Paint Calculator
Gallons of paint needed for a room, after deducting doors and windows, for one or more coats.
Paint maths usually fails in one of two directions. Buy by the room and you finish the second coat with a quarter gallon to spare on three walls. Buy by guess and you make a second trip to the store mid-job, which is exactly when the tinted batch you need is out of stock.
The rule of thumb is 350 sq ft per gallon of interior latex, but that number assumes a primed, previously painted surface. Bare drywall drinks 20 to 30% more. Textured walls, deep base colours and spraying instead of rolling all push consumption up — when in doubt, round up one gallon rather than down.
How this calculator works
Every field is editable and the result updates as you type — nothing is submitted anywhere, all the maths runs in your browser. That matters for a number you are about to spend money on: you can see exactly which input moved the answer, and by how much.
What this calculation assumes
Assumes previously painted, primed walls at one standard coat thickness. Bare drywall, plaster and skim coats absorb 20 to 30% more paint, and going from a dark colour to a light one needs a primer coat that is not counted here.
Treat the result as an order-of-magnitude check rather than a final quantity. Product packaging changes, regional labour rates vary by a factor of two, and no calculator has seen your actual site.
Frequently asked questions
One coat or two?
Two coats is standard for a colour change, and non-negotiable going from dark to light. One coat is only enough when you are refreshing the same colour, or using a paint-and-primer-in-one over a similar tone. Two thin coats always cover better than one thick one.
Do I need to subtract doors and windows?
Yes, if you want an accurate number. A standard interior door is about 21 sq ft and a window about 15 sq ft. This calculator deducts both — the savings are small per opening but add up across a whole house.
Should I buy an extra gallon?
Keep a quart of each colour for touch-ups — it costs little and saves a full repaint after a scuff. Beyond that, leftover gallons are wasted money. Paint stores fine for years if the lid is sealed and it is kept from freezing.
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