Paint Calculator
Work out how much paint a room needs from its length, width and height, minus doors and windows, with coats, coverage rate and the number of cans to buy.
Wall area = 2 × (length + width) × height, less door and window openings, multiplied by the number of coats. A 5% allowance is added, then the volume is rounded up to whole containers. Coverage varies a lot by product, colour change and surface porosity — always check the figure printed on the tin. This is an estimate, not a decorator's take-off.
What is the Paint Calculator?
The ByteTools Paint Calculator turns a room's measurements into a shopping list. Enter the length, width and wall height, say how many doors and windows to deduct, choose the number of coats, and it works out the paintable area, the litres or gallons required and how many tins that comes to at the container size you plan to buy.
- Imperial and metric input — gallons or litres
- Deducts doors and windows with editable opening sizes
- Optional ceiling, multiple coats and a touch-up allowance
- Rounds up to whole tins at the container size you choose
- Estimated cost from your price per tin
- Runs entirely in your browser, no signup and nothing uploaded
How to use the Paint Calculator
- 1
Pick imperial or metric and enter the room length, width and wall height.
- 2
Enter how many doors and windows to deduct, adjusting the area of each if yours are unusual.
- 3
Set the number of coats and the coverage rate printed on the paint tin.
- 4
Choose your container size and price, and tick the ceiling box if you are painting it too.
- 5
Read the paintable area, the volume of paint needed and the number of tins to buy.
About the Paint Calculator
The ByteTools Paint Calculator turns a room's measurements into a shopping list. Enter the length, width and wall height, say how many doors and windows to deduct, choose the number of coats, and it works out the paintable area, the litres or gallons required and how many tins that comes to at the container size you plan to buy.
It is built for anyone decorating a room themselves, as well as landlords and small contractors pricing a repaint. You can switch between imperial and metric, edit the coverage rate to match the figure printed on your tin, add the ceiling, and set an allowance for touch-ups.
Everything is calculated locally in your browser — nothing is uploaded and no measurements leave your device. Coverage varies a lot between products, colours and surfaces, so treat the result as a planning estimate rather than a decorator's measured take-off.
Frequently asked questions
How much paint do I need for a 12x12 room?
A 12 ft by 12 ft room with 8 ft ceilings has about 384 sq ft of wall, roughly 349 sq ft once a door and a window are deducted. At 350 sq ft per gallon that is one gallon per coat, so two gallons for two coats. Enter your own measurements above for an exact figure.
How many square feet does a gallon of paint cover?
Most interior wall paints state 350 to 400 square feet per US gallon for a single coat, which is roughly 10 to 12 square metres per litre. Porous, textured or previously unpainted surfaces soak up noticeably more, so check the coverage printed on the tin and adjust the figure in the calculator.
Do I subtract windows and doors when calculating paint?
Yes, for anything larger than a small window it is worth deducting. This calculator uses 20 sq ft (1.9 m²) per door and 15 sq ft (1.4 m²) per window by default, and both figures are editable. Very small openings are often left in as a built-in safety margin.
How many coats of paint do I need?
Two coats is the normal answer for a durable, even finish. You may get away with one when refreshing a similar colour over a sound surface, and you may need three when covering a dark colour with a pale one, or painting bare plaster after a mist coat.
Should I buy extra paint?
A little extra is worth having for touch-ups and for the batch to match later. The calculator adds a 5% allowance by default and then rounds up to whole tins, which usually leaves enough in the bottom of the can for repairs.
Is this calculator accurate enough to order from?
It is a good planning estimate, but coverage genuinely varies by product, colour change, roller nap and surface porosity. Treat the result as a starting point, buy from one batch where you can, and confirm anything expensive against the manufacturer's own figures.
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