Paint Calculator

How to use this paint calculator

What this calculator does

This calculator estimates the paint needed for interior walls. It first finds gross wall area, subtracts doors and windows that will not be painted, multiplies the remaining surface by the number of coats, adds the selected waste allowance, and divides by the paint's stated coverage rate. It reports both the exact mathematical requirement and a whole-gallon purchase quantity. The result is a planning estimate, not a guarantee of jobsite consumption; porous drywall, heavy texture, sprayer loss, major color changes, and product-specific application instructions can all change real use.

When to use it

Use it when budgeting a bedroom repaint, comparing one-coat and two-coat plans, checking whether leftover paint is likely to be enough, or estimating materials before requesting contractor quotes. For homes built before 1978, review the EPA's Renovation, Repair and Painting Program guidance before disturbing old painted surfaces.

How to calculate

1. The calculator opens with a complete 12 ft by 10 ft room example, 8 ft walls, one door, two windows, two coats, 350 sq ft/gal coverage, a 10% allowance, and $42 per gallon. Its XLSX workbook is ready immediately.
2. Choose Area method. Keep Room dimensions for a rectangular room, or choose Direct wall area when you already measured all wall surfaces.
3. Replace the sample dimensions and opening counts. Enter decimal points in U.S. format, such as 6.5; commas are accepted only as thousands separators, not decimal separators.
4. Set Number of coats, Paint coverage, Waste allowance, and optional Price per gallon. Read the live results and breakdown table.
5. Select Download Excel to save the current assumptions and results. Reset clears the demonstration values and disables export until a complete valid state is entered again.

Input guide

Area method is required and determines whether wall area comes from geometry or a direct measurement. In room-dimensions mode, Room length, Room width, and Wall height are required positive measurements in feet; 12, 10, and 8 are realistic examples. Longer, wider, or taller rooms increase gross area. Do not enter floor area in place of wall area. In direct mode, Gross wall area is required in square feet; use the sum of all wall rectangles before deductions.

Doors and Windows are required nonnegative whole-number counts. Their corresponding Door width, Door height, Window width, and Window height are positive feet measurements when the count is above zero. The sample uses one 3 ft × 6.67 ft door and two 3 ft × 4 ft windows. Larger or more numerous openings reduce paintable area. Enter zero for an opening count when none should be deducted; do not subtract trim unless it truly will remain unpainted.

Number of coats is a required whole number from 1 to 10; the sample uses 2. More coats increase paint proportionally. Paint coverage is required in square feet per gallon; 350 is a common planning example, but the product label is the authoritative value. Waste allowance is optional from 0% to 50%; 10% is useful for normal touch-ups and application loss. Price per gallon is optional in U.S. dollars and must be nonnegative; $42 produces the sample cost. A blank price keeps the material quantity valid but displays cost as not set.

Output guide

Paint to purchase is the exact gallon requirement rounded up to a whole gallon. Exact paint required is the unrounded estimate. Estimated paint cost multiplies whole gallons by price per gallon. Gross wall area is the starting surface before deductions; Openings area combines door and window deductions; Paintable wall area is the net surface for one coat; and Coated area with waste is the final coverage demand after coats and allowance. The summary pills repeat paintable area, coats, and coverage so assumption changes are easy to spot. The breakdown table shows each intermediate line and whether it adds to or subtracts from demand.

Worked example

For the startup room, gross wall area is 2 × (12 + 10) × 8 = 352.00 sq ft. The door removes 1 × 3 × 6.67 = 20.01 sq ft, and the windows remove 2 × 3 × 4 = 24.00 sq ft. Net paintable area is therefore 307.99 sq ft. Two coats require 615.98 sq ft of coverage; adding 10% gives 677.58 sq ft. Dividing by 350 sq ft/gal gives 1.94 gal, so the purchase estimate is 2 gal. At $42 per gallon, estimated paint cost is $84.00.