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 primer
2
Net wall area
408
sq ft
Exact gallons before rounding
1.36
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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 coats of primer.
- 5Enter coverage per gallon (sq ft).
- 6Read your gallons of primer on the right — it updates as you type.
- 7Hit Share to keep the scenario or send it to someone.
About this calculator
Primer typically covers slightly less area per gallon than finish paint — usually 300-350 sq ft per gallon versus 350-400 for topcoat — because primer is formulated to soak into and seal porous or patched surfaces like new drywall, bare wood, or repaired plaster rather than to lay flat and glossy. This calculator works exactly like an interior paint calculator: take your wall area (from perimeter times height, minus door and window openings), decide whether you need one coat of primer (standard for most repaints) or two (recommended for major color changes, stains, or bare drywall that will otherwise drink up your finish coat unevenly), then divide by the coverage rate and round up. Skipping or skimping on primer is one of the most common reasons a paint job needs a surprise third coat of finish paint, since unprimed patches and bare drywall absorb paint at a different rate than the surrounding wall, leaving visible flashing once it dries. Tinted primer (gray for dark topcoats, white for light ones) can also reduce the number of topcoat coats needed, which is worth factoring into your total project cost.
Worked example
Using the values the calculator loads with:
Inputs
- Room perimeter: 56 ft
- Wall height: 8 ft
- Doors & windows area: 40 sq ft
- Coats of primer: 1
- Coverage per gallon: 300 sq ft
Results
- Gallons of primer: 2
- Net wall area: 408
- Exact gallons before rounding: 1.36
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.
- Coats of primer
- The coats of primer used in the calculation. Starts at 1 so you have a working example on load. Accepted range: 1–2.
- Coverage per gallon (sq ft)
- The coverage per gallon used in the calculation, measured in sq ft. Starts at 300 sq ft so you have a working example on load. Accepted range: 150–400 sq ft.
Results
- Gallons of primer
- 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
- 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
Do I always need to prime before painting?
Not always — repainting a similar color over sound existing paint often doesn't need primer, but new drywall, bare wood, stains, major color changes, and patched areas all benefit significantly from a primer coat first.
Why does primer cover less area per gallon than paint?
Primer is formulated to be absorbed into porous surfaces like raw drywall or wood, which means more of it soaks in rather than sitting on the surface, reducing effective coverage compared to a sealed-surface topcoat.
Can tinted primer reduce the number of topcoat coats I need?
Yes, having your primer tinted toward your final color (gray-tinted for dark colors, for example) can let you get full coverage in one topcoat instead of two, which can offset the extra cost and time of priming.
Is primer necessary over glossy paint?
Yes, primer or at minimum a deglossing step is important over high-gloss or semi-gloss existing paint since new paint won't adhere well to a slick glossy surface without it.
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
Interior Paint Gallons Calculator
Convert wall square footage into the exact number of gallons and coats you need to buy.
Ceiling Paint Calculator
Figure out gallons of ceiling paint from room dimensions and coat count.
Drywall Sheets Calculator
Convert wall and ceiling square footage into the number of 4x8 drywall sheets to order.
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Primer Gallons Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/primer-gallons-calculator
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/primer-gallons-calculator" target="_blank" rel="noopener">Primer Gallons Calculator — RevenueLab</a> (2026).</p>
Source: [Primer Gallons Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/primer-gallons-calculator) (2026).