Lowe’s Paint Calculator: How Much Paint You Need

The Lowe’s paint calculator is a free online tool that estimates the number of gallons of paint you need for a room or exterior surface. You enter the room’s dimensions (length, width, height) and the number of doors and windows, and it returns the total square footage to cover and the recommended paint quantity. This article explains how the calculator works, how to use it correctly, and what to do when your project doesn’t fit the standard assumptions.

Formula

The Lowe’s paint calculator uses the standard wall-painting formula:

Total wall area (sq ft) = (2 × (length + width) × height) − (door area × number of doors) − (window area × number of windows)

Paint needed (gallons) = (Total wall area ÷ 350) × number of coats

Variables and units:

• Length, width, height – room dimensions in feet (or meters, if you convert first).
• Door area – assumed to be 21 square feet per standard interior door.
• Window area – assumed to be 12 square feet per standard window.
• 350 – the coverage rate in square feet per gallon for one coat of typical latex paint on primed drywall.
• Coats – the number of paint layers you plan to apply (usually 2 for a color change, 1 for a repaint of the same color).

Worked Example

Using the default values from the calculator above (room 12 ft × 10 ft with 8 ft ceilings, 1 door, 1 window, 2 coats):

1. Calculate the perimeter: 2 × (12 + 10) = 44 ft.
2. Multiply by ceiling height: 44 × 8 = 352 sq ft (gross wall area).
3. Subtract door area: 352 − 21 = 331 sq ft.
4. Subtract window area: 331 − 12 = 319 sq ft (net paintable area).
5. Divide by coverage per gallon: 319 ÷ 350 = 0.911 gallons for one coat.
6. Multiply by coats: 0.911 × 2 = 1.82 gallons.

The calculator rounds this to 1.8 gallons. Since paint is sold in whole gallons (and sometimes quarts), you would buy 2 gallons for this room. If you were only doing one coat, you’d need just 1 gallon (0.91 gallons rounds to 1).

How the Lowe’s Calculator Differs from Ours

The actual Lowe’s website calculator asks for the same basic inputs (room length, width, height, number of doors and windows) and applies the same 350 sq ft per gallon coverage rate. The main difference is that Lowe’s also lets you choose the number of coats and sometimes includes a ceiling paint option. Our version above adds the number of coats directly and gives a decimal result so you can see exactly how close you are to the next gallon. Both tools assume you are painting walls only, not trim, ceilings, or textured surfaces.

How to Measure Your Room Correctly

Accurate measurements are the most important part of using any paint calculator. Here’s what to do:

• Measure the length and width of the room at the floor, in feet, using a tape measure. Round to the nearest 0.5 ft.
• Measure the ceiling height from floor to ceiling. Standard US ceilings are 8 ft, but many newer homes have 9 or 10 ft.
• Count the number of interior doors that are fully in the room. A closet door counts if it’s on the wall you’re painting.
• Count the number of windows that are fully in the room. Bay windows or sliding glass doors should be counted as windows if they’re larger than a typical window.
• For non-rectangular rooms (L-shaped, alcoves), break the space into rectangles, calculate each separately, and add the results.

A common mistake is measuring the room’s floor area (length × width) instead of the wall area. The calculator needs the perimeter (2 × (length + width)) multiplied by height. If you enter 120 sq ft (the floor area) instead of 352 sq ft (the wall area), you’ll underbuy by about 70%.

When the Calculator Underestimates

The 350 sq ft per gallon coverage rate is an average for smooth, primed drywall. Several situations will make you need more paint than the calculator predicts:

• Rough or porous surfaces – unprimed drywall, old plaster, brick, or stucco can absorb 20–50% more paint.
• Dark or bold color changes – going from dark to light (or vice versa) often requires 3 coats instead of 2.
• Textured walls – knockdown or orange peel texture increases surface area and paint consumption.
• Unprimed new drywall – the first coat acts as a primer and soaks in much more than subsequent coats.
• Heavy roller nap – a thick roller leaves more paint on the wall per pass, which can reduce coverage per gallon.

If any of these apply, add 10–20% to the final number. For example, if the calculator says 1.8 gallons for a textured wall, plan on 2.2 gallons (round up to 3).

What About Ceilings, Trim, and Primer?

