Paint Calculator

What is a paint calculator?

A paint calculator estimates how much paint you need to cover a room’s walls, given in both gallons and litres. You enter the room’s length, width and ceiling height, the number of doors and windows, and how many coats you want — and it works out the paintable wall area and the amount of paint to buy. It saves you from guessing at the store, so you avoid both running out mid-wall and over-buying half-empty cans you’ll never use.

How is the paint amount calculated?

The tool works in two steps: first it finds the paintable wall area, then it converts that area into paint volume.

wall area = 2 × (length + width) × ceiling height − doors − windows

The first part, 2 × (length + width) × height, is the perimeter of the room multiplied by the wall height — the total surface of all four walls. From that it subtracts the openings you will not paint:

• 21 sq ft per door (a standard 3 ft × 7 ft door)
• 15 sq ft per window (a typical window opening)

Then it converts area into paint volume:

gallons = wall area × number of coats ÷ 350

One gallon is assumed to cover about 350 sq ft in a single coat, so two coats simply doubles the paint required. The result is also shown in litres using litres = gallons × 3.785, because one US gallon equals 3.785 litres. All lengths are in feet and area is in square feet.

Examples

Here are three examples you can reproduce in the calculator exactly.

Example 1 — 12 × 10 ft room, 9 ft ceiling, 1 door, 2 windows, 2 coats. Gross wall area = 2 × (12 + 10) × 9 = 2 × 22 × 9 = 396 sq ft. Subtract 21 for the door and 30 for the two windows (15 × 2): 396 − 51 = 345 sq ft paintable. Paint = 345 × 2 ÷ 350 = 1.97 gallons (about 7.5 litres).

Example 2 — 15 × 12 ft room, 8 ft ceiling, 2 doors, 3 windows, 1 coat. Gross wall area = 2 × (15 + 12) × 8 = 2 × 27 × 8 = 432 sq ft. Subtract 42 for the doors (21 × 2) and 45 for the windows (15 × 3): 432 − 87 = 345 sq ft paintable. Paint = 345 × 1 ÷ 350 = 0.99 gallons (about 3.7 litres).

Example 3 — 20 × 16 ft room, 10 ft ceiling, 2 doors, 4 windows, 2 coats. Gross wall area = 2 × (20 + 16) × 10 = 2 × 36 × 10 = 720 sq ft. Subtract 42 for the doors and 60 for the windows (15 × 4): 720 − 102 = 618 sq ft paintable. Paint = 618 × 2 ÷ 350 = 3.53 gallons (about 13.4 litres).

How much paint per room size?

The table below shows the paint needed for two coats on a common 9 ft ceiling, with one door and two windows subtracted (51 sq ft removed). Use it as a quick reference, and the tool for your exact figures.

Room size (ft)	Gross wall area	Paintable area	Paint for 2 coats
10 × 10	360 sq ft	309 sq ft	1.77 gal (6.7 L)
12 × 10	396 sq ft	345 sq ft	1.97 gal (7.5 L)
12 × 12	432 sq ft	381 sq ft	2.18 gal (8.2 L)
14 × 12	468 sq ft	417 sq ft	2.38 gal (9.0 L)
16 × 14	540 sq ft	489 sq ft	2.79 gal (10.6 L)

For instance, a 14 × 12 room has a 52 ft perimeter, so its walls are 52 × 9 = 468 sq ft gross and 417 sq ft after the openings — needing 2.38 gallons for two coats.

When should you use a paint calculator?

Use it any time you’re buying paint and want to order the right quantity. Common cases include:

• Repainting a bedroom, living room or office before booking a weekend project.
• Estimating a whole-house repaint by running each room separately and adding the gallons together.
• Comparing one coat versus two, to see how much an extra coat will cost in paint.
• Quoting a job as a painter or handyman who needs a fast, defensible material figure.

It pairs naturally with other estimating tools in our construction calculators collection, like the concrete block calculator for walls or the gravel calculator for outdoor projects.

Tips and common mistakes

A few habits keep your estimate accurate and your order clean:

• Buy a little extra. Round up to the next full gallon (or quart) so you have touch-up paint that matches the batch exactly.
• Add a coat for colour changes. Going from dark to light, or covering bare drywall, often needs primer plus two coats — set coats to 2 or 3 and prime first.
• Don’t subtract small openings. The 21 and 15 sq ft deductions assume standard doors and windows; tiny windows you’ll still paint around add negligible savings.
• Mind the surface. Rough, textured or porous walls drink up paint, so real coverage can drop well below 350 sq ft per gallon.

Limitations and accuracy

This calculator estimates wall paint only and assumes a rectangular room with four flat walls of equal height — the same assumption built into the widget. It does not include the ceiling, trim, or closets, and it uses fixed deductions of 21 sq ft per door and 15 sq ft per window, so unusually large openings will be slightly under-counted. The 350 sq ft per gallon coverage is a typical figure for smooth, primed interior walls; primers, deep colours and textured surfaces can all reduce it.

Treat the result as a planning estimate, not a guarantee. Always check the coverage printed on your specific paint can, and confirm quantities before buying. For irregular rooms, calculate each wall or section separately and add the areas together before dividing by 350.