Walls & Finishes calculator

Paint CalculatorGallons for a Room

Calculate gallons of interior paint from room size, wall height, doors, windows, coats, coverage and waste. Get a practical rounded purchase estimate.

Updated August 20, 2026

✓Instant estimateNo sign-up required

✓Clear formulaSee how the result is built

✓Practical unitsBoxes, bags, sheets or volume

✓Editable wasteAdjust for your project

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Project details

Enter your measurements and material details.

Planning estimate only. Actual quantities vary with cuts, layout, site conditions and product specifications.

💡Tip:

Fresh drywall, masonry and heavily textured walls can absorb more paint than smooth previously painted drywall.

How the Paint Calculator works

This calculator estimates wall area first, subtracts a practical allowance for doors and windows, then applies the number of coats and the coverage rate you enter. It is intended for ordinary rectangular rooms and gives both the calculated gallons and a rounded purchase quantity.

Formula

Paintable wall area = 2 × (length + width) × wall height − doors − windows. Gallons = wall area × coats ÷ coverage × waste factor.

Measure before you buy

Measure the room length and width along the floor, then measure wall height from finished floor to ceiling. If the room has unusually large doors, sliders or windows, use the result as a starting point and reduce the paintable area manually. Paint coverage varies by product, surface texture, color change and application method, so the coverage figure on the can should take priority.

Fresh drywall, masonry and heavily textured walls can absorb more paint than smooth previously painted drywall.
A major color change may need primer or an additional finish coat.
Keep a small amount of the same batch for future touch-ups when color consistency matters.

Common questions

How much does one gallon of paint cover?

Many interior paints list about 350–400 square feet per gallon for one coat, but the manufacturer label is the better number to enter.

Should I subtract doors and windows?

Yes for large openings, although leaving a small allowance can help cover trim mistakes, touch-ups and uneven surfaces.

How many coats should I calculate?

Two finish coats are common for uniform color, while some repaint jobs can cover in one and dramatic color changes may need more.

Does the calculator include ceiling paint?

No. The main calculation is for walls so ceiling paint can be planned separately.

Should I round gallons up?

Usually yes. Paint is difficult to match perfectly later, and a little extra is useful for touch-ups.

Worked example: Paint Calculator

Using the example inputs shown in the calculator — Room length (ft) = 14, Room width (ft) = 12, Wall height (ft) = 8, Doors (count) = 1, Windows (count) = 2 — the result is 2.19 gallons. This demonstrates the formula with a complete set of numbers rather than a hidden assumption.

The result breakdown also shows Paintable wall area: 365 sq ft; Recommended purchase: 3 gal. Replace the example values with field measurements and the exact product coverage, yield or package size you plan to buy.

For this paint calculator, the inputs with the greatest practical effect are room length, room width and wall height. Recalculate when any of those change; do not reuse a result from a different room, run, layer depth or product specification.