Paint Footage Calculator: How Much Paint Do I Need?

This paint footage calculator works out the total wall area you need to cover and estimates the gallons (or liters) of paint required for one or two coats. You enter your room dimensions, the number of doors and windows, and the number of coats, and it returns the paintable square footage and the paint volume needed. It uses the standard rule that one gallon covers 350 square feet with a single coat.

Formula

The calculator uses the standard interior painting formula:

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

Then:

Paint (gallons) = Wall area ÷ 350 × number of coats

Where:

• length – room length in feet
• width – room width in feet
• height – wall height (floor to ceiling) in feet
• doors – number of standard interior doors (each subtracts ~21 sq ft)
• windows – number of standard windows (each subtracts ~15 sq ft)
• coats – number of paint coats you plan to apply
• 350 – average coverage of one gallon of latex paint on primed drywall (sq ft per gallon)

If you measure in meters, multiply the final gallons by 3.785 to get liters, or use the metric equivalent: 1 liter covers about 8.5 square meters per coat.

Worked Example

Using the default values: room 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 = 352 sq ft.
2. Subtract doors: 2 × 21 = 42 sq ft. Subtract windows: 1 × 15 = 15 sq ft. Adjusted area = 352 − 42 − 15 = 295 sq ft.
3. Paint for one coat: 295 ÷ 350 = 0.843 gallons.
4. For two coats: 0.843 × 2 = 1.686 gallons.

So you need about 1.69 gallons for two coats. In practice, you would buy 2 gallons (a full gallon plus a quart, or two gallons if you want extra for touch-ups).

Why 350 Square Feet Per Gallon?

The 350 sq ft per gallon figure is the industry standard for latex paint on smooth, primed drywall. It comes from the average spread rate that most paint manufacturers list on their cans. However, actual coverage varies:

• Primed drywall – about 350–400 sq ft per gallon
• Unprimed drywall – absorbs more, only 250–300 sq ft per gallon
• Textured walls – 200–250 sq ft per gallon
• Rough stucco or brick – 100–150 sq ft per gallon
• Previously painted smooth walls – 350–400 sq ft per gallon

If your walls are heavily textured or very porous, reduce the coverage number by 20–30% in your own estimate. The calculator assumes smooth, primed surfaces.

How to Measure Your Room Correctly

Accurate measurements matter more than the math. Here are the steps:

1. Measure the length and width of the room at the baseboard level, in feet. Round to the nearest 0.1 ft.
2. Measure the wall height from floor to ceiling. If you have crown molding or a tray ceiling, measure to the highest point of the wall.
3. Count the number of standard interior doors (about 3 ft × 6.5 ft = ~20 sq ft each, we use 21 to include the frame).
4. Count the number of standard windows (about 3 ft × 5 ft = ~15 sq ft each). For larger picture windows, measure and subtract the actual area.
5. Decide how many coats you need. One coat for a same-color repaint, two coats for a color change or over primer, three coats for dramatic color shifts or covering dark colors.

Do not subtract areas for baseboards, trim, or outlets – they are small and the extra paint is useful for touch-ups.

Common Mistakes and Misconceptions

One frequent error is forgetting to multiply the perimeter walls by 2. The formula uses 2 × length + 2 × width because a rectangular room has two of each wall. Another mistake is subtracting doors and windows twice – you only subtract them once from the total wall area.

A widespread misconception is that a gallon of paint always covers exactly 400 sq ft. That number comes from the theoretical maximum on a perfectly smooth, non-porous surface. Real walls are rarely that ideal, so 350 is a safer average. Also, paint coverage is usually listed for one coat – if you apply two coats, you need double the paint (as the calculator does).

Another common issue: people measure the floor area and multiply by something like 4 to get wall area. That only works if the room is a perfect cube and has no doors or windows – which never happens. The perimeter formula is more accurate.

Finally, don’t forget that the ceiling is not included in this calculation. If you plan to paint the ceiling, add its area separately (length × width) and use the same coverage rate.

How to Use the Result

The calculator gives you a decimal number of gallons (e.g., 1.69). Paint is sold in gallons (US) or liters (metric). You cannot buy 1.69 gallons, so round up to the nearest container size:

• If the result is under 1 gallon, buy 1 gallon (or a quart if it’s less than 0.25 gal).
• If it’s between 1 and 2 gallons, buy 2 gallons.
• If it’s between 2 and 5 gallons, buy the number of full gallons that covers it, plus one extra quart for touch-ups.

For large projects, paint is often sold in 5-gallon buckets, which are cheaper per gallon. If your total is over 4 gallons, consider buying a 5-gallon bucket and a quart for small areas.

Always add 10% extra for waste, drips, and future touch-ups. For the example above, 1.69 gallons × 1.1 = 1.86 gallons, so 2 gallons is the right purchase.

When to Use a More Complex Estimate

This calculator works well for rectangular rooms with standard doors and windows. For irregular rooms (L-shaped, vaulted ceilings, multiple window sizes), you should break the room into rectangular sections and calculate each separately, then add the areas.

If you are painting cabinets, furniture, or exterior siding, the coverage rate changes. Exterior paint covers less because of rough surfaces – typically 250–300 sq ft per gallon. Primer also covers less than paint – about 300 sq ft per gallon. For those cases, adjust the coverage number in your own calculation.

Also note that the calculator assumes you are painting only the walls, not the ceiling or trim. If you are painting trim, add about 20% more paint or buy a separate smaller can.

Frequently Asked Questions

Does this calculator include the ceiling?

No, it only calculates wall area. To include the ceiling, multiply the room length by the width and add that to the wall area. A 12×10 ft ceiling is 120 sq ft, which would need about 0.34 gallons for one coat.

Why does my paint can say 400 sq ft coverage, but the calculator uses 350?

400 sq ft is the theoretical maximum on a perfectly smooth, non-porous surface. In real homes, walls have texture, patch spots, and varying porosity, so 350 is a more realistic average. Using 350 also gives you a small buffer for touch-ups.

How many coats do I really need?

If you are painting over the same color with a similar finish, one coat may suffice. For a color change, especially from dark to light or light to dark, two coats are standard. If you are covering a bold color like red or black, you may need three coats or a tinted primer first.

What if my room has large windows or a sliding glass door?

Measure the actual width and height of the glass and frame, multiply to get square feet, and subtract that number instead of the standard 15 sq ft per window. For a sliding door, treat it as a window and subtract its full area.

Should I buy extra paint for future touch-ups?

Yes, it is wise to buy 10% more than the calculated amount. Store the leftover paint in a labeled jar with the room name and date. It will last for years if sealed properly, and it will match exactly when you need to patch a scuff.

Does the type of paint (matte, eggshell, satin) change the coverage?

Not significantly. The main factor is the surface texture, not the sheen. However, flat paints tend to be slightly more porous and may absorb a bit more, while glossy paints cover slightly less due to their higher resin content. The difference is usually less than 10%.

Related

• Drywall Takeoff Calculator: How to Estimate
• Duct Bank Concrete Calculator: Volume & Cost
• Drywall Repair Estimate Calculator: Costs & Pricing

in length | Room length | ft | 12 | 0
in width | Room width | ft | 10 | 0
in height | Wall height | ft | 8 | 0
in doors | Number of doors | - | 2 | 0
in windows | Number of windows | - | 1 | 0
in coats | Coats of paint | - | 2 | 1
out area | Paintable wall area | sq ft | (2 * length * height + 2 * width * height) - (doors * 21 + windows * 15) | 0
out paint_gal | Paint needed (1 coat) | gallons | area / 350 | 2
out paint_total | Paint needed (all coats) | gallons | (area / 350) * coats | 2