PAINT

Interior paint planning: calculate wall area before gallons

Floor area is not paintable wall area. Measure the surfaces being painted, subtract large openings when useful, multiply by coats, then divide by the selected paint's stated coverage.

Worked 12×15 room example

With 8-ft walls, the room perimeter is 2×(12+15) = 54 ft. Gross wall area is 54 × 8 = 432 ft² before subtracting doors and windows.

54 ft perimeter × 8 ft = 432 ft²

If the measured doors/windows total 55 ft², net wall area is 377 ft². Two coats create 754 ft² of coat-area. Primer is a separate coverage calculation if required.

Coverage is an input, not a promise

Sherwin-Williams notes typical paint coverage around 350–400 ft² per gallon, while actual coverage varies with surface, application and product. Using 375 ft²/gal only as a planning example, 754 ÷ 375 = 2.01 gallons, so the purchase decision must account for whole-container sizes and touch-up needs.

Do not forget scope differences

Surface	Track separately
Walls	net area × coats
Ceiling	length × width × coats
Trim/doors	linear or unit-based estimate
Primer	product coverage and number of coats

6. Floor area is not paintable wall area

A “2,000 ft² house” describes floor area, not the square footage of walls and ceilings. For a room, start with wall perimeter × wall height, then handle doors, windows, ceilings and accent walls deliberately. This is more checkable than applying a multiplier to floor area.

12×15 room with 8 ft walls: perimeter = 54 ft. Gross wall area = 54 × 8 = 432 ft². A 12×15 ceiling adds 180 ft² if it is being painted. Openings may be deducted, but small openings are often offset by cutting-in loss and leftover paint; decide consistently rather than subtracting everything automatically.

7. Coverage is an input, not a universal constant

Sherwin-Williams notes typical gallon coverage around 350–400 ft², but actual coverage varies with surface texture, porosity, color change, application method and product. Use the label/data sheet for the selected paint. Primer and finish coats should be estimated separately when their coverage differs.

8. Common mistakes that change the purchase

Using floor square footage as wall square footage.
Forgetting a second coat or treating “paint and primer” marketing as proof one coat will cover every color change.
Ignoring textured or porous surfaces that consume more material.
Rounding every room upward separately when the same paint can be shared between rooms.
Combining walls, ceilings and trim even though they use different products.

9. A better way to round cans

Calculate the total gallons for all surfaces using the same product and sheen, then convert the total to available container sizes. This can reduce unnecessary leftovers compared with rounding each wall or room independently. Keep a small labeled remainder when future touch-up is likely.

10. Verify before buying

Measure wall perimeter and height for each room.
Separate ceilings, trim and differently colored walls.
Choose the actual product and read its stated coverage.
Set the number of coats based on the surface and color transition.
Compare your manual area total with the calculator result before rounding to containers.

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Primary reference

Sherwin-Williams Paint Calculator — manufacturer planning tool and coverage guidance.