Estimate paint and primer for up to three rooms, or enter a measured gross wall area for an irregular space. Include ceilings if needed, subtract doors and windows, set separate coverages and coat counts, and round purchases to the container sizes and prices you enter.
Worked example
A 12 ft × 10 ft room with 8 ft walls has 352 ft² of wall area. Subtract one 20 ft² door and two 15 ft² windows to get 302 ft². Two paint coats create 604 ft² of coverage demand. At 400 ft² per gallon, mathematical paint need is 1.51 gallons, which rounds to 2 one-gallon containers. One primer coat at 300 ft² per gallon needs 1.01 gallons, which rounds to 2 one-gallon primer containers.
Coverage and purchasing
Use the actual product label or technical data sheet. Sherwin-Williams states that a gallon of paint commonly covers about 350–400 ft² and primer about 200–300 ft², while also warning that application method and surface condition change requirements. Its paint calculator guidance is the source for those example ranges. The defaults are editable assumptions, not guarantees for a particular coating.
Texture, porosity, repairs, dramatic color changes, application loss, and an uneven substrate can increase use. Common mistakes include using floor area as wall area, forgetting coats, subtracting openings twice, combining primer and paint coverage, or rounding gallons before multiplying by coats.
Examples
Manual wall area
If careful measurement gives 750 ft² after the entered openings are subtracted and the project needs two coats, the net coverage demand is 1,500 ft². At 375 ft²/gal, required paint is 4 gal.
Formula
For each rectangular room:
wall area = 2 × (length + width) × wall height
paintable area = walls + selected ceilings − doors − windows
paint volume = paintable area × paint coats ÷ paint coverage
primer volume = paintable area × primer coats ÷ primer coverage
containers = ceil(required volume ÷ entered container size)
US and metric room dimensions convert internally. Door and window area inputs are square feet so their unit is explicit even when room dimensions use metres.
Limits of the estimate
Coverage is a product and surface property. Sherwin-Williams notes that its 400 ft² example is theoretical because texture and paint left on applicators are outside the arithmetic. The calculator also does not decide whether a surface needs primer, special preparation, or additional coats. Confirm the chosen product data sheet, substrate condition, application method, and color change before purchase.
Separate surfaces and mixed container purchases
Room/manual-area mode remains available for a single finish using one container size. Coverage can be entered as ft² per US gallon or m² per liter for paint and primer. Changing the coverage selector changes the interpretation; enter the matching number from the label. Container units are independent.
Choose Separate surfaces / products for different colours, wall/ceiling
finishes or primer. Each line is surface, product, colour, net area, coats, coverage.
For example, with m² and m²/L selected: Bedroom walls, Wall paint, Blue, 30, 2, 10
requires 6 L. Subtract openings before entering net area; room-mode openings and
primer controls do not apply to this list. Add primer as its own product row.
Do not use commas inside names.
Enter available packages as product, size, price, using the selected container
unit. Product names must match exactly. For 6 L, options of 1 L at $15 and 5 L
at $55 produce a $70 purchase of one of each. Different products and colours
are purchased separately; matching rows consolidate before rounding.
Lowest cost minimizes entered price, breaking ties by leftover volume. Least waste minimizes leftover volume, breaking ties by price. Sizes accept up to three decimals and prices two decimals. The search supports up to ten sizes per product and rejects oversized searches with a clear message. Prices exclude unentered tax, labor and fees. Tables are included in saved/shared calculations and printable/exported results.