Paint Coverage Calculator
This calculator estimates how much paint you need by computing paintable surface area, accounting for doors and windows, the number of coats, manufacturer coverage per gallon, waste allowance, and chosen can sizes. Use the defaults as a starting point and adjust coverage and waste based on surface texture and paint type.
Results include net paintable area, gallons required (before rounding), recommended number of cans based on your selected can size, total gallons to purchase, and an estimated cost. The methodology follows standard surface-area calculations used by building science and extension services.
Inputs
Results
Net paintable area (sq ft)
301
Total paint area for coats (sq ft)
602
Gallons required (before rounding)
1.9111
Recommended cans (count)
—
Gallons to purchase
—
Estimated paint cost
—
| Output | Value | Unit |
|---|---|---|
| Net paintable area (sq ft) | 301 | sq ft |
| Total paint area for coats (sq ft) | 602 | sq ft |
| Gallons required (before rounding) | 1.9111 | gallons |
| Recommended cans (count) | — | — |
| Gallons to purchase | — | gallons |
| Estimated paint cost | — | USD |
Visualization
Methodology
Measure room dimensions (length, width, height). Wall area = 2 × (length + width) × height. Add ceiling area if painting the ceiling. Subtract the area of doors and windows to get net paintable area.
Multiply net area by number of coats to get total paint area. Divide total paint area by effective coverage per gallon. Effective coverage = manufacturer coverage × (1 − waste allowance). Round up to whole cans when purchasing.
Defaults (coverage and waste) reflect typical professional guidance; actual coverage varies with paint formulation, primer use, surface porosity and texture. For technical unit definitions and measurement standards see NIST and extension resources listed below.
Expert Q&A
How accurate is the coverage estimate?
This tool uses manufacturer coverage and a user-specified waste allowance to estimate paint needed. Coverage varies by porosity, texture, color change, primer use, and application method. For final ordering add a modest buffer (commonly 5–15%).
What coverage per gallon should I use?
Start with the manufacturer's published coverage (sq ft per gallon). Typical values for interior paints range from about 300–400 sq ft per gallon. Use higher values for smooth, primed surfaces and lower for rough or unprimed surfaces.
Should I count primer separately?
If you will apply a dedicated primer, treat it as a separate product: calculate primer gallons using the same area but the primer's coverage rate. Some coatings combine primer and paint; follow manufacturer guidance.
How do I measure odd-shaped rooms or cabinets?
Break complex shapes into rectangles and circles, calculate each area, then sum them. Subtract fixed openings (doors/windows). For cabinetry or furniture, measure surface area of each face and add edges as needed.
How should I dispose of leftover paint?
Follow local hazardous-waste guidance. The U.S. EPA provides recommendations for household hazardous waste and paint disposal—check local government programs for recycling or drop-off options.
What safety precautions should I follow?
Use adequate ventilation, follow product safety data sheets (SDS), and wear appropriate respirators, gloves and eye protection for solvent- or aerosol-based paints. Consult OSHA/NIOSH guidance for workplace safety when using industrial coatings.
Sources & citations
- NIST — SI units & measurement resources — https://www.nist.gov/pml/weights-and-measures/metric-si-units
- U.S. EPA — Household hazardous waste and paint disposal guidance — https://www.epa.gov/hw/household-hazardous-waste
- Cooperative Extension resources — measurement and painting guidance — https://extension.org