Answer first
What this calculator tells you
Estimate the gallons of paint a room needs from its size, doors, windows and coats. Buy the right number of cans once, with the doors and windows already taken out of the area. Formula: Wall area = 2 × (length + width) × height − 20 sq ft per door − 15 sq ft per window; gallons = wall area × coats ÷ coverage per gallon. At the worked-example inputs, the gallons needed is 1.9. Holding every other input steady, moving room length (ft) from 11 to 17 moves the result from 1.6 to 2.2.
Transparent method
The formula
Buy the right number of cans once, with the doors and windows already taken out of the area.
Worked example
Example inputs
How to interpret the result
Paint is bought by the gallon but covers by the square foot, so the job is turning a room into wall area. Four walls of a 14 by 12 room at 8 feet give 416 square feet, less about 85 for two doors and three windows. Two coats over 331 square feet needs about 1.9 gallons at 350 square feet per gallon, so two cans cover it with little left.
At the worked-example inputs the gallons needed is 1.9. It rises with coats of paint, wall height (ft), room length (ft) and room width (ft) and falls as coverage per gallon (sq ft), number of doors and number of windows increase.
These are material-quantity estimates. Always add a waste and overage margin, and confirm actual coverage with your specific product and supplier before purchasing.
Before you rely on it
What to check
Read the coverage on your own can and enter it. Rough or dark surfaces take more, and coverage is quoted for smooth, primed walls.
The common error
Where people go wrong with paint calculator
Skipping the second coat in the math. Many colors need two for even coverage, and buying for one leaves the job unfinished.
Sensitivity evidence
How room length (ft) changes the gallons needed
Holding every other input at the worked-example value, moving room length (ft) from 11 to 17 moves the gallons needed from 1.6 to 2.2: a spread of 0.549, or 29% of the worked-example result.
| Room length (ft) | Gallons needed | Paintable wall area (sq ft) | Total area to cover (sq ft) |
|---|---|---|---|
| 11 | 1.6 | 283 | 566 |
| 13 | 1.8 | 315 | 630 |
| 14worked example | 1.9 | 331 | 662 |
| 15 | 2 | 347 | 694 |
| 17 | 2.2 | 379 | 758 |
Every input, tested
Which input moves the gallons needed most
Of the 7 inputs, coats of paint moves the gallons needed most (1.9 across the range tested) and number of windows moves it least (0.171).
| Input | Tested from | To | Gallons needed at each end | Swing |
|---|---|---|---|---|
| Coats of paint | 1 | 3 | 0.946 to 2.8 | 1.9 (100%) |
| Wall height (ft) | 7 | 9 | 1.6 to 2.2 | 0.594 (31%) |
| Coverage per gallon (sq ft) | 315 | 385 | 2.1 to 1.7 | 0.382 (20%) |
| Number of doors | 1 | 3 | 2 to 1.8 | 0.229 (12%) |
| Room length (ft) | 13 | 15 | 1.8 to 2 | 0.183 (9.7%) |
| Room width (ft) | 11 | 13 | 1.8 to 2 | 0.183 (9.7%) |
| Number of windows | 2 | 4 | 2 to 1.8 | 0.171 (9.1%) |
Two variables at once
Gallons needed by room length (ft) and room width (ft)
Across the grid the gallons needed runs from 1.4 to 2.3. Moving room length (ft) from 11 to 17 shifts it by 0.549 at the middle column, and moving room width (ft) from 10 to 14 shifts it by 0.366 at the middle row, so room length (ft) is the bigger lever here.
| Room length (ft) \ Room width (ft) | 10 | 12 | 14 |
|---|---|---|---|
| 11 | 1.4 | 1.6 | 1.8 |
| 13 | 1.6 | 1.8 | 2 |
| 14 | 1.7 | 1.9 | 2.1 |
| 15 | 1.8 | 2 | 2.2 |
| 17 | 2 | 2.2 | 2.3 |
The highlighted cell is the worked example: 1.9.
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Room length (ft) | 14 | Enter the room length (ft) used in this calculation. |
| Room width (ft) | 12 | Enter the room width (ft) used in this calculation. |
| Wall height (ft) | 8 | Enter the wall height (ft) used in this calculation. |
| Number of doors | 2 | Each door is counted as 20 square feet. |
| Number of windows | 3 | Each window is counted as 15 square feet. |
| Coats of paint | 2 | Enter the coats of paint used in this calculation. |
| Coverage per gallon (sq ft) | 350 | Printed on the paint can. 350 is a common figure for smooth walls. |
| Gallons needed | 1.9 | |
| Paintable wall area (sq ft) | 331 | |
| Total area to cover (sq ft) | 662 | |
Inputs, definitions and assumptions
Room length (ft)
Enter the room length (ft) used in this calculation. The prefilled worked-example value is 14.
Room width (ft)
Enter the room width (ft) used in this calculation. The prefilled worked-example value is 12.
Wall height (ft)
Enter the wall height (ft) used in this calculation. The prefilled worked-example value is 8.
