for one coat. The calculator does not supply a coverage rate or guarantee the amount used.
Are ceilings and primer included?
No. The area is the four walls of one rectangular room, minus the openings you enter. Ceilings, floors, trim and primer are separate jobs. Each comparison assumes the same number of coats for both products.
Does this find the cheapest mix of can sizes?
No. Each product uses one can size, rounded up to a whole number of cans. Compare separate quotes for different sizes. Taxes, shipping, tools and labor are excluded.