Paint Coverage Calculator: Gallons, Liters & Wall Area
Calculate exact paint needed for any room. Deducts doors and windows, factors in 1 or 2 coats, spreading rate (sq ft/gal or m²/L), and estimates can purchase cost.
1-Click Room Archetype Presets
Instantly populate standard room sizes, doors, windows, and spreading rates:
Room Dimensions & Painting Specifications
Professional Architectural Painting Formulas
Wall Area & Opening Deductions:
Gross Wall Area = 2 × (Length + Width) × Height
Net Paintable Area = Gross Wall Area - (Doors × 21 sq ft) - (Windows × 15 sq ft)
Gallons Required with Multi-Coat Multiplier:
Gallons = (Net Paintable Area × Number of Coats) / Spreading Rate (sq ft/gal)
Frequently Asked Questions: Paint Coverage & Estimating
How do you calculate the wall surface area to paint a room?
To find total gross wall area, multiply room perimeter by ceiling height: $\text{Perimeter} = 2 \times (\text{Length} + \text{Width})$, then $\text{Gross Area} = \text{Perimeter} \times \text{Height}$. Then deduct non-painted openings: subtract standard doors (roughly $20\text{–}21\text{ sq ft}$ or $1.9\text{ m}^2$ each) and standard windows (roughly $12\text{–}15\text{ sq ft}$ or $1.1\text{–}1.4\text{ m}^2$ each). The result is net paintable surface area.
How many square feet does one gallon of paint cover?
As an industry rule of thumb, one standard US gallon ($3.785\text{ liters}$) of interior architectural latex or acrylic paint covers between $350$ and $400\text{ sq ft}$ ($32.5\text{–}37.2\text{ m}^2$) on smooth, pre-primed drywall. On textured surfaces (knockdown, orange peel, rough masonry), coverage drops to $250\text{–}300\text{ sq ft/gal}$ due to surface absorption and increased microscopic surface area.
Why are two coats of paint almost always recommended?
A single coat rarely achieves full optical opacity, uniform color saturation, or consistent sheen. Two coats ensure complete chemical film coalescing, conceal substrate patches, and deliver the manufacturer's rated scrub resistance and durability. For drastic color shifts (e.g. going from deep navy to soft ivory), a tinted primer plus two topcoats is standard practice.
How do you convert between gallons (sq ft) and liters (sq meters)?
In metric regions, paint spreading rates are stated in square meters per liter ($m^2/L$), typically $10\text{–}12\text{ m}^2/L$. Because $1\text{ gallon} \approx 3.785\text{ liters}$ and $1\text{ m}^2 \approx 10.764\text{ sq ft}$, $375\text{ sq ft/gal}$ is mathematically identical to $\frac{375}{10.764 \times 3.785} \approx 9.2\text{ m}^2/L$.
How many whole cans of paint should I buy?
Always round fractional gallons up to the next nearest whole unit to account for brush/roller absorption, wall irregularities, cutting-in overlap, and future touch-ups. If your project requires $2.3$ gallons, purchase two 1-gallon cans plus one 1-quart can, or three 1-gallon cans. For projects requiring $4.2+$ gallons, purchasing a single 5-gallon contractor bucket is typically $15\text{–}25\%$ cheaper per gallon than individual cans.
Does unpainted or bare drywall require extra paint?
Yes. Bare porous drywall, new plaster, or raw wood readily absorbs paint binders and solvents, resulting in an uneven, blotchy finish (known as 'flashing'). Applying a dedicated PVA (polyvinyl acetate) drywall primer seals the paper facing and joint compound, allowing the subsequent colored finish coats to achieve maximum spreading efficiency ($400\text{ sq ft/gal}$).
Should I include the ceiling in this wall paint calculation?
This calculator focuses on vertical walls. If you are also painting the ceiling, calculate the ceiling separately: $\text{Ceiling Area} = \text{Length} \times \text{Width}$. Because ceilings are generally painted with flat, non-reflective white paint formulated to minimize roller spatter, ceiling paint is purchased separately from wall finish enamel.
How much buffer or overage should I include in my paint order?
Professional painting contractors routinely add a $10\%$ waste buffer to the calculated total. This compensates for roller nap retention ($10\text{–}15\%$ retained in a 3/8" sleeve), paint tray residue, cut-in feathering, and ensures you retain a matching half-quart for scuff repairs over subsequent years.
Authoritative Paint Industry & Contractor References
Master Painters Institute (MPI) - Architectural Painting Standards
Authoritative industry benchmark detailing spreading rates, dry film thickness (DFT), and surface preparation guidelines.
Sherwin-Williams - Measuring and Estimating Paint Coverage
Manufacturer technical reference specifying square-foot coverage per gallon across flat, satin, semi-gloss, and primer products.
Benjamin Moore - Paint Coverage and Calculation Guide
Comprehensive guidelines on door/window deductions, multi-coat application factors, and roller nap absorption.
Related Calculators
Mortgage Payment Calculator
Home and HouseholdHome Equity Loan Calculator
Home and HouseholdRent vs. Buy Calculator
Home and HouseholdHome Improvement Budget Calculator
Home and HouseholdFurniture Arrangement Calculator
Home and HouseholdSquare Footage Calculator
Home and HouseholdRoom Paint Coverage Calculator
Home and HouseholdHome Energy Savings Calculator
Home and HouseholdLaundry Load Calculator
Home and HouseholdCleaning Time Estimator
Home and HouseholdQuick Tips
- 1Track results over time
- 2Consider personal factors
- 3Set realistic goals
- 4Share progress with others