Calculate how much paint you need for walls or ceilings using the surface area, doors and windows, number of coats, paint coverage, and waste allowance.
Paint Quantity Calculator
Surface dimensions
Use 4 for four walls with the same dimensions.
Doors and windows
Paint coverage and waste
Use the coverage stated on the paint manufacturer's label when available.
Optional. Used to estimate the number of containers to buy.
How the Paint Calculator Works
The calculator first finds the gross surface area from the length, height, and number of surfaces. Doors and windows are then deducted because those areas do not need paint.
The remaining area is multiplied by the number of coats. The resulting coat area is divided by the paint's coverage rate to estimate the paint volume required.
Finally, the selected waste allowance is added to create a practical purchasing estimate.
Paint calculation formula
Gross area = length × height × number of surfaces
Net area = gross area − doors and windows
Total coat area = net area × number of coats
Paint required = total coat area ÷ coverage rate
Paint to buy = paint required × (1 + waste percentage ÷ 100)
Example: Painting a 6 m × 2.7 m Wall
Suppose one wall measures 6 m × 2.7 m. It has one 0.9 m × 2.1 m door and two 1.2 m × 1.2 m windows. You want two coats of paint with a stated coverage of 10 m2/L and a 10% waste allowance.
| Measurement | Value |
|---|---|
| Gross wall area | 16.20 m2 |
| Door area | 1.89 m2 |
| Window area | 2.88 m2 |
| Net paint area | 11.43 m2 |
| Two-coat area | 22.86 m2 |
| Paint before waste | 2.29 L |
| With 10% waste | About 2.52 L |
How Much Paint Coverage Should You Use?
Paint coverage varies between products. A smooth, sealed surface often uses paint differently from a rough or porous surface. New plaster, masonry, and heavily textured walls can require more material.
| Surface condition | Planning consideration |
|---|---|
| Smooth previously painted surface | Often closer to the manufacturer's stated coverage |
| New plaster or porous surface | Primer and additional paint consumption might be required |
| Rough or textured surface | More paint can be needed because of the increased surface area |
| Major colour change | Additional coats might be needed for consistent coverage |
How Many Coats of Paint Do You Need?
The number of coats depends on the product, surface condition, existing colour, new colour, primer, and the finish required.
One coat does not always provide the same coverage as two coats. Follow the paint manufacturer's application instructions for drying times, recoating, dilution, and recommended coverage.
Common Paint Calculation Mistakes
- Calculating gross wall area without subtracting doors and windows.
- Forgetting to multiply the area by the number of coats.
- Using a generic coverage rate instead of checking the paint label.
- Ignoring porous or textured surfaces.
- Buying the exact calculated volume with no allowance for waste.
- Mixing litres and gallons without converting the units.
- Assuming one coat will always provide complete coverage.
Paint Calculator FAQs
How much paint do I need for a room?
Measure the walls, calculate the total surface area, subtract doors and windows, multiply the remaining area by the number of coats, then divide by the paint's coverage rate. Add a reasonable waste allowance.
Should I subtract doors and windows when calculating paint?
Yes. Subtracting openings gives a closer estimate because those areas normally do not require the same wall paint.
How many coats of paint should I use?
The required number varies by product and surface. Two coats are common for many decorative painting projects, but follow the manufacturer's instructions for the specific paint and surface.
How much extra paint should I buy?
A modest waste allowance helps account for application losses, spills, touch-ups, surface variation, and paint left in rollers or containers. The appropriate allowance depends on the project.
Does primer count as a coat?
Primer is normally calculated separately from the finish coats because it is a different product with its own coverage and application requirements.
Can I use this calculator for ceilings?
Yes. Enter the ceiling length and width as the surface dimensions. For a ceiling, you can treat the ceiling area as one surface.