Home improvement
Paint Calculator
Works paint quantity, where coats and coverage matter more than area.
Result
Coverage varies with surface texture and colour change. Rough or previously unpainted surfaces can drop coverage by a third, and a dramatic colour change usually needs primer plus two coats.
Get a Free Business AuditHow the calculation works
Wall area from the perimeter times the height, less large openings, divided by the coverage per unit, multiplied by the number of coats. Coverage figures are stated by the manufacturer for a smooth sealed surface, and a porous, textured or previously unpainted surface takes meaningfully more.
What moves the number
- Number of coats, which doubles or triples the quantity.
- Surface porosity and texture.
- Whether primer is needed.
- Colour change, which affects the coats required.
Where this one goes wrong
A significant colour change needs a tinted primer plus two coats rather than two coats alone, because coverage over a strongly contrasting colour is poor and a third finish coat costs more than the primer would have. Plan the system, not just the paint. Texture is the other factor, since a textured surface has far more area than its measured dimensions and the coverage figure assumes a smooth one.
Before you quote
- Count the coats, including primer where needed.
- Allow extra coverage for porous or textured surfaces.
- Use a tinted primer for a significant colour change.
Questions
About this calculator
How many coats do I need?
Two finish coats as standard, with a primer where the surface is new, porous, stained or being changed significantly in colour. Over a strongly contrasting colour a tinted primer plus two coats beats three finish coats on both cost and result.
Why does coverage fall short?
Because the stated figure assumes a smooth sealed surface. A porous, textured or previously unpainted surface takes meaningfully more, and a textured surface has far more actual area than its measured dimensions suggest.
Should I deduct windows and doors?
Large openings yes, small ones generally not, since cutting in around them uses more paint per square foot than open wall. Deducting every opening tends to leave you short, and running out mid-wall shows.
The formula
gallons = (paintable area × coats) ÷ coverage per gallon
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How to use this calculator
- Total wall length (ft) (ft). Enter your own figure; the page starts at 48 ft.
- Wall height (ft) (ft). Enter your own figure; the page starts at 9 ft.
- Coverage per gallon (sq ft) (sq ft). Enter your own figure; the page starts at 350 sq ft.
- Waste allowance (%). Usually left at zero. Use the coats field instead.
- Cost per gallon ($). Enter your own figure; the page starts at $45.
- Number of coats. Enter your own figure; the page starts at 2.
The result updates as you type, so you can try a figure, see what it does to gallons needed, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Total wall length (ft) | 48 ft |
| Wall height (ft) | 9 ft |
| Coverage per gallon (sq ft) | 350 sq ft |
| Waste allowance | 0% |
| Cost per gallon | $45 |
| Number of coats | 2 |
| Doors and windows to deduct (sq ft) | 60 sq ft |
What comes out
| Result | Value |
|---|---|
| Gallons needed main result | 3 |
| Area | 432.0 |
| Area including waste | 744.0 |
| Estimated material cost | $135 |
| Paintable area | 372.0 |
Painting 372.0 sq ft with 2 coats needs 3 gallons, costing about $135.
How sensitive the answer is
Holding everything else steady and changing only cost per gallon, this is what happens to gallons needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per gallon ($) | Gallons needed |
|---|---|
| $22.5 | 3 |
| $33.75 | 3 |
| $45 example above | 3 |
| $67.5 | 3 |
| $90 | 3 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Total wall length (ft) (ft)
- Measured in ft, starting at 48 ft.
- Wall height (ft) (ft)
- Measured in ft, starting at 9 ft.
- Coverage per gallon (sq ft) (sq ft)
- Measured in sq ft, starting at 350 sq ft.
- Waste allowance (%)
- Usually left at zero. Use the coats field instead.
- Cost per gallon ($)
- A money amount, starting at $45.
- Number of coats
- A number, between 0 and 6, starting at 2.
- Doors and windows to deduct (sq ft) (sq ft)
- Measured in sq ft, starting at 60 sq ft.
What the results tell you
| Result | What it is |
|---|---|
| Gallons needed | The headline result. A number. |
| Area | A decimal figure. |
| Area including waste | A decimal figure. |
| Estimated material cost | A money figure. |
| Paintable area | A decimal figure. |
Keep going
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