Number of doors
Each door is counted as 20 square feet. The prefilled worked-example value is 2.
Number of windows
Each window is counted as 15 square feet. The prefilled worked-example value is 3.
Coats of paint
Enter the coats of paint used in this calculation. The prefilled worked-example value is 2.
Coverage per gallon (sq ft)
Printed on the paint can. 350 is a common figure for smooth walls. The prefilled worked-example value is 350.
How to use this calculator
- 1Verify the inputs. Gather room length (ft), room width (ft), wall height (ft), number of doors, number of windows, coats of paint and 1 more from your own documents; the prefilled values are examples.
- 2Save a baseline. The worked example puts the gallons needed at 1.9. Store your own version of it as Scenario A.
- 3Test one change. Start with coats of paint, the input with the biggest effect here: moving coats of paint from 1 to 3 takes the gallons needed from 0.946 to 2.8, a swing of 100% of the worked-example figure.
- 4Check the extremes. At half the example coats of paint (1) the gallons needed is 0.946; at double (4) it is 3.8.
People also ask
Frequently asked questions
How do you calculate paint?
Wall area = 2 × (length + width) × height − 20 sq ft per door − 15 sq ft per window; gallons = wall area × coats ÷ coverage per gallon. At the worked-example inputs the gallons needed is 1.9.
What does the paint result mean?
Buy the right number of cans once, with the doors and windows already taken out of the area. At the worked-example inputs the gallons needed is 1.9. It rises with coats of paint, wall height (ft), room length (ft) and room width (ft) and falls as coverage per gallon (sq ft), number of doors and number of windows increase.
How much does room length (ft) change the gallons needed?
Holding every other input at the worked-example value, moving room length (ft) from 11 to 17 moves the gallons needed from 1.6 to 2.2, a spread of 0.549.
What are the limits of this paint calculator?
These are material-quantity estimates. Always add a waste and overage margin, and confirm actual coverage with your specific product and supplier before purchasing. The tables on this page test room length (ft) only from 11 to 17; a value outside that range is not tabulated here.
Which input moves the gallons needed most in the paint calculator?
Ranked by how far each moves the gallons needed across the range tested: coats of paint (1.9, 100%), wall height (ft) (0.594, 31%), coverage per gallon (sq ft) (0.382, 20%) and number of doors (0.229, 12%).
If I double coats of paint in the paint calculator, does the gallons needed double?
Doubling it from 2 to 4 takes the gallons needed from 1.9 to 3.8, which is 2.00 times the worked-example figure. So the result scales almost exactly in proportion. Halving it to 1 gives 0.946.
How much does room width (ft) matter in the paint calculator?
The worked example uses 12. With the other inputs left at the worked example, moving room width (ft) from 11 to 13 takes the gallons needed from 1.8 to 2, a swing of 9.7% of the worked-example figure.
How much does wall height (ft) matter in the paint calculator?
The worked example uses 8. Holding every other input at its worked-example value, moving wall height (ft) from 7 to 9 takes the gallons needed from 1.6 to 2.2, a swing of 31% of the worked-example figure.
How much does number of doors matter in the paint calculator?
The worked example uses 2. Holding every other input at its worked-example value, moving number of doors from 1 to 3 takes the gallons needed from 2 to 1.8, a swing of 12% of the worked-example figure.
How much does number of windows matter in the paint calculator?
The worked example uses 3. Holding every other input at its worked-example value, moving number of windows from 2 to 4 takes the gallons needed from 2 to 1.8, a swing of 9.1% of the worked-example figure.
How much does coats of paint matter in the paint calculator?
The worked example uses 2. With the other inputs left at the worked example, moving coats of paint from 1 to 3 takes the gallons needed from 0.946 to 2.8, a swing of 100% of the worked-example figure.
How much does coverage per gallon (sq ft) matter in the paint calculator?
The worked example uses 350. With the other inputs left at the worked example, moving coverage per gallon (sq ft) from 315 to 385 takes the gallons needed from 2.1 to 1.7, a swing of 20% of the worked-example figure.
Which inputs change the paintable wall area (sq ft) in the paint calculator?
At the worked-example inputs it is 331. Room length (ft) takes it from 315 to 347, room width (ft) takes it from 315 to 347, wall height (ft) takes it from 279 to 383 and number of doors takes it from 351 to 311.
Which inputs change the total area to cover (sq ft) in the paint calculator?
At the worked-example inputs it is 662. Room length (ft) takes it from 630 to 694, room width (ft) takes it from 630 to 694, wall height (ft) takes it from 558 to 766 and number of doors takes it from 702 to 622.
Why should I round up a material estimate?
Real coverage is never perfectly uniform: uneven ground, waste from cutting, and spillage all consume more material than the flat geometric calculation suggests. Rounding up avoids a second supply trip, which usually costs more in time than the extra material costs in money.
Sources and evidence
Free Calculators Online is independent and is not affiliated with or endorsed by the source organizations. Educational estimates only.