The standard Lowe’s calculator and our version above only estimate wall paint. Ceilings, baseboards, door frames, and window trim are separate surfaces with their own coverage calculations:

• Ceiling – multiply length × width to get the ceiling area. A 12×10 ft room has 120 sq ft of ceiling. At 350 sq ft per gallon, one gallon covers about 2.9 coats of that ceiling. Most people need 1 gallon for a single coat of ceiling paint.
• Trim – measure the linear feet of baseboard, casing, and crown molding. A typical 12×10 room has about 44 linear feet of baseboard. At 4–6 inches wide, that’s roughly 15–20 sq ft. A quart of trim paint usually suffices for one coat.
• Primer – if you’re painting new drywall, stained wood, or a drastic color change, use a separate primer. Primer covers about 300–400 sq ft per gallon. For the example room, one gallon of primer is enough for two coats on the walls.

If you’re painting the ceiling the same color as the walls, add the ceiling area to the wall area before dividing by 350. That changes the example from 319 sq ft to 439 sq ft, requiring 2.5 gallons for two coats.

Common Mistakes and How to Avoid Them

Even with a calculator, these errors lead to too little or too much paint:

1. Mixing units – if you measure in feet but enter inches, the result will be wildly off. Stick to one unit throughout.
2. Forgetting the second coat – most paint jobs need two coats for even color and durability. The calculator includes this, but if you’re repainting the same color, one coat may be enough – adjust the coats field accordingly.
3. Ignoring doors and windows – subtracting their areas is standard, but if you have a large sliding glass door (say 6 ft × 7 ft = 42 sq ft), the default 12 sq ft per window is too small. Manually subtract the actual area.
4. Rounding too early – always keep the decimal until the end. Rounding 1.82 gallons to 2 gallons is fine, but rounding 1.82 to 1.8 and then to 1 gallon would be a mistake.
5. Not buying extra for touch-ups – even a perfect calculation leaves no paint for future repairs. Buy at least one extra quart, or 10% more, for the same batch color.

Another misconception is that a gallon of paint always covers exactly 350 sq ft. In reality, coverage varies by brand, sheen, and application method. The 350 figure is a safe average – most premium paints claim 400 sq ft per gallon, but actual coverage on textured walls can drop to 250 sq ft. When in doubt, buy 10% more than the calculator says.

Frequently Asked Questions

Does the Lowe’s paint calculator include ceilings?

No, the standard Lowe’s calculator only estimates wall paint. It does not have a ceiling option in its basic form. You must calculate ceiling area separately (length × width) and add it to the wall area if you plan to paint the ceiling.

How many gallons do I need for a 12×12 bedroom?

Using the formula with an 8 ft ceiling, 1 door, 1 window, and 2 coats: perimeter = 2×(12+12)=48 ft, wall area = 48×8=384 sq ft, minus door (21) and window (12) = 351 sq ft. Divide by 350 = 1.0 gallon per coat, times 2 coats = 2.0 gallons. You would buy 2 gallons.

Should I use primer before painting?

Primer is recommended for new drywall, stained surfaces, or when changing from dark to light colors. A gallon of primer covers about 350 sq ft, so for a typical 12×10 room, one gallon is enough for one coat. If you skip primer, you may need an extra coat of paint, so the calculator’s two-coat estimate may become three.

Why does the calculator say I need 1.8 gallons but the store sells only whole gallons?

Paint is sold in gallons and quarts. For 1.8 gallons, you would buy 2 gallons (or 1 gallon + 3 quarts, if available). It’s always better to have a little extra than to run out mid-project – you can save the leftover for touch-ups.

Can I use the calculator for exterior painting?

Yes, but the coverage rate may differ. Exterior surfaces like siding, stucco, or brick are more porous and often require more paint. Use the same formula but reduce the coverage to 250–300 sq ft per gallon, and subtract the areas of doors and windows as usual.

What if my room has vaulted ceilings?

For vaulted or cathedral ceilings, measure the height at the tallest point and the width at the base. The wall area calculation becomes more complex – you may need to break the wall into rectangles and triangles. A simpler approach is to measure the actual square footage of each wall with a tape measure and add them up, then subtract doors and windows.

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in length | Room Length | ft | 12 | 0
in width | Room Width | ft | 10 | 0
in height | Room Height | ft | 8 | 0
in doors | Number of Doors | - | 1 | 0
in windows | Number of Windows | - | 1 | 0
in coats | Number of Coats | - | 2 | 1
out area | Total Wall Area | sq ft | (2 * (length + width) * height) - (doors * 21) - (windows * 12) | 0
out paint | Paint Needed | gallons | area / 350 * coats | 1