Paint Calculator
Estimate paint for a rectangular room from wall dimensions, excluded wall area, coats and entered coverage. Include the ceiling optionally and round to whole cans.
YOUR CALCULATION
Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Find wall area as perimeter times height, subtract the entered excluded wall area, and optionally add one flat ceiling. Multiply by coat count, divide by entered per-coat coverage, add allowance and round to whole equal-size cans.
wall area=2(L+W)H−openings; paint=(wall area+optional LW)×coats/coverage×(1+allowance/100)
Worked example
A 5 m × 4 m room with 2.5 m walls has 45 m² of walls. Subtract 4 m² of openings: 41 m². Two coats at 10 m²/L plus 10% allowance need 9.02 L. Four 2.5 L cans provide 10 L.
Assumptions and input details
Enter coverage from the chosen product for one coat. No local paint catalogue, recommended coat count or price is assumed. Exclusions apply only to walls. Ceiling uses the same coverage and coats; calculate it separately if a different paint is used. The model excludes sloped ceilings, trim, floor, substrate absorption differences and application losses beyond your entered allowance. US liquid and Imperial gallons differ. Whole-can rounding allows for floating-point noise near integers. Zero net area needs zero cans; the maximum supported estimate is one billion cans.
Unit definitions
Unit selection is independent of language. Each selector contains only units for its physical quantity. Calculations use defined base units internally.
Choose from 23 length definitions, including explicitly labelled regional measures. Area conversions square the length factor; geometric volumes cube it.
The 36 volume definitions distinguish US liquid, US dry and Imperial measures. Cups carry an explicit definition. Japanese shō is 2401/1331 L and a gō is one tenth of it; the explicitly labelled 180 mL rice cup is separate.
Regional measures are not interchangeable merely because their names look similar. This is a documented selection, not a claim to include every historical unit.
NIST measurement tables · UK Imperial definitions · Japanese traditional units · Mainland China and Hong Kong unit distinctions
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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