Paint Calculator
Use our free online paint calculator to find out exactly how much paint you need for your project. Enter your room dimensions, and this painting calculator will estimate the total paint required for walls, ceilings, and doors. It saves you time, money, and unnecessary trips to the store.
How to Use This Paint Calculator
Using this online paint calculator is simple:
- Enter the length, width, and height of your room in feet or metres.
- Enter the number of doors and windows to be excluded.
- Enter the paint coverage rate from your paint can label.
- Select the number of coats you plan to apply.
- Click Calculate to see the total gallons or litres of paint required.
What Is a Paint Calculator?
A paint calculator is an online tool that estimates how many gallons of paint you need to cover a surface based on its dimensions. It takes into account the total paintable area, the number of coats required, and the coverage rate of the paint you are using.
People use a painting calculator before starting interior or exterior painting projects to avoid buying too much or too little paint. Running out of paint mid-project or returning unused cans is a common and avoidable problem.
This calculator painting tool works for rooms, individual walls, ceilings, and exterior surfaces.
How Much Paint Do I Need?
The amount of paint you need depends on the total surface area you are painting and the coverage rate of your paint. Most standard interior paints cover 350 to 400 square feet per gallon. Primer typically covers 200 to 300 square feet per gallon.
Paint Needed Formula:
How to Calculate Paint for a Room
To calculate paint for a full room, you need to find the total wall area and subtract the area of doors and windows.
Step 1: Calculate total wall area.
Step 2: Subtract doors and windows.
Each standard door covers approximately 20 square feet. Each standard window covers approximately 15 square feet.
Step 3: Divide by coverage rate.
Step 4: Multiply by the number of coats.
For a room measuring 12 ft long, 10 ft wide, and 8 ft tall with 1 door and 2 windows and a paint coverage of 400 sq ft per gallon, the total wall area is 352 sq ft. After subtracting the area for the door and windows, the final paintable area is 302 sq ft, meaning that for two coats of paint, the calculated total paint needed is 1.51 gallons, and the recommended purchase is 2 gallons.
How to Calculate Paint for a Ceiling
Ceiling paint is calculated separately from wall paint because ceilings often use a different paint type and finish.
Example: For a ceiling measuring 12 ft × 10 ft with a paint coverage of 400 sq ft per gallon and requiring one coat, the ceiling area is calculated as 12 × 10 = 120 sq ft; dividing this by the coverage rate (120 ÷ 400) results in 0.3 gallons needed, meaning one quart of paint is sufficient for the project.
How to Calculate Paint for a Single Wall
To calculate paint for just one wall, multiply the width by the height to get the surface area.
Example: A wall measuring 15 ft wide and 9 ft tall with one window.
- Step 1: Wall area = 15 × 9 = 135 sq ft
- Step 2: Subtract window = 135 − 15 = 120 sq ft
- Step 3: Gallons needed = 120 ÷ 400 = 0.3 gallons per coat
Paint Coverage Guide
Paint coverage varies by paint type, surface texture, and application method. Use the table below as a general reference.
| Paint Type | Coverage per Gallon |
|---|---|
| Standard interior paint | 350 – 400 sq ft |
| Primer | 200 – 300 sq ft |
| Exterior paint | 250 – 350 sq ft |
| Paint and primer in one | 300 – 350 sq ft |
| Textured or porous surfaces | 150 – 250 sq ft |
Always check the coverage information on the paint can label for the most accurate figure.
How Many Coats of Paint Do I Need?
The number of coats required depends on the colour change and the surface condition.
One coat is usually sufficient when painting a similar colour over a well-prepared surface or using a paint and primer combined product.
Two coats are recommended for most standard painting projects, especially when changing from a dark colour to a light one or painting over bare walls.
Three or more coats may be needed when painting white or a very light colour over a dark existing colour, or when working on a rough or porous surface.
How to Calculate Paint Ratio for Mixing
When mixing paint with a thinner, water, or another paint, you need to calculate the correct ratio to achieve the right consistency or colour.
Paint Ratio Formula:
Example: You need 2 litres of a 70:30 paint-to-thinner mix.
- Step 1: Paint = 2 × (70 ÷ 100) = 1.4 litres
- Step 2: Thinner = 2 × (30 ÷ 100) = 0.6 litres
- Step 3: Combined = 1.4 + 0.6 = 2 litres total
This calculate paint ratio online method works for any two or three-part mix.
FAQs
How much paint do I need for a room?
Measure the total wall area using 2 × (Length + Width) × Height, subtract the area of doors and windows, then divide by the paint coverage rate. Multiply by the number of coats to get the total amount needed.
What is a paint calculator?
A paint calculator is an online tool that estimates the amount of paint needed based on the dimensions of the surface, the paint coverage rate, and the number of coats required.
How many square feet does a gallon of paint cover?
Most standard interior paints cover 350 to 400 square feet per gallon. Primer covers 200 to 300 square feet per gallon. Always check the label on your specific paint can for the exact figure.
Do I need to calculate ceiling paint separately?
Yes. Ceilings are typically painted with a different finish or paint type. Calculate the ceiling area separately by multiplying the room length by the room width.
How do I account for doors and windows?
Subtract the area of each door (approximately 20 sq ft) and each window (approximately 15 sq ft) from your total wall area before calculating how much paint you need.
How many coats of paint do I need?
Most painting projects require two coats. One coat may be enough when painting a similar colour over a prepared surface. Three or more coats may be needed when covering a very dark colour with a light one.
What does calculating paint ratio mean?
It means calculating how much of each component to use when mixing paint with a thinner, water, or another paint to achieve a specific consistency or finish.
Should I buy extra paint?
It is good practice to buy 10–15% more paint than your estimate to account for touch-ups, uneven surfaces, and waste during application.
Does surface texture affect how much paint I need?
Yes. Rough or porous surfaces absorb more paint and reduce coverage per gallon significantly. Use the lower end of the coverage range for textured surfaces.
Is this paint calculator free?
Yes. Our online paint calculator is completely free with no sign-up required.