Wall Paint Calculator (Gallons, Litres and Cans)
Work out how much paint your walls need. Add each wall or the room perimeter, subtract doors and windows, choose the coats, and get gallons or litres, suggested cans and cost.
Enter the wall sizes, or the room perimeter and wall height.
About the wall paint calculator
The Wall Paint Calculator answers "how much paint do I need for my walls?" Measure the room, subtract the doors and windows you will not paint, choose the number of coats, and you get the amount of paint in US gallons and litres, plus a suggested set of cans. It is built for interior rooms, from a small bathroom to a large living area.
There are two ways to enter walls. If the room is a simple box, type the perimeter (the distance around the room) and the wall height. If the room has alcoves, a sloping or taller wall, or different heights, add each wall as its own row with a width and a height. A ceiling can be added with a checkbox.
How to work out paint for the walls of a room
Steps
- Pick US units (feet, gallons) or metric (metres, litres).
- For a plain room, enter the perimeter and the wall height. For an irregular room, add one row per wall.
- Enter the number of doors and windows. The defaults deduct 21 ft² per door and 15 ft² per window. Edit them to match your openings.
- Set the coats, normally 2, and the coverage from your paint can.
- Optionally add the price per gallon or litre to see the cost of the suggested cans.
The perimeter shortcut and opening deductions
Walls form a loop, so their combined area is the perimeter times the height. A room 20 ft by 16 ft has a perimeter of 2 x (20 + 16) = 72 ft, which saves measuring four walls separately.
The number of doors is and windows is . Their areas are subtracted because you do not paint them. Large openings such as French doors should be entered with their real area.
Worked examples
Large living room. A room 20 ft by 16 ft with 9 ft walls has a perimeter of 72 ft and 648 ft² of wall. Two doors (42 ft²) and three windows (45 ft²) leave 561 ft². Two coats at 350 ft² per gallon need 3.206 gallons. The suggestion is 3 one-gallon cans and 1 quart, 3.25 gallons. At 146.25.
Metric bedroom. A room with a 20 m perimeter and 2.4 m walls has 48 m². One door of 1.9 m² and two windows of 1.44 m² each leave 43.22 m², which is about 465 ft². At 10 m² per litre, two coats need 8.64 litres, or about 2.28 gallons. The cans suggested are four 1 litre cans and one 5 litre can, 9 litres in total.
How suggested cans work
The calculator finds the standard can sizes with the smallest total volume that covers your need, then the fewest cans among those. US sizes are 1 quart, 1 gallon and 5 gallons. Metric sizes are 1, 2.5, 5 and 10 litres. Any extra beyond your need is touch-up paint, which is useful because a matching batch is hard to reproduce later.
Tips for painting walls
- Textured, new or porous walls soak up more paint. Lower the coverage figure to match.
- Bare surfaces, stains and strong colour changes call for primer. Calculate it separately, usually as 1 coat.
- Painting dark over light, or the reverse, may need a third coat.
- Skirting boards, window frames and radiators are not included unless you add them as extra wall area.
- Buy all the paint for a room at once so the colour is consistent.
To measure irregular floor plans, use the Area Calculator. If you also want to paint the ceiling, or work from other angles, the Paint Calculator covers the same tool with a general focus. For converting a measurement between feet and metres, the Length Converter can help.
Accuracy and limits
This is an estimate. Coverage depends on the paint, the surface, the roller and your technique. Check the manufacturer's stated coverage and add a small margin for cutting in and waste.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device.
Frequently asked questions
How do I calculate the amount of paint for my walls?
Multiply the room perimeter by the wall height, subtract doors and windows, multiply by the coats and divide by the coverage. A room with a 72 ft perimeter and 9 ft walls has 648 ft². After two doors and three windows, 561 ft² remain, which needs 3.2 gallons for two coats at 350 ft² per gallon.
When should I use the perimeter instead of individual walls?
Use the perimeter when every wall has the same height. Enter each wall separately when heights differ, such as in a room with a gable wall, a raised section or an alcove.
Should I subtract doors and windows?
Yes, because you do not paint them. The defaults are 21 ft² per door and 15 ft² per window. Change the areas for unusual openings. Painted frames and trim are not included automatically.
How many coats should I allow for?
Two coats is the default and suits most repaints over a similar colour. Allow a third coat for a big change from a dark colour to a light one, or if the first two coats leave the old colour showing through.
Will the tool tell me how many cans to buy?
Yes. It suggests the standard can sizes with the smallest total volume that covers your need, and among those the fewest cans. A need of 3.2 gallons gives 3 one-gallon cans and 1 quart, or 3.25 gallons.
Can I use litres and square metres?
Yes. Switch to metric to enter areas in square metres and get litres, with 10 m² per litre as the default coverage. The result also shows gallons, where 1 US gallon equals 3.785411784 litres.
Is my data sent anywhere?
No. The wall paint sums run in your browser, so your room measurements and prices are never uploaded or stored on a server. They stay on this device and are not shared with anyone.