Paint Calculator

Who Uses a Paint Calculator and Why

A paint calculator is used by homeowners, renters, and DIY decorators who want to buy the right amount of paint the first time. Buying too little means a second trip to the store and the risk of a slightly different color batch. Buying too much wastes money and leaves cans of paint that dry out in the garage. A quick estimate takes the guesswork out of the shopping list.

Professionals lean on the same math. Painting contractors, property managers, and handypeople use it to write accurate quotes and to order materials in bulk. Real estate agents preparing a home for sale and landlords turning over a rental also use it to budget a repaint. Anyone who needs to turn a room size into a number of gallons benefits from a fast, repeatable calculation.

What You Need Before You Start

• Wall dimensions: The length and width of the room and the ceiling height, or a total wall area you have already measured.
• Number of coats: Most jobs need two coats, especially over a new or very different color.
• Doors and windows: A count of each so their area can be subtracted from the paintable surface.
• Paint coverage: The square feet per gallon printed on the can, if it differs from the standard 350.
• A waste buffer: A small extra percentage for spills, uneven walls, and future touch-ups.

How Paint Is Calculated

The tool first finds the wall area. When you enter a room, it uses the formula wall area equals 2 times length plus width, times the ceiling height. It then subtracts 21 square feet for each door and 15 square feet for each window to get the paintable area. That paintable area is multiplied by the number of coats and divided by the coverage rate of 350 square feet per gallon per coat. The result is rounded up to whole gallons, and quarts are shown for smaller amounts.

These numbers are averages, not guarantees. Rough or porous walls, a big color change, and thick versus thin application all change how far a gallon goes, so always confirm the final amount with your paint supplier and consider ordering a little extra for touch-ups.

Paint Coverage Reference

Common Mistakes and Tips

The most common mistake is forgetting the second coat. One coat almost never looks even, especially over a bold color, so plan for two unless you are refreshing the same shade. Another error is skipping primer on new drywall or dark walls, which forces you to add coats and use more paint than you expected. Measuring in the wrong units also trips people up, so double check that your length and height match the unit you selected.

Round up and keep a little extra for future scuffs, since a small leftover can matches your wall exactly. If you are planning a bigger project, pair this with the tile calculator for flooring and backsplashes, the length converter to switch between feet and meters, and the concrete calculator for patios and footings.

People Also Ask

How many gallons of paint do I need for one room?

An average bedroom with about 350 to 400 square feet of wall usually needs 2 gallons for two coats. Enter your exact size above for a precise number.

How many square feet does a gallon of paint cover?

A gallon of interior wall paint covers roughly 350 to 400 square feet in one coat. This calculator uses 350 as a safe average.

Should I paint one coat or two?

Two coats is standard for a smooth, even finish. Use one coat only when refreshing the same color over a clean, primed surface.

Do I subtract doors and windows?

Yes. This tool removes 21 square feet per door and 15 square feet per window so you do not buy paint for areas you will not cover.

How much extra paint should I buy?

A waste buffer of 5 to 10 percent covers spills and touch-ups. Add that in the waste field to raise the estimate slightly.

Does this calculator include primer?

No. It estimates finish paint only. Primer has its own coverage rate, so calculate it separately if your walls need it.

Can I use this for exterior walls?

You can, but exterior surfaces are often rougher and cover less per gallon. Lower the coverage or add waste to be safe.

How do I measure wall area myself?

Multiply each wall length by its height, add the walls together, then subtract doors and windows. Or let this tool do it from room size.