How to use
- Measure the room’s length, width, and wall height.
- Count doors and windows, then adjust the deducted area per opening if your measurements differ.
- Choose the number of coats and enter the product’s coverage per US gallon.
- Include the ceiling only if you want it counted with the same paint and coat assumptions.
How the estimate is calculated
Wall area is 2 × (length + width) × height, less door and window deductions. Negative wall area is clamped to zero and flagged for review. A selected ceiling adds length × width. Paint gallons equal total area × coats ÷ coverage. The package suggestion uses whole gallons and enough quarts to cover the remainder; four quarts become another gallon.
Example: a 12 ft × 10 ft room with 8 ft walls has 352.0 sq ft of wall area. One 20 sq ft door and two 15 sq ft windows leave 302.0 sq ft. Two coats at 350 sq ft per gallon need 1.73 US gallons, rounded for purchase to 1 US gallon + 3 US quarts.
Coverage to check before buying
| Material | Typical coverage per US gallon | Source and use |
|---|---|---|
| Paint | 350–400 sq ft | Sherwin-Williams paint calculator |
| Primer | 200–300 sq ft | Sherwin-Williams primer estimate |
| Door deduction | 20 sq ft per door | Editable planning input, not a product standard |
| Window deduction | 15 sq ft per window | Editable planning input, not a product standard |
Sherwin-Williams gives coverage as a typical range, not a promise for every coating. The calculator starts at 350 sq ft per gallon for paint. If you are estimating primer, use the primer label and run it separately rather than combining primer and finish coats in one total.
Texture, ceilings, and package sizes
A rough surface may not match a smooth-wall coverage figure. Use the product instructions for the substrate and keep a record of the rate you chose. If walls and ceilings use different products, calculate them separately so each has its own coverage and coat count.
The gallon-and-quart result is a volume rounding suggestion. It does not check which package sizes or colors a store stocks, and it does not compare package prices. Retain a product record if you expect future touch-ups.
Common mistakes
- Coverage is per coat. Do not reduce the label coverage for two coats and also enter two coats.
- Do not leave the ceiling checked when you are buying only wall paint.
- Unusually large opening deductions can hide a measurement error; review the warning even if a ceiling still produces a positive total.
Sources
Typical paint and primer coverage: Sherwin-Williams Paint Calculator. Opening sizes and the initial two-coat choice are user-editable assumptions.
Related calculators
- Square Footage Calculator — measure separate surface shapes
- Drywall Calculator — estimate the wallboard underneath
Frequently asked questions
Do I need a separate estimate for primer?
Yes, when the primer has a different coverage or coat count. Enter its own label coverage and calculate it as a separate material.
Why does the result include quarts?
The estimator rounds the fractional gallon upward to quarter-gallon increments. It combines a four-quart remainder into a whole gallon.
Are doors and windows fixed sizes?
No. The initial 20 and 15 square foot deductions are planning defaults. Measure your openings and edit those areas when needed.
Does metric mode use liters?
No. It uses meters and square meters for surfaces while keeping coverage and package quantities per US gallon, as labeled.