Calculated result
3gallons
Paint purchase
Formula
PAINT-INTERIOR-002(2 × (L + W) × H − door deductions − window deductions) × coats ÷ coverage
448 ft²
Gross wall area
35 ft²
Opening deductions
413 ft²
Paintable area
Painting calculator
Paint calculator
Room wall area, opening deductions, coats and rounded paint purchase quantity.
By Feroz Sheikh · Formula PAINT-INTERIOR-002 v2.1.0 · Tested formula
Formula
PAINT-INTERIOR-002v2.1.0Project inputs
Enter your measurements. Switching units converts the same project, including coverage and unit rates. Package yields keep the unit printed beside each field.
Refine measurements and rates at any time. Your current values are included in the material-list PDF.
Planning estimate, not engineering advice. Confirm site measurements and product specifications. Calculation limits.
This calculator returns whole gallons for interior walls from room dimensions, coats, openings and labelled coverage. Formula PAINT-INTERIOR-002 v2.1.0. Waste stays visible. Add gallons to an estimate and send a quote. Ceilings and primer are separate tools.
- Measure finished room length, width and ceiling height.
- Confirm or edit door and window deductions using actual opening areas when needed.
- Use the coverage stated on the selected interior or exterior paint product label.
Size chart
Interior paint room chart (2 coats)
Interior walls. Ceilings and primer are different tools. Replace the coverage with the can in your hand.
| Room | Gallons | Check |
|---|---|---|
| 10 × 10 × 8 ft | 2 gallons | 320 ft² |
| 12 × 12 × 8 ft | 2 gallons | 384 ft² |
| 12 × 16 × 8 ft | 3 gallons | 448 ft² |
| 16 × 20 × 8 ft | 3 gallons | 576 ft² |
| 20 × 20 × 9 ft | 4 gallons | 720 ft² |
Metric example — 4 × 3.5 × 2.4 m room. A 4 × 3.5 × 2.4 m room, two coats, labelled coverage, returns 2 gallons. Coverage is entered as square metres per US gallon; the package unit stays explicit.
Bags vs bulk. Gallons versus leftover: whole cans only. A quart is a touch-up, not a second wall.
- Label coverage. Room length, width, height, doors and windows. Coverage is the labelled rate on the can, not a hidden Construclytic constant. Benjamin Moore — paint calculator
- Coverage range. A typical gallon covers up to about 400 ft² on a primed, smooth surface. Texture, colour change and sheen move that number. Glidden — paint coverage guidance
Add your project costs
Enter supplier prices and any labour, equipment or delivery costs. Pick a currency — badges, inputs, and totals follow.
Costs use the can rate you type. Construclytic does not fetch paint prices. Primer, ceiling gallons and cut-and-roll labour stay on their own lines. For GCC projects, select QAR, AED, SAR, KWD, OMR or BHD and enter local supplier rates.
How this calculator works
A transparent interior paint calculator takeoff.
Feroz SheikhWritten by Feroz Sheikh · founder
This calculator measures all four interior room walls, deducts editable door and window areas, then applies selected coats and product-label coverage before whole-gallon rounding.
Paint gallons come from wall area × coats ÷ the label coverage you enter. Construclytic does not invent a hiding power. Dark colours, new drywall and textured plaster all change the real spread rate — keep that on the input, not in marketing copy.
Worked example. Example: a 16 ft × 12 ft room at 8 ft high has 448 ft² of gross wall area. One 20 ft² door and one 15 ft² window leave 413 ft²; two coats at 375 ft² per gallon round to 3 gallons.
See the example inputs and calculated result
This separate example uses the calculator's starting values in imperial units. It stays unchanged when you edit the tool above. Example rates, yields and dimensions are assumptions, not recommendations for your project.
| Input | Example value |
|---|---|
| Room length | 16 ft |
| Room width | 12 ft |
| Ceiling height | 8 ft |
| Doors | 1 count |
| Windows | 1 count |
| Door deduction | 20 ft² |
| Window deduction | 15 ft² |
| Number of coats | 2 count |
| Coverage per gallon | 375 ft² / US gal |
Example result: Paint purchase — 3 gallons.
(2 × (L + W) × H − door deductions − window deductions) × coats ÷ coverage
Use the labelled units above when reproducing this example. To work in metric, switch the calculator to Metric; applicable measurements are converted, while product-specific units remain labelled.
- 1. Measure finished room length, width and ceiling height.
- 2. Confirm or edit door and window deductions using actual opening areas when needed.
- 3. Use the coverage stated on the selected interior or exterior paint product label.
Field notes that change the number
- Measure wall length × height, then deduct doors and windows you will not paint, or leave them in if you want a conservative order.
- A second coat is not automatically 100% of the first. Many jobs still buy two full coats; do not hide a 70% second-coat guess unless you will explain it.
- Primer is its own product. New drywall usually wants it.
Common mistakes
- Using floor area as wall area.
- Trusting “400 ft² per gallon” on a deep red or a hot, porous block wall.
- Skipping the extra gallon that lets you cut in without a mid-wall colour break.
Metric coverage is litres and square metres. A 10 L pail is not “two gallons and a bit” — switch units rather than mixing US gallons into a Doha material list.
Need the broader workflow? Read the painting planning guide.
How-to: How much ceiling paint: area, coats and sloped surfaces · Trim paint coverage: convert width and length into gallons · How to count drywall sheets · How to estimate interior paint · How to Estimate Exterior Paint: Gallons by Wall Area
Questions
Questions about interior paint calculator
Are windows and doors deducted?
Yes. Standard deductions are visible and editable so they can be replaced with measured opening areas.
Does primer count as a coat?
No. Create a separate estimate line when primer uses a different coverage or product.
How much paint for a 12x12 room?
A 12 × 12 × 8 ft room, two coats, labelled coverage, one door and one window, returns 2 gallons. Replace coverage with the can in your hand.
Does this include the ceiling?
No. Interior walls only. Ceiling paint is a separate calculator.
What is the formula ID?
This paint calculator publishes formula PAINT-INTERIOR-002, version 2.1.0. The ID stays on the result and on the estimate line.