Paint Calculator
Estimate wall paint after subtracting standard door and window areas and applying your selected number of coats.
What this calculator does
Paint quantity equals net paintable wall area × number of coats ÷ product coverage. Enter the coverage printed on the exact paint because formulation, surface, and application affect spread rate.
Paint coverage
Each door subtracts 21 ft² and each window subtracts 15 ft². Replace these standard allowances with a custom opening-area calculator when exact opening sizes matter.
Paint = (wall perimeter × height − standard opening areas) × coats ÷ container coverage rate.
The arithmetic uses the exact values shown above. Verify field measurements, product pack sizes, layout constraints, and local requirements before ordering or building.
How to interpret the result
The result is liquid volume, not a guarantee of coverage. Porous surfaces, texture, color changes, primer, application method, and touch-up needs can increase actual use.
How the result is calculated
Paint = ((room perimeter × height) − openings) × coats ÷ coverage.
Worked example
A 14 × 12 × 8 ft room with one door, two windows, two coats, and 350 ft²/gal coverage needs roughly 2.1 gallons.
How to use this calculator
- Measure room length, width, and wall height.
- Count standard doors and windows or calculate openings separately.
- Choose the planned number of finish coats.
- Enter the exact manufacturer coverage rate.
- Round to available container sizes and estimate primer separately.
Supported units
Feet and square feet per gallon in imperial mode; meters and square meters per liter in metric mode. Results show both gallons and liters.
Practical guidance
- Use the coverage rate on the exact product.
- Plan extra for porous or heavily textured surfaces.
- Round purchasing quantities to available can sizes.
Assumptions and limitations
Standard opening deductions are used. Surface texture, color change, primer, application method, and paint quality can affect actual coverage.
Methodology
Gross wall area = 2 × (length + width) × height. Standard opening areas are deducted; net area is multiplied by coats and divided by entered coverage.
Read the complete calculation methodology and testing policy.
Sources and technical references
References support terminology, units, or practical constraints. They do not imply endorsement or project approval.
- NIST Special Publication 811 — Guide for the Use of the International System of UnitsReference for SI units, symbols, and conversion-factor practice.
- American Coatings Association — coverage-rate researchIndustry research showing that applied amount and formulation affect achieved coverage; product directions should control.
Frequently asked questions
Does this include the ceiling?
No. This version estimates room walls only.
Should primer be counted as a paint coat?
Estimate primer separately because its coverage can differ.
Why subtract doors and windows?
They reduce the wall area that normally receives paint.