Paint Calculator Interior: How Much Paint Do You Need?

This paint calculator works out how much paint you need for an interior room. You enter the room’s dimensions, the number of doors and windows, and the number of coats you plan to apply; it returns the total wall area in square feet (or square meters) and the estimated gallons (or liters) of paint required. The result includes a 10% buffer for waste and touch-ups, so you can buy with confidence.

Formula

The calculator uses the standard interior painting formula:

Wall area = (2 × length × height) + (2 × width × height) − (doors × 21 sq ft) − (windows × 15 sq ft)

Paint needed (gallons) = (Wall area × coats) ÷ 350 × 1.1

Where:

• length, width, height are room dimensions in feet
• doors = number of standard interior doors (each ≈ 21 sq ft including frame)
• windows = number of standard windows (each ≈ 15 sq ft)
• coats = number of paint coats (usually 2)
• 350 = square feet covered by one gallon of paint (typical for interior latex on smooth, primed drywall)
• 1.1 adds a 10% buffer for waste, uneven surfaces, and touch-ups

Worked Example

Using the default values from the calculator (length = 12 ft, width = 10 ft, height = 8 ft, 2 doors, 1 window, 2 coats):

1. Calculate the perimeter walls: 2 × 12 × 8 = 192 sq ft (two long walls) and 2 × 10 × 8 = 160 sq ft (two short walls). Total = 192 + 160 = 352 sq ft.
2. Subtract openings: 2 doors × 21 = 42 sq ft; 1 window × 15 = 15 sq ft. Total openings = 57 sq ft.
3. Net wall area = 352 − 57 = 295 sq ft.
4. Multiply by coats: 295 × 2 = 590 sq ft of coverage needed.
5. Divide by coverage per gallon: 590 ÷ 350 = 1.6857 gallons.
6. Add 10% waste: 1.6857 × 1.1 = 1.854 gallons.

The calculator rounds to 1.85 gallons, which means you would need 2 gallons for the job (since paint is sold in whole gallons).

How to Measure Your Room Correctly

Accurate measurements are the foundation of a good estimate. Here is what to do:

• Measure the length and width of the room at the baseboard level, in feet. If you have an L-shaped room, divide it into rectangles and measure each separately.
• Measure the wall height from the floor to the ceiling. For most homes this is 8 ft, but older or custom homes may have 9 ft or 10 ft ceilings.
• Count all doors and windows that are fully inside the wall area you are painting. Standard interior doors are about 6.67 ft × 3 ft = 20 sq ft; the calculator uses 21 sq ft to include the frame. Standard windows are roughly 3 ft × 5 ft = 15 sq ft.
• If you are painting a ceiling, measure the floor area (length × width) and use a separate calculation – this calculator is for walls only.
• For trim, baseboards, and crown molding, add about 10% to your total or measure their linear footage and multiply by the height of the trim (usually 0.33 ft for baseboards).

Coverage Rates and Paint Types

The 350 sq ft per gallon figure is a typical average for interior latex paint on smooth, primed drywall. However, actual coverage varies:

• Smooth, primed drywall: 350–400 sq ft per gallon
• Textured walls (e.g., orange peel, knockdown): 250–300 sq ft per gallon
• Unprimed drywall or bare wood: 300–350 sq ft per gallon (more absorbent)
• Dark or bold colors: may require an extra coat or a tinted primer, reducing effective coverage
• High-quality paints often have higher solids content and cover better, but always check the can’s label for the manufacturer’s stated coverage.

If your walls are rough, unpainted, or you are making a drastic color change (e.g., from dark red to white), add an extra coat or use a primer. The calculator’s 10% buffer helps, but it is not a substitute for proper surface preparation.

Common Mistakes and How to Avoid Them

Even with a calculator, people make errors. Here are the most frequent ones:

• Forgetting to subtract doors and windows. A room with many openings can have 15–20% less wall area than a blank box. The calculator does this for you, but if you measure an existing room, remember to count.
• Mixing units. If you measure in meters, do not use the same numbers in a formula designed for feet. This calculator expects feet; if you have meters, convert first (1 m = 3.281 ft).
• Rounding too early. Round your final answer, not intermediate steps. For example, 1.85 gallons rounds to 2 gallons, but if you rounded 1.68 to 1.7 and then added waste, you would get 1.87 – still 2 gallons, but for larger rooms the error can push you to buy an extra gallon unnecessarily.
• Ignoring the number of coats. A single coat covers far less than two. Most interior paints require two coats for even color and durability, especially if you are covering a lighter or darker shade.
• Not adding a waste buffer. Paint gets spilled, dripped, and absorbed unevenly. The 10% buffer is a safety margin – do not remove it unless you are an experienced painter.

