Paint Calculator
Free Paint Calculator. Free online tool with accurate results using verified formulas. Includes worked examples, FAQ, and instant calculations.
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Formula
Calculate the room perimeter and multiply by ceiling height for gross wall area. Subtract door openings (21 sq ft each) and window openings (15 sq ft each) for net paintable area. Multiply by the number of coats, then divide by 350 sq ft per gallon (standard interior paint coverage). Round up to the nearest whole gallon.
Last reviewed: December 2025
Worked Examples
Example 1: Standard Bedroom
Example 2: Large Living Room
Background & Theory
The Paint Calculator — Gallons, Coverage & Cost Estimator applies the following established principles and formulas. Structural and construction engineering is governed by fundamental load analysis, material science, and regulatory standards that ensure the safety and durability of built structures. The primary distinction in load analysis is between dead loads — the permanent self-weight of structural elements, finishes, and fixed equipment — and live loads, which represent variable occupancy, furniture, and environmental forces such as wind and snow. These are combined using factored load equations, such as the ASCE 7 formula U = 1.2D + 1.6L, where D is dead load and L is live load. Concrete mix design is governed by the water-cement (w/c) ratio, which is the primary determinant of compressive strength and durability. A w/c ratio of 0.40–0.45 typically yields concrete with 28-day compressive strengths of 30–40 MPa. Common mix ratios by weight for structural concrete are approximately 1 part cement : 1.5–2 parts sand : 3 parts coarse aggregate. Structural steel is characterized by its yield strength (the stress at which permanent deformation begins, typically 250–350 MPa for mild steel) and ultimate tensile strength (typically 400–500 MPa). Mid-span deflection of a simply supported beam under a central point load is given by δ = FL³ / (48EI), where F is force, L is span length, E is Young's modulus, and I is the second moment of area. Building insulation is rated by R-value, a measure of thermal resistance in units of m²·K/W (SI) or ft²·°F·h/BTU (imperial). Higher R-values indicate greater resistance to heat flow. Foundation design depends on the allowable bearing capacity of the underlying soil, which ranges from approximately 75 kPa for soft clay to over 10,000 kPa for bedrock. Drainage gradients for surface water are typically specified as a minimum of 1–2% slope away from building foundations to prevent hydrostatic pressure and water infiltration.
History
The history behind the Paint Calculator — Gallons, Coverage & Cost Estimator traces back through the following developments. The history of construction engineering spans thousands of years of accumulated empirical knowledge and, more recently, rigorous scientific analysis. The ancient Egyptians built the Great Pyramid of Giza around 2560 BCE using an estimated 2.3 million stone blocks, demonstrating sophisticated logistics, geometry, and workforce organization. Roman engineers advanced the field dramatically through the use of pozzolanic concrete — a mixture of volcanic ash, lime, and seawater — enabling the construction of the Pantheon dome (43.3 m diameter, completed around 125 CE) and a vast network of aqueducts and roads across the empire. Cast iron emerged as a structural material during the Industrial Revolution, first used prominently in the Iron Bridge at Coalbrookdale, England, completed in 1779. Wrought iron and later steel allowed far greater spans and heights. The Eiffel Tower, completed in 1889, demonstrated the structural possibilities of wrought iron at scale and influenced the development of steel-frame skyscraper construction in Chicago and New York. Reinforced concrete was systematically developed by Joseph Monier, a French gardener, who patented iron-reinforced concrete pots and panels in the 1860s, and later by engineers including François Hennebique who created the first comprehensive reinforced concrete framing system in the 1890s. The 1906 San Francisco earthquake caused widespread devastation and galvanized the engineering profession to develop seismic design provisions. Subsequent earthquakes — including the 1971 San Fernando and 1994 Northridge events — drove successive improvements in seismic codes, base isolation technology, and ductile detailing of reinforced concrete and steel frames. Building codes became increasingly standardized in the twentieth century, with the International Building Code (IBC) first published in 2000 providing a unified model code adopted across much of the United States. Building Information Modeling (BIM) emerged in the 2000s as a digital workflow integrating architectural, structural, and MEP design into a unified three-dimensional model, fundamentally changing coordination practices across the industry.
