What this calculator does
Plan one paint product for the four walls of a rectangular room. Subtract doors and windows, use the coverage stated on the paint container and choose your number of coats. The calculator rounds the amount up to full cans so the cost reflects the actual purchase.
Formula, assumptions and sources
Wall area = 2 × (length + width) × height − openings. Paint = wall area × coats ÷ coverage per litre (Metric) or US gallon (US customary), then add allowance. Cans = round up(paint ÷ can size). Price is per can. Use coverage from the product label; changing the unit selector does not convert entered measurements. Ceilings, trim and primer are excluded.
Worked example
This example uses the initial values shown in the form. It is illustrative and is not a saved personal result.
Example inputs
- Measurement system
- Metric
- Room length (m or ft)
- 5
- Room width (m or ft)
- 4
- Wall height (m or ft)
- 2.5
- Total door and window area (m² or ft²)
- 4
- Number of coats
- 2
- Coverage per litre or US gallon (m² or ft²)
- 10
- Can size (litres or US gallons)
- 2.5
- Extra material allowance (%)
- 10
- Currency
- USD
- Price per purchase unit (optional)
- 0
Example result
The difference is paint left over after your allowance.
View chart data
| Item | Value |
|---|---|
| Needed with allowance | 9.02 |
| Whole cans | 10 |
Material only; excludes delivery, labour and tax. A zero price means cost has not been entered. Coverage varies with surface texture and colour changes. Calculate different colours separately.
Frequently asked questions
Does this include the ceiling?
No. The result covers the four walls only, excluding the openings you enter.
Does choosing US units convert my measurements?
No. The labels change, but you must enter feet, square feet, coverage per US gallon and can size in US gallons. Imperial UK gallons are not supported.
Method and example checked 4 November 2026. How we check calculations