Paint Gallons Calculator

What it does

Calculate exactly how many gallons of paint to buy for a room — no half-finished walls, no second trip to the hardware store. Enter room dimensions (length × width × ceiling height), the number of coats (typically 2 for quality results), and the spread rate from the paint can (usually 350-400 square feet per gallon for most interior latex paints), and the tool returns: total square footage to paint, gallons needed, recommended purchase (rounded up to whole gallons or quarts).

The math: total wall area = perimeter × ceiling height for the four walls, then optionally add ceiling area (length × width). Subtract doors and windows if you want tight estimates (a typical interior door is ~21 sq ft, a window ~15 sq ft). Multiply by the number of coats. Divide by spread rate. Round up to whole gallons (you can’t buy partial).

Why two coats: color uniformity and coverage. One coat over an existing painted wall in a similar color sometimes looks fine. One coat over a dramatically different color (white over dark blue, light over red) will show through and look streaky. Primer is a separate decision — needed when going from dark to light, painting new drywall, or covering stains. Allow primer + 2 finish coats for those scenarios; budget gallons accordingly.

How to use it

1. Measure room dimensions: length × width of floor + ceiling height. A 12×14×8 room means 12-foot wall × 14-foot wall × 8-foot ceiling height.
2. Enter dimensions in feet (or switch to meters).
3. Set the spread rate from your paint can — typically 350-400 sq ft per gallon for interior latex; 250-300 for exterior or rough surfaces.
4. Set coats (2 is standard for quality work; 3 for dramatic color changes).
5. Optionally tick 'paint ceiling' if you're painting that too.
6. Optionally enter doors / windows count to subtract their areas (more accurate, prevents over-buying).
7. Read gallons needed. Round up to whole gallons OR plan to buy 1-quart cans for small touch-ups.

When to use this tool

• Planning an interior room paint job (single room or whole house, room by room).
• Estimating paint cost as part of a renovation budget.
• Buying just the right amount — too little = second trip, too much = leftover wasted gallons.
• Comparing whether a paint upgrade (better coverage = lower spread rate quote) is worth the higher per-gallon cost.

When not to use it

• Exterior painting — different surface absorbency (siding, brick, stucco) and weather exposure mean different spread rates and coat counts. Use exterior-specific calculators.
• Specialty finishes (textured, faux, color-washing, glazing) — those have unique coverage characteristics not modeled here.
• Cabinet refinishing — different prep and surface area; calculate by linear feet of doors/drawers, not square footage of room.
• Commercial / industrial paints — those have very different spread rates and application methods.

Common use cases

• Educational use — demonstrating the underlying concept
• Onboarding a colleague who needs the same calculation/conversion
• Verifying a number or output before passing it on
• Quick calculation during a typical workday