Frequently Asked Questions
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy
Paint Calculator Formula
Gallons = (Net Wall Area × Coats) ÷ 350
Calculate the room perimeter and multiply by ceiling height for gross wall area. Subtract door openings (21 sq ft each) and window openings (15 sq ft each) for net paintable area. Multiply by the number of coats, then divide by 350 sq ft per gallon (standard interior paint coverage). Round up to the nearest whole gallon.
Paint Calculator — Worked Examples
Example 1: Standard Bedroom
Problem: Calculate paint for a 12×10 ft bedroom with 8-ft ceilings, 1 door, 2 windows, 2 coats.
Solution: Perimeter = 2 × (12 + 10) = 44 ft\nGross wall area = 44 × 8 = 352 sq ft\nNet area = 352 - 21 - 30 = 301 sq ft\nTotal with 2 coats = 602 sq ft\nGallons = 602 / 350 = 1.72 → 2 gallons
Result: 2 gallons, 301 sq ft wall area, $80 estimated cost
Example 2: Large Living Room
Problem: Calculate paint for a 20×16 ft room with 9-ft ceilings, 2 doors, 4 windows, 2 coats at $50/gallon.
Solution: Perimeter = 2 × (20 + 16) = 72 ft\nGross wall area = 72 × 9 = 648 sq ft\nNet area = 648 - 42 - 60 = 546 sq ft\nTotal with 2 coats = 1,092 sq ft\nGallons = 1,092 / 350 = 3.12 → 4 gallons\nCost = 4 × $50 = $200
Result: 4 gallons, 546 sq ft wall area, $200 estimated cost
Paint Calculator — Frequently Asked Questions
How much paint do I need for a room?
Calculate the total wall area (perimeter × height), subtract doors (21 sq ft each) and windows (15 sq ft each), then multiply by the number of coats. Divide by the paint coverage rate — typically 350-400 sq ft per gallon. For a 12×15 room with 8-foot ceilings, 1 door, and 2 windows: net wall area = 381 sq ft. With 2 coats, you need 762 sq ft of coverage, or about 2.2 gallons. Always round up to ensure you have enough.
How many coats of paint should I apply?
Two coats is the standard for most interior paint jobs. This ensures complete coverage, uniform color, and proper durability. A single coat may suffice when repainting with the same or very similar color. Three coats may be needed when covering dark colors with light ones, using high-quality flat paints, or painting new drywall (1 primer + 2 finish coats). Using a tinted primer when making dramatic color changes can reduce finish coats needed.
How much area does a gallon of paint cover?
One gallon of interior paint typically covers 350-400 square feet per coat on smooth, primed surfaces. Rough or textured surfaces may only yield 250-300 sq ft per gallon. Exterior paint covers 250-350 sq ft per gallon due to rougher surfaces. Coverage also depends on paint quality — premium paints with higher solids content cover better than economy paints. Primer covers about 300-400 sq ft per gallon. Always check the specific product label for manufacturer coverage estimates.
How much does it cost to paint a room?
DIY paint costs typically run $100-$300 per room depending on size and paint quality. Budget paint costs $20-$30 per gallon, mid-range $30-$50, and premium $50-$80+. A typical bedroom needs 2-3 gallons ($60-$240 for paint). Add $20-$40 for supplies (rollers, tape, drop cloths, brushes). Professional painting costs $300-$800+ per room including labor and materials. For a whole house interior, expect $1,500-$5,000+ DIY or $3,000-$10,000+ professional.
How do I estimate paint coverage for a room?
One gallon covers approximately 350-400 square feet per coat. Calculate wall area by multiplying perimeter by ceiling height, then subtract window and door openings. Most jobs need two coats, so divide total area by 175-200 per gallon. Add 10% for touch-ups and waste.
Can I use Paint Calculator on a mobile device?
Yes. All calculators on NovaCalculator are fully responsive and work on smartphones, tablets, and desktops. The layout adapts automatically to your screen size.