Engineering

Paint calculator formula: net wall square footage, coats & whole gallons

Estimate interior/exterior paint gallons by subtracting door and window openings from room perimeter × height, multiplying by coats, and rounding up.

Use measured openings and product coverage

The rectangular room example has 56 feet of perimeter and 9-foot wall height, for 504 square feet. Assumed door and window areas of 21 and 15 square feet subtract 87 square feet for two doors and three windows.

These opening sizes and 350 square feet per gallon are examples, not universal architectural or paint specifications. Use measured opening areas in G2/H2 and the product’s coverage for the surface, application method and number of coats.

The calculation covers walls only and excludes ceiling, trim, primer and an extra waste allowance. A negative net area indicates inconsistent inputs and is flagged rather than hidden by clamping to zero.

Paint Take-Off Step (56' Perim × 9' High)	Formula	Result
Gross Wall Area	=A2 * B2	504 sq ft
Opening Deductions (2 Doors, 3 Windows)	=C2 * 21 + D2 * 15	87 sq ft (42 + 45)
Net Paintable Area × 2 Coats	=MAX(0, 504 - 87) * E2	834 coated sq ft
Whole Gallons @ 350 sq ft/gal	=ROUNDUP(834 / F2, 0)	3 gallons (exact 2.38 gal)

Whole paint gallons required (with opening deductions and coat multiplier)

Rounds modeled paint volume up to whole gallons. Check package sizes and product coverage; actual absorption and application losses can differ.

=IF(OR(COUNT(A2:H2)<>8,MIN(A2:H2)<0,F2<=0,MOD(C2,1)<>0,MOD(D2,1)<>0,MOD(E2,1)<>0,A2*B2<C2*G2+D2*H2),"Invalid paint inputs",ROUNDUP(((A2 * B2 - C2 * G2 - D2 * H2) * E2) / F2, 0))

Single-coat net paintable wall square footage

Useful for pricing labor per square foot independently of paint coverage.

=IF(OR(COUNT(A2:H2)<>8,MIN(A2:H2)<0,F2<=0,MOD(C2,1)<>0,MOD(D2,1)<>0,MOD(E2,1)<>0,A2*B2<C2*G2+D2*H2),"Invalid paint inputs",(A2 * B2 - C2 * G2 - D2 * H2))

Try the example

Use the original values or change them to test your understanding.