Interior Paint Calculator: Square Feet Guide

An interior paint calculator for square feet works out how much paint you need for a room by taking your wall and ceiling dimensions and subtracting areas you won’t paint, like doors and windows. You enter the room’s length, width, and height, plus the number of doors and windows, and the calculator returns the total paintable area in square feet and the estimated gallons of paint required. This guide explains the math behind the calculator, how to use it correctly, and common pitfalls to avoid so you buy the right amount of paint the first time.

Formula

The calculator uses a standard formula that estimates the paintable wall area of a rectangular room:

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

Where:

• length (l) – the room’s longest wall dimension, in feet
• width (w) – the room’s shorter wall dimension, in feet
• height (h) – floor-to-ceiling height, in feet
• doors – number of standard interior doors (each subtracts about 21 sq ft)
• windows – number of standard windows (each subtracts about 15 sq ft)

The first part, 2 × h × (l + w), is the perimeter of the room (2l + 2w) times the height, which gives the total wall area. Then you subtract the areas you won’t paint: a typical interior door is about 3 ft × 7 ft = 21 sq ft, and a typical window is about 3 ft × 5 ft = 15 sq ft. The result is the net wall area in square feet.

To convert to paint volume, divide by the coverage rate of the paint, usually 350 square feet per gallon for one coat. Most paint brands state a coverage of 250–400 sq ft per gallon, so 350 is a reasonable middle ground.

Worked Example

Using the default values from the calculator – a room 12 ft long, 10 ft wide, 8 ft high, with 2 doors and 1 window – we can walk through the calculation step by step.

1. Calculate the perimeter: 2 × (12 + 10) = 2 × 22 = 44 ft
2. Multiply by height for total wall area: 44 × 8 = 352 sq ft
3. Subtract doors: 2 × 21 = 42 sq ft
4. Subtract windows: 1 × 15 = 15 sq ft
5. Net paintable area: 352 − 42 − 15 = 295 sq ft
6. Paint needed for one coat: 295 ÷ 350 = 0.84 gallons

So for this room, you’d need about 0.84 gallons for one coat, or roughly 1.7 gallons for two coats. Since paint is sold in full gallons, you’d buy 1 gallon for one coat or 2 gallons for two coats.

How to Use the Calculator Correctly

To get an accurate estimate, measure your room carefully with a tape measure. Follow these steps:

• Measure the length and width of the room at the baseboard level, in feet. If you have an L-shaped room, break it into rectangles and calculate each separately, then add the areas.
• Measure the height from the floor to the ceiling. Standard ceilings are 8 ft, but older homes may have 9 or 10 ft ceilings.
• Count the number of doors and windows that are in the walls you plan to paint. If you’re not painting the ceiling, don’t include it in the calculation.
• For non-standard doors or windows (e.g., sliding glass doors, large picture windows), adjust the subtraction: measure the actual width × height and subtract that number instead of the default 21 or 15.
• If you’re painting the ceiling, add the ceiling area (length × width) to the wall area before converting to gallons.

The calculator gives a rough estimate. For a more precise number, measure the actual door and window dimensions and adjust the subtraction accordingly.

Common Mistakes and How to Avoid Them

Several errors can throw off your paint estimate:

• Mixing units – Always use feet for length, width, and height. If you measure in inches, convert to feet by dividing by 12. Mixing meters and feet will give a wildly wrong answer.
• Forgetting to subtract doors and windows – This can overestimate paint by 10–20%. A standard room with two doors and one window has about 57 sq ft of non-paintable area, which is roughly 15% of the wall area.
• Not accounting for multiple coats – The calculator gives the amount for one coat. If you’re painting a darker color over a lighter one, or using a primer, you’ll need more coats. Multiply the gallons by the number of coats.
• Ignoring texture and porosity – Rough surfaces like stucco or unpainted drywall absorb more paint. If your walls are textured, add 10–20% to the final estimate.
• Rounding too early – Always keep the full decimal until the end, then round up to the nearest gallon. You can’t buy a fraction of a gallon, so always round up.

When to Use This Calculator

This calculator is ideal for planning a painting project, whether you’re a DIY homeowner or a contractor estimating a job. Use it to:

• Determine how many gallons of paint to buy for a single room or an entire house.
• Compare the cost of different paint brands by multiplying the gallons by the price per gallon.
• Plan a budget for supplies – knowing the square footage helps you estimate primer, paint, and even the amount of painter’s tape needed.

It’s also useful for other projects like wallpapering, where you need the same wall area calculation.

Limitations of the Formula

The formula assumes a rectangular room with standard doors and windows. Real rooms have variations:

• Cathedral ceilings or vaulted ceilings add extra wall area that the simple height × perimeter formula doesn’t capture.
• Built-in bookshelves, fireplaces, or wainscoting reduce the paintable area.
• Sliding glass doors are much larger than a standard door – subtract their actual area.
• Bay windows or alcoves add to the perimeter, so you’d need to measure those separately.

For such irregular rooms, it’s best to measure each wall individually and add the areas, then subtract the non-paintable sections. The calculator is a starting point, not a substitute for careful measurement.

Frequently Asked Questions

How many square feet does a gallon of paint cover?

Most paint cans state a coverage of 250 to 400 square feet per gallon for one coat. The industry standard is 350 sq ft per gallon, which is what this calculator uses. However, the actual coverage depends on the paint’s viscosity, the surface texture, and the application method. A roller typically covers more than a brush, and a smooth surface covers more than a textured one.

Should I include the ceiling in the paint calculation?

Only if you’re painting the ceiling. If you’re painting only the walls, leave the ceiling out. If you are painting the ceiling, add the length × width (ceiling area) to the wall area. For the default example, the ceiling would add 120 sq ft, making the total 415 sq ft, which would require about 1.2 gallons for one coat.

How much paint do I need for two coats?

Simply multiply the one-coat amount by 2. For the default example, 0.84 gallons × 2 = 1.68 gallons, so you’d buy 2 gallons. If you’re painting a light color over a dark one, you might need three coats, so multiply accordingly.

What if my room has different wall heights?

If your room has a tray ceiling or a partial wall, measure each wall segment individually. Calculate the area of each segment (width × height) and add them together. Then subtract doors and windows. This is more accurate than using a single height.

Does primer count as a coat?

Yes, primer is a coat and covers roughly the same square footage as paint, so you’ll need the same amount. If you’re priming new drywall or covering a stain, you’ll need a separate gallon of primer. Some paints are self-priming, but they still require a full coat.

Why does the calculator say I need 0.84 gallons but I can only buy full gallons?

Paint is sold in full gallons (or quarts in some regions). Always round up to the nearest full gallon. For 0.84 gallons, you’d buy 1 gallon. For two coats, 1.68 gallons rounds up to 2 gallons. It’s better to have a little extra for touch-ups than to run out mid-project.

Related

• Davies Paint Calculator: How It Works
• Cubic Yard Calculator Sand: Volume & Weight
• Crown Paint Calculator: How Much Paint Do You Need?

in l | Room length | ft | 12 | 0
in w | Room width | ft | 10 | 0
in h | Room height | ft | 8 | 0
in d | Number of doors | - | 2 | 0
in win | Number of windows | - | 1 | 0
out area | Painable area | sq ft | (2 * h * (l + w)) - (d * 21) - (win * 15) | 0
out gallons | Paint needed (1 coat) | gallons | area / 350 | 1