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Quick answer
What does the Paint quantity calculate?
How much paint is needed after openings, coats, and waste? This calculator uses room perimeter, wall height, doors & windows area, number of coats, and coverage per litre to estimate paint to buy immediately in your browser.
With the values currently entered, the result is 1.8 gal — paint to buy. It also shows painted wall area, coverage across all coats, and calculated amount.
How to use the Paint quantity
- Replace the example values with your own numbers.
- Review the result and supporting figures as they update automatically.
- Check the formula and assumptions before using the estimate for a decision.
Inputs used
- Room perimeter — entered in m
- Wall height — entered in m
- Doors & windows area — entered in m²
- Number of coats — entered in coats
- Coverage per litre — entered in m²/L
Paint quantity formula
((Perimeter × height) − openings) × coats ÷ coverage
Assumptions
- Coverage follows the paint manufacturer.
- Ceilings, trim, and primer are separate.
Practical guide
Paint quantity example and edge cases
Paint coverage starts with wall area. Coats, doors, windows, and the product coverage rate change how many cans you need.
Example: Painting a medium room
Use a 18-meter wall perimeter, 2.5-meter walls, 6 square meters of openings, two coats, and coverage of 10 square meters per liter.
- Room perimeter
- 18 m
- Wall height
- 2.5 m
- Doors & windows area
- 6 m²
- Number of coats
- 2 coats
- Coverage per litre
- 10 m²/L
Calculated result2.1 galpaint to buy
Round up to the available can size. A small amount left over is useful for touch-ups.
Example results use the default display profile. The calculator above follows your selected country and units.
How to read the result
- Coverage printed on the can is a guide. Rough or porous walls often use more paint.
- Strong color changes may need another coat or a separate primer.
Edge cases worth checking
The room has sloped walls
Measure each unusual section separately. Add those areas before calculating the total.
You are painting trim too
Calculate trim with its own product and coverage rate. Do not hide it inside the wall estimate.
What changes the result most
Room perimeter
Measure room perimeter with the same unit shown beside the input. Convert first if your source uses another unit.
Wall height
Measure wall height with the same unit shown beside the input. Convert first if your source uses another unit.
Doors & windows area
Measure doors & windows area with the same unit shown beside the input. Convert first if your source uses another unit.
Try a different scenario
Small changes show whether the answer is stable or sensitive.
Room perimeter: 10% lower
14 m1.6 galpaint to buy
Room perimeter: 10% higher
18 m2.1 galpaint to buy
Wall height: 10% higher
2.75 m2.1 galpaint to buy
Common mistakes
Check room perimeter
Coverage follows the paint manufacturer. Make sure this matches the number you enter.
Keep coverage per litre consistent
Ceilings, trim, and primer are separate. Use the same units and time period throughout the calculation.
Do not rely on one paint quantity scenario
Run a cautious case and an optimistic case. The range is often more useful than one exact-looking number.
Use this result well
How much paint is needed after openings, coats, and waste?
Confirm measurements, pack sizes, and product instructions before ordering materials.