Paint Calculator — Complete Guide
Paint is one of those jobs where guessing goes wrong in both directions: buy too little and you make a second trip mid-wall, buy too much and you are storing a half-empty tin for years. A quick area calculation removes the guesswork.
What this calculator does
Enter the room's length, width and wall height, the number of doors and windows, how many coats you plan and the paint's coverage. It returns the paintable area and the litres or gallons you need.
The formula
wall area = 2 × (length + width) × height
ceiling area = length × width
paintable = walls + ceiling − openings
paint = paintable × coats ÷ coverage
Openings are deducted at roughly 1.8 m² per door and 1.5 m² per window (20 and 15 ft² in imperial).
A worked example
A 5 m × 4 m room with 2.4 m walls, one door and two windows: walls = 2 × 9 × 2.4 = 43.2 m², ceiling = 20 m², openings = 1.8 + 3.0 = 4.8 m². Paintable area is 43.2 + 20 − 4.8 = 58.4 m². Two coats at 12 m² per litre needs 58.4 × 2 ÷ 12 = 9.73 litres, so buy 10 litres.
| Room | Paintable | 2 coats, 12 m²/L |
|---|
| 3 m × 3 m | 32.6 m² | 5.4 L |
| 4 m × 4 m | 45.6 m² | 7.6 L |
| 5 m × 4 m | 58.4 m² | 9.7 L |
Where the estimate drifts
- Colour changes cost more. Going from dark to light can need an extra coat or a tinted primer.
- Porous surfaces drink paint. New plaster and bare wood need more than a previously painted wall.
- Texture matters. Brick, render and heavy wallpaper all reduce coverage.
- Batch numbers. If you might need to touch up later, keep a little extra from the same batch — colours can differ slightly between batches.
涂料用量计算器完整使用指南
刷墙这件事，靠猜往往两头出错：买少了刷到一半还得再跑一趟，买多了则要在储物间里放着一桶半空的漆好几年。简单算一下面积，就能把这份猜疑去掉。
这个计算器能做什么
输入房间的长度、宽度、墙高，门窗数量，计划涂几层，以及涂料的涂布率。它会给出可涂面积，以及需要的升数或加仑数。
计算公式
墙面面积 = 2 × (长 + 宽) × 墙高
天花板面积 = 长 × 宽
可涂面积 = 墙面 + 天花板 − 门窗
涂料用量 = 可涂面积 × 涂层数 ÷ 涂布率
门窗按每扇门约 1.8 平方米、每扇窗约 1.5 平方米扣除（英制为 20 与 15 平方英尺）。
算例演示
一个 5 米 × 4 米、墙高 2.4 米的房间，一扇门、两扇窗：墙面 = 2 × 9 × 2.4 = 43.2 平方米，天花板 = 20 平方米，门窗 = 1.8 + 3.0 = 4.8 平方米。可涂面积 = 43.2 + 20 − 4.8 = 58.4 平方米。按每升 12 平方米、涂两层计算，需要 58.4 × 2 ÷ 12 = 9.73 升，买 10 升即可。
| 房间 | 可涂面积 | 两层 @12 m²/L |
|---|
| 3 米 × 3 米 | 32.6 平方米 | 5.4 升 |
| 4 米 × 4 米 | 45.6 平方米 | 7.6 升 |
| 5 米 × 4 米 | 58.4 平方米 | 9.7 升 |
估算会在哪里偏离
- 换色更费漆。 深色改浅色可能需要多刷一遍，或先上底漆。
- 多孔基层吃漆。 新抹灰墙面和裸木比刷过的墙需要更多涂料。
- 纹理有影响。 砖墙、拉毛墙面和厚重壁纸都会降低涂布率。
- 注意批次。 若日后可能要补墙，最好多留一点同批次的漆 —— 不同批次的颜色可能略有差异。