# How much paint do I need?

Computes how many gallons (or litres) of paint a room needs from its size, less doors and windows, with the number of coats, the coverage per gallon and an extra share for waste.

- Page: https://www.acalculator.org/construction/paint-calculator
- JSON spec: https://www.acalculator.org/construction/paint-calculator.json
- Version: ff68a0191a14

## Default answer

Example with the default inputs (Room length 12 ft, Room width 14 ft, Wall height 8 ft, Coats 2, Coverage per gallon 350 ft², Extra for waste 10%, Doors 1, Area of one door 20 ft², Windows 2, Area of one window 15 ft², Paint the ceiling too No): Paint needed for 366 ft²: 2.3 gal (8.71 L); whole gallons to buy: 3.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| l | Room length | The length of the room, wall to wall. |
| w | Room width | The width of the room, wall to wall. |
| h | Wall height | The height of the walls, floor to ceiling. |
| coats | Coats | How many coats of paint. |
| cov | Coverage per gallon | The area one US gallon of paint covers in one coat, as printed on the can. |
| extra | Extra for waste | Paint to add, in percent, for rough walls, touch-ups and paint left in the tray. |
| doors | Doors | How many doors are in the walls (not painted). |
| da | Area of one door | The area of one door: 20 ft² for a 36 × 80 in door. |
| win | Windows | How many windows are in the walls (not painted). |
| wa | Area of one window | The area of one window: 15 ft² for a 3 × 5 ft window. |
| ceil | Paint the ceiling too | Whether to add the ceiling (length × width) to the area, with the same paint and coats. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| gallons | Gallons of paint | The paint needed, in US gallons: area × coats × (1 + extra) ÷ coverage. |
| buy | Gallons to buy | The paint needed rounded up to whole gallons. |
| liters | Litres of paint | The paint needed, in litres. |
| squareFeet | Area to paint, ft² | The wall area less doors and windows, plus the ceiling if chosen, for one coat. |
| squareMeters | Area to paint, m² | The same area in square metres. |

## Method

Area = 2 × (length + width) × height − doors × door area − windows × window area (+ length × width for the ceiling); gallons = area × coats × (1 + extra ÷ 100) ÷ coverage per gallon.

## Assumptions

- The room is a rectangle with walls of one height; each coat covers the area printed on the can.
- Doors and windows are not painted. By default a door is 20 ft² (36 × 80 in) and a window 15 ft² (3 × 5 ft).
- Extra for waste is 10% by default, for rough walls, touch-ups and paint left in the tray.
- Gallons are US gallons (3.785411784 L).

## Worked examples

1. l = 3.6576, w = 4.2672, h = 2.4384, coats = 2, cov = 32.516064, extra = 10%, doors = 1, da = 1.858061, win = 2, wa = 1.393546, ceil = no gives squareFeet = 366, squareMeters = 34.002513, gallons = 2.300571, buy = 3, liters = 8.70861. Source: NIST Handbook 44, Appendix C (1 US gallon = 3.785411784 L, 1 ft = 0.3048 m).
2. l = 3.048, w = 3.048, h = 2.4384, coats = 1, cov = 37.161216, extra = 0%, doors = 0, da = 1.858061, win = 0, wa = 1.393546, ceil = yes gives squareFeet = 420, gallons = 1.05, buy = 2. Source: 2 × 20 × 8 + 10 × 10 = 420 ft² ÷ 400 = 1.05 gal (NIST Handbook 44 units).
3. l = 3.048, w = 3.048, h = 2.667, coats = 1, cov = 32.516064, extra = 0%, doors = 0, da = 1.858061, win = 0, wa = 1.393546, ceil = no gives squareFeet = 350, gallons = 1, buy = 1. Source: 2 × 20 × 8.75 = 350 ft² ÷ 350 = 1 gal (NIST Handbook 44 units).
4. l = 4, w = 3, h = 2.5, coats = 2, cov = 37.854118, extra = 0%, doors = 1, da = 1.8, win = 1, wa = 1.5, ceil = no gives squareMeters = 31.7, liters = 6.34, gallons = 1.674851. Source: NIST Handbook 44, Appendix C (1 US gallon = 3.785411784 L, so 10 m² per litre is 37.85411784 m² per gallon).

## FAQ

### How do I calculate how much paint I need for a room?

Add up the wall area: 2 × (length + width) × height. Subtract the doors and windows, multiply by the number of coats, and divide by the area one gallon covers. A 12 × 14 ft room with 8 ft walls has 416 ft² of wall; less one door (20 ft²) and two windows (30 ft²) that is 366 ft². Two coats at 350 ft² per gallon need 366 × 2 ÷ 350 = 2.09 gallons, or 2.30 with 10% extra.

### How much area does a gallon of paint cover?

It depends on the paint and the surface, so use the number on the can. The calculator starts at 350 ft² per gallon for one coat. A rough or porous wall, a big colour change or a primer can cover less. In metric, 10 m² per litre is 37.85 m² per gallon (1 gal = 3.785411784 L).

### Do I need two coats of paint?

Often, yes: a second coat evens out the colour and the sheen, and many paint makers suggest two coats. Set the number of coats to what you plan; each coat needs the full area again.

### Should I subtract doors and windows?

Yes, if they are not painted. The calculator takes 20 ft² for each door (36 × 80 in) and 15 ft² for each window (3 × 5 ft) by default; type the sizes of yours for a closer answer. If you paint the doors too, set the number of doors to 0.

### How do I include the ceiling?

Turn on “Paint the ceiling too”. The calculator adds length × width to the area and uses the same coats and coverage. Ceiling paint often differs from wall paint, so run the calculator twice if your coverage differs.

### Why buy more paint than the exact amount?

Paint is sold in whole cans, and some is lost in the tray and on the roller. The calculator adds 10% for waste by default and rounds the result up to whole gallons. Keep a little left over for touch-ups.

## Sources

- NIST Handbook 44 (2024), Appendix C, General Tables of Units of Measurement: 1 US gallon = 231 in³ = 3.785411784 L; 1 ft = 0.3048 m; 1 ft² = 144 in². https://www.nist.gov/pml/owm/nist-handbook-44-current-edition
- OpenStax, Prealgebra 2e, §9.4 Use Properties of Rectangles, Triangles, and Trapezoids (area and perimeter of a rectangle). https://openstax.org/books/prealgebra-2e/pages/9-4-use-properties-of-rectangles-triangles-and-trapezoids