Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Gallons of paint
3
Net wall area
408
sq ft
Total painted area (all coats)
816
sq ft
Exact gallons before rounding
2.33
Psst — share this and help Rex grow
One click, a permanent link with your numbers baked in.
How to use this
- 1Enter room perimeter (ft).
- 2Enter wall height (ft).
- 3Enter doors & windows area (sq ft).
- 4Enter number of coats.
- 5Enter coverage per gallon (sq ft).
- 6Read your gallons of paint on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Interior wall paint typically covers 350-400 square feet per gallon on a single smooth coat, a figure printed on nearly every paint can label, but that number only tells half the story since almost every interior job needs two coats for even color and hide, especially over patched drywall, a color change, or a bold new hue going over white primer. This calculator takes your room's total wall length (perimeter) and ceiling height to compute gross wall area, subtracts an allowance for doors and windows so you're not buying paint for openings, then divides by your chosen coverage rate and multiplies by the number of coats. It rounds up to the nearest whole gallon since paint stores sell in gallon and 5-gallon increments and half-used cans of custom-tinted color are hard to match later. The default 350 sq ft/gallon coverage rate is a conservative real-world number that accounts for roller texture and slightly thicker application than the theoretical 400 sq ft ideal, so if you're a careful painter with a quality roller you may land under this estimate, but it's a safer number to shop with than the optimistic can label.
Worked example
Using the values the calculator loads with:
Inputs
- Room perimeter: 56 ft
- Wall height: 8 ft
- Doors & windows area: 40 sq ft
- Number of coats: 2
- Coverage per gallon: 350 sq ft
Results
- Gallons of paint: 3
- Net wall area: 408
- Total painted area (all coats): 816
- Exact gallons before rounding: 2.33
What each field means
Inputs
- Room perimeter (ft)
- The room perimeter used in the calculation, measured in ft. Starts at 56 ft so you have a working example on load.
- Wall height (ft)
- The wall height used in the calculation, measured in ft. Starts at 8 ft so you have a working example on load. Accepted range: 6–20 ft.
- Doors & windows area (sq ft)
- The doors & windows area used in the calculation, measured in sq ft. Starts at 40 sq ft so you have a working example on load.
- Number of coats
- The number of coats used in the calculation. Starts at 2 so you have a working example on load. Accepted range: 1–3.
- Coverage per gallon (sq ft)
- The coverage per gallon used in the calculation, measured in sq ft. Starts at 350 sq ft so you have a working example on load. Accepted range: 200–450 sq ft.
Results
- Gallons of paint
- Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Net wall area
- sq ft
- Total painted area (all coats)
- sq ft
- Exact gallons before rounding
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How much wall area does a gallon of paint cover?
Most quality interior paints cover 350-400 sq ft per gallon on a smooth wall for one coat; 350 is a safer real-world planning number since texture, roller nap, and touch-ups eat into the theoretical maximum.
Do I really need two coats?
Almost always yes for a true color change, dark colors, or patched walls; a single coat over a similar existing color with a quality paint-and-primer product can sometimes suffice, but two coats is the standard for even, streak-free coverage.
Should I subtract doors and windows from wall area?
Yes, subtracting standard openings (a door is roughly 20 sq ft, a window roughly 15 sq ft) keeps you from overbuying, though many painters skip small deductions since trim work and cutting-in still use some paint.
Is it better to round up or buy exactly what's calculated?
Always round up to the next full gallon and consider buying slightly extra of a custom tint color, since color-matching a partial gallon later rarely produces an identical match.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Ceiling Paint Calculator
Figure out gallons of ceiling paint from room dimensions and coat count.
Primer Gallons Calculator
Work out how much primer to buy before you ever open the topcoat can.
Exterior Paint Gallons Calculator
Estimate gallons of exterior house paint from wall dimensions and siding texture.
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Interior Paint Gallons Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/interior-paint-gallons-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/interior-paint-gallons-calculator" target="_blank" rel="noopener">Interior Paint Gallons Calculator — RevenueLab</a> (2026).</p>
Source: [Interior Paint Gallons Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/interior-paint-gallons-calculator) (2026).