Kilz Paint Calculator: How Much Primer You Need

This calculator estimates how much Kilz primer you need for a painting project. You enter the total square footage of the surfaces you plan to prime, the number of coats, and the porosity of the surface (low, medium, or high). The calculator returns the estimated gallons of primer required, along with the number of quarts and liters for easier purchasing.

Formula

The calculation uses the manufacturer’s standard coverage rate for Kilz primers, which is approximately 300 square feet per gallon per coat. The formula is:

Gallons needed = (Total area ÷ 300) × Number of coats × Porosity factor

Where:

• Total area = the sum of all wall, ceiling, or surface areas to be primed, in square feet.
• 300 = the average coverage of one gallon of Kilz primer on a smooth, primed, or previously painted surface (square feet per gallon).
• Number of coats = how many layers of primer you plan to apply (usually 1, sometimes 2 for heavy stains or drastic color changes).
• Porosity factor = an adjustment for how much primer the surface absorbs: 1 for low (smooth, sealed), 1.5 for medium (drywall, wood), 2 for high (bare masonry, unsealed drywall, or heavily stained areas).

Worked Example

Using the defaults from the calculator: a room with 400 square feet of walls, one coat, and medium porosity (drywall).

1. Start with the area: 400 sq ft.
2. Divide by coverage: 400 ÷ 300 = 1.333 gallons for one coat on a smooth surface.
3. Multiply by coats (1) and porosity factor (1.5): 1.333 × 1 × 1.5 = 2.0 gallons.

The calculator rounds to 2.00 gallons. That equals 8 quarts or about 7.57 liters. Most hardware stores sell Kilz in 1-gallon and 5-gallon pails, so you would likely buy a 5-gallon pail (or two 1-gallon cans plus a quart) to have a little extra for touch-ups.

How Kilz Primer Coverage Works

Kilz is a brand of primers made by PPG (Pittsburgh Paints). The actual coverage varies by product line: Kilz Original (oil-based), Kilz 2 (water-based), Kilz Premium (water-based), and Kilz Restoration (for odor and stain blocking). All of them list a coverage of around 300 sq ft per gallon, but that assumes a properly prepared surface and application with a roller or sprayer. If you use a thick nap roller or apply heavily, coverage drops. If you spray, you may lose 10–20% to overspray. The 300 sq ft figure is for a single coat at the recommended film thickness (about 3–4 mils wet).

How to Measure Your Surface Area

To use the calculator accurately, you need the square footage of the surfaces you’ll prime. For walls, measure the length of each wall and multiply by the height (floor to ceiling). Add all walls together. Subtract areas for doors and windows (typically 20 sq ft for a door, 15 sq ft for a standard window). For ceilings, multiply length by width. For trim, measure the linear feet and multiply by the width of the trim (in feet) to get square footage.

Example: a 12×15 ft room with 8 ft ceilings has two walls of 12×8 = 96 sq ft each and two walls of 15×8 = 120 sq ft each, for a total of 432 sq ft. Subtract one door (20) and two windows (30) = 382 sq ft. That’s the number you’d enter.

Choosing the Right Porosity Factor

The porosity factor is the most common source of error. Here’s a quick guide:

• Low (1.0): Painted walls in good condition, glossy surfaces, metal, laminate, or previously primed surfaces. These don’t soak up much primer.
• Medium (1.5): New drywall (joint compound is more porous than paper), bare wood, or previously painted surfaces with a flat finish. This is the default for most rooms.
• High (2.0): Bare masonry, brick, unsealed drywall (especially if you skip the drywall primer), or surfaces with heavy stains (water stains, smoke, graffiti). These can absorb twice as much primer.

Common Mistakes and How to Avoid Them

One big mistake is forgetting to multiply by the number of coats. For example, if you’re priming over a dark red wall to paint it white, you might need two coats of primer. The calculator will show that doubling the coats doubles the gallons. Another mistake is using the wrong area: people often measure the floor area and use that for walls. A 200 sq ft floor can easily have 600 sq ft of walls. Also, don’t round down—always round up to the nearest quart or gallon. It’s better to have leftover primer than to run out mid-project.

Kilz vs. Other Primers: Coverage Differences

Most general-purpose primers (like Zinsser Bullseye 1-2-3 or Behr Premium Plus) also claim about 300 sq ft per gallon. Kilz isn’t unusual in that regard. However, Kilz Original (oil-based) is a stronger stain blocker than water-based primers, so it may require a different number of coats on tough stains. The calculator works for any primer with 300 sq ft coverage, so you can use it even if you choose a different brand. Just check the label on your specific primer—some high-hide primers cover only 250 sq ft per gallon, and some high-build products cover 400 sq ft.

Limitations of This Calculator

This calculator gives a theoretical estimate based on average conditions. Real-world coverage depends on the applicator (roller, brush, sprayer), the texture of the surface (knockdown texture absorbs more than smooth), the temperature and humidity (which affect drying and viscosity), and the skill of the painter. For example, a ½-inch nap roller puts down more primer than a ¼-inch nap. Also, if you’re priming a ceiling, expect to use more primer because you’re working overhead and may apply thicker coats. Always buy 10–15% extra to account for these variables.

Frequently Asked Questions

How many square feet does a gallon of Kilz cover?

Most Kilz primers cover about 300 square feet per gallon per coat. That’s the number used in this calculator. Some specialty formulas (like Kilz Restoration) may cover slightly less due to their thicker consistency, so always check the label.

Do I need to use a primer at all?

Primer is recommended when painting new drywall, bare wood, metal, or when covering stains, odors, or a dramatic color change. It seals the surface, improves adhesion, and helps the topcoat look uniform. If you’re repainting the same color over a sound painted surface, you can skip primer.

Can I use the calculator for exterior surfaces?

Yes, but exterior surfaces like wood siding, stucco, or brick are often more porous and may need a higher porosity factor (2.0). Also, exterior primers sometimes have different coverage rates—check the label. The calculator’s formula still applies if you adjust the porosity.

How do I know if I need two coats of primer?

You need two coats if the surface is heavily stained, if you’re going from a very dark color to a very light one, or if the primer soaks in unevenly (like on new drywall without a separate drywall primer). After the first coat dries, if you can still see stains or the old color bleeding through, apply a second coat.

Is oil-based Kilz better than water-based?

Oil-based Kilz Original is a stronger stain and odor blocker, especially for water stains, smoke, and pet odors. Water-based Kilz 2 and Premium dry faster, clean up with soap and water, and have lower VOCs. For most interior walls, water-based is fine. Use oil-based for tough stains or on surfaces that need maximum sealing.

What if the calculator says I need 0.4 gallons?

That’s about 1.5 quarts. Since Kilz is sold in quart (0.25 gal) and gallon sizes, you’d buy a quart and a gallon, or just a gallon and keep the extra for touch-ups. Never buy exactly the calculated amount—always round up to the nearest available container size.

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in area | Total surface area | sq ft | 400 | 0
in coats | Number of coats | - | 1 | 1
sel porosity | Surface porosity | Low=1, Medium=1.5, High=2
out gallons | Estimated primer | gallons | (area / 300) * coats * porosity | 2
out quarts | Equivalent quarts | qt | gallons * 4 | 2
out liters | Equivalent liters | L | gallons * 3.78541 | 2