Quick answer
Calculate surface area before paint volume
For a rectangular room, wall area is the perimeter multiplied by wall height. Add the ceiling only when it is included, subtract measured doors and windows, multiply by coats, then divide by the product's coverage rate. Apply any extra allowance before rounding the final container count upward.
Calculate my room nowThe formula
Room paint calculation step by step
2 × (length + width) × wall heightPaintable areawalls + included ceiling − measured openingsCoated areapaintable area × number of coatsPaint volume(coated area ÷ product coverage) × (1 + extra ÷ 100)Keep every area in the same unit as the coverage rate. Use square feet with ft²/gal or square meters with m²/L; do not mix systems inside one division.
Product input
Coverage is not a universal constant
Sherwin-Williams reports typical gallon coverage of about 350 to 400 square feet and notes that texture and desired coverage can change the result. Its estimating instructions also direct users to measure wall area, subtract large openings, and check the selected paint label.
Sources: Sherwin-Williams Paint Calculator FAQs and How Much Paint to Buy.
Worked example
A 12 ft × 10 ft room with an 8 ft ceiling
Include the ceiling, subtract one 21 ft² door and two windows totaling 30 ft², use two coats, 400 ft²/gal coverage, and 10% extra.
- Walls: 2 × (12 + 10) × 8 = 352 ft².
- Ceiling: 12 × 10 = 120 ft².
- Paintable area: 352 + 120 − 51 = 421 ft².
- Two coats: 421 × 2 = 842 coat-ft².
- Base paint: 842 ÷ 400 = 2.105 gal.
- With 10% extra: 2.105 × 1.10 = 2.3155 gal.
If the coating is sold only in one-gallon containers, the final purchase result is three complete cans. A different package mix may be more economical, so compare available sizes and prices.
Purchase quantity
Round once, at the final container step
Do not round wall area, coated area, or paint volume between steps. Divide the final volume by the selected container size, then round complete containers upward. For separate wall and ceiling products, calculate each product independently rather than combining unlike coverage rates.
A calculator estimates quantity, not product suitability. Check primer needs, surface preparation, color change, sheen, dry time, ventilation, and application instructions for the exact coating.
Before purchase
Verify what room dimensions cannot predict
- Re-measure length, width, height, and every excluded opening.
- Confirm whether walls and ceiling use the same product and coat count.
- Replace the example coverage with the current label value.
- Choose extra allowance for texture, application loss, and touch-ups.
- Follow the manufacturer's preparation, safety, ventilation, and application instructions.
Common questions
Room paint FAQ
How many square feet does a gallon of paint cover?
Sherwin-Williams says one gallon typically covers about 350 to 400 square feet. The exact coating, surface texture, porosity, color change, and application can alter coverage, so use the current product label for the final estimate.
Do I multiply the room area by the number of coats?
Yes. First find the paintable surface after subtracting measured openings, then multiply that area by the planned number of coats before dividing by product coverage.
Should I subtract doors and windows?
Subtract only openings that will not be painted, using their measured combined area. Do not rely on a generic door or window size when an accurate measurement is available.
Should the ceiling be included?
Include length multiplied by width only when the ceiling is part of the same coating estimate. Use a separate calculation if its product, color, coverage, or coat count differs from the walls.
Open the BuildMeasure Paint Calculator and replace the default coverage with your product value.