Cost Considerations (Without Specific Prices)

Paint prices vary widely by brand, quality, and finish. Generally, premium paints cost more per gallon but may require fewer coats and last longer. A gallon of interior paint typically covers 350 sq ft, so a 12×10 room with 8 ft ceilings and two coats needs about 2 gallons. For a larger room (say 16×14), you might need 3–4 gallons. To estimate cost, multiply the number of gallons by the price per gallon at your local store. Remember that primer is separate – if you need to prime bare drywall or stained walls, add about 1 gallon of primer per 400 sq ft of wall area.

Also consider the cost of supplies: brushes, rollers, painter’s tape, drop cloths, and trays. These are not included in the calculator but are essential for the job. Many stores allow you to return unopened paint cans, so buying one extra gallon is safer than running out mid-project.

Safety and Preparation Tips

Interior painting is generally safe, but follow these precautions:

• Ventilation: Open windows and use fans to reduce fumes, especially if using oil-based paints. Water-based latex paints have lower VOCs, but still ventilate.
• Protective gear: Wear gloves and old clothing. Use a respirator or mask if you are sensitive to fumes or sanding dust.
• Lead paint: If your home was built before 1978, the paint may contain lead. Do not sand or scrape old paint without testing for lead and following EPA guidelines.
• Ladder safety: Use a sturdy step ladder, never stand on the top step, and keep your hips between the rails.
• Cleanup: Clean brushes and rollers with soap and water for latex paint, or mineral spirits for oil-based. Dispose of paint responsibly – never pour it down drains.

Frequently Asked Questions

How much paint do I need for a 12×12 room?

Assuming 8 ft ceilings, 1 door, 1 window, and 2 coats, the wall area is (2×12×8 + 2×12×8) − (21 + 15) = 384 − 36 = 348 sq ft. With 2 coats, you need 348 × 2 = 696 sq ft of coverage. Divide by 350 = 1.99 gallons, plus 10% waste = 2.19 gallons, so you would buy 2 gallons (or 3 if you want extra for touch-ups).

Does the calculator include ceilings?

No, this calculator is for walls only. To estimate ceiling paint, multiply the room’s length by width to get the ceiling area (e.g., 12 × 10 = 120 sq ft). Divide by 350 per gallon, add waste, and round up. Most ceilings need one coat of ceiling paint, but if you are covering stains, you may need a second coat.

What if my walls have texture?

Textured walls (popcorn, knockdown, etc.) absorb more paint, reducing coverage to about 250–300 sq ft per gallon. You can adjust by using a lower coverage rate in the formula. For a heavily textured wall, increase the paint estimate by 20–30%.

How do I convert from gallons to liters?

One US gallon is approximately 3.785 liters. If your paint is sold in liters, multiply the gallons by 3.785. For example, 2 gallons ≈ 7.57 liters, so you would buy an 8-liter can or two 4-liter cans.

Why does the calculator add 10% waste?

The 10% buffer accounts for paint lost to roller absorption, spills, uneven application, and future touch-ups. It is a standard industry practice to avoid running out mid-job. If you are an experienced painter and your walls are smooth, you might reduce it to 5%, but for most DIYers, 10% is prudent.

Can I use this calculator for exterior painting?

No, exterior painting requires different calculations because you are covering siding, trim, and possibly masonry, which have different coverage rates. Exterior paint also covers less (about 250–300 sq ft per gallon) due to rough surfaces. Use a dedicated exterior paint calculator.

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in l | Room length | ft | 12 | 0
in w | Room width | ft | 10 | 0
in h | Wall height | ft | 8 | 0
in d | Number of doors | - | 2 | 0
in win | Number of windows | - | 1 | 0
in coats | Coats of paint | - | 2 | 1
out area | Wall area (no openings) | sq ft | (2*l*h + 2*w*h) - (d*21 + win*15) | 0
out gal | Paint needed (incl. 10% waste) | gallons | (area * coats / 350) * 1.1 | 2