🎨 Paint Calculator
By ToolNimba Editorial Team · Updated 2026-06-19
Assumes one door is about 1.9 sq m and one window about 1.5 sq m. Adjust the coverage value to match your paint can's stated spread rate.
This paint calculator works out how much paint you need for a project. Enter the room dimensions or a known wall area, choose how many coats you plan to apply, set the coverage your paint achieves, and subtract any doors and windows you are not painting. The tool returns the volume of paint in litres or gallons and an estimate of how many standard cans to buy. It supports both metric and imperial units.
What is the Paint Calculator?
Estimating paint comes down to one idea: total area divided by coverage. First you find the surface you are painting. For four walls you take the room perimeter, which is twice the length plus twice the width, and multiply it by the wall height. A 4 m by 5 m room with 2.4 m walls has a perimeter of 18 m and a gross wall area of 18 × 2.4 = 43.2 sq m. You then subtract the openings you will not paint, such as doors and windows, to get the net paintable area.
Next you account for coats. Most jobs need two coats for an even, fully opaque finish, especially over a different colour or fresh filler, so the area to cover is the net wall area multiplied by the number of coats. A primer or undercoat counts as its own coat. Finally you divide the total area to cover by the coverage rate of your paint. Coverage, sometimes called spread rate, is how much area one litre or one gallon covers, and it is printed on the tin. A common default is about 10 sq m per litre, or roughly 350 sq ft per US gallon, but textured, porous, or dark-to-light surfaces drink more paint and cover less.
The result is the volume of paint you need. Because paint is sold in fixed can sizes (commonly 5 litre tins or 1 gallon cans), the calculator also rounds up to whole cans so you do not run short mid-wall. It is wise to keep a little extra for touch-ups, so rounding up rather than buying the exact litre figure is the safer choice.
When to use it
- Working out how many litres or gallons to buy before repainting a bedroom, hallway, or living room.
- Budgeting a decorating job by turning wall area and coats into a concrete paint quantity.
- Checking a decorator quote by estimating the paint volume yourself from the room size.
- Planning a feature wall or ceiling where you only need to cover a single known area.
How to use the Paint Calculator
- Pick metric (metres, litres) or imperial (feet, gallons) units.
- Choose "By room size" to enter length, width, and wall height, or "By wall area" to type a known area.
- Set the number of coats (two is typical) and the coverage printed on your paint tin.
- Enter how many doors and windows to subtract, then read off the paint volume and the number of cans to buy.
Formula & method
Worked examples
A 4 m by 5 m room with 2.4 m walls, 2 coats, one door subtracted, coverage 10 sq m per litre.
- Wall area = 2 × (4 + 5) × 2.4 = 43.2 sq m
- Subtract one door at 1.9 sq m: net area = 43.2 − 1.9 = 41.3 sq m
- Area to cover = 41.3 × 2 coats = 82.6 sq m
- Paint volume = 82.6 ÷ 10 = 8.26 litres
Result: 8.26 litres of paint (about two 5 litre tins)
A known wall area of 40 sq m, 2 coats, no openings, coverage 10 sq m per litre.
- Area to cover = 40 × 2 coats = 80 sq m
- Paint volume = 80 ÷ 10 = 8 litres
Result: 8 litres of paint
A 350 sq ft wall in imperial units, 1 coat, one door and one window subtracted, coverage 350 sq ft per gallon.
- Subtract one door at 20 sq ft and one window at 16 sq ft: net area = 350 − 36 = 314 sq ft
- Area to cover = 314 × 1 coat = 314 sq ft
- Paint volume = 314 ÷ 350 = 0.90 gallons
Result: 0.90 gallons (buy one 1 gallon can)
Typical paint coverage by surface and paint type (check your tin for the exact rate)
| Surface or paint | Approx. coverage per litre | Approx. coverage per US gallon |
|---|---|---|
| Smooth, primed interior wall | 11–13 sq m | 430–510 sq ft |
| Standard emulsion / latex | 10 sq m | 350–400 sq ft |
| Fresh plaster (first coat) | 6–8 sq m | 240–310 sq ft |
| Textured or rough render | 5–7 sq m | 200–270 sq ft |
| Gloss / trim paint | 12–16 sq m | 470–620 sq ft |
Standard opening sizes used to subtract doors and windows
| Opening | Metric area | Imperial area |
|---|---|---|
| Single interior door | 1.9 sq m | 20 sq ft |
| Standard window | 1.5 sq m | 16 sq ft |
Common mistakes to avoid
- Forgetting to multiply by the number of coats. One coat rarely gives a solid, even finish. Most jobs need two, plus a primer over bare or patched surfaces. Calculate paint for every coat, or you will buy roughly half of what you need.
- Using a generic coverage figure instead of the tin. Coverage varies a lot by paint and surface. Porous plaster and dark colours soak up far more than a smooth primed wall. Use the spread rate printed on your specific tin rather than a one-size default.
- Buying the exact calculated litres with nothing spare. Paint comes in fixed can sizes and you want a little left over for touch-ups. Round up to whole cans rather than buying the precise litre figure, which can leave you short on the final wall.
- Subtracting every small opening. Subtracting a large patio door makes sense, but skipping tiny windows is usually fine and gives you a safety margin. Over-subtracting openings can leave you a little short of paint.
Glossary
- Coverage (spread rate)
- How much area a unit of paint covers, for example 10 sq m per litre or 350 sq ft per gallon. Printed on the tin.
- Coat
- A single layer of paint applied over the whole surface. Two coats are typical for an even, opaque finish.
- Net paintable area
- The wall area left after subtracting openings like doors and windows that you are not painting.
- Primer / undercoat
- A preparatory coat that seals the surface and helps the topcoat adhere and cover. It counts as one coat for paint quantity.
- Gross wall area
- The full wall area before any openings are subtracted, found from room perimeter times wall height.
Frequently asked questions
How much paint do I need for a room?
Multiply the room perimeter by the wall height to get the wall area, subtract doors and windows, multiply by the number of coats, then divide by the coverage on your tin. A typical 4 m by 5 m room with 2.4 m walls and two coats needs roughly 8 to 9 litres.
How much area does one litre or one gallon of paint cover?
A common rule of thumb is about 10 sq m per litre, or roughly 350 sq ft per US gallon, for a single coat on a smooth surface. Rough, porous, or dark surfaces cover less, so always check the spread rate on the tin.
How many coats of paint should I use?
Two coats is standard for an even, fully opaque finish, especially over a new colour or fresh filler. Add a primer or undercoat over bare plaster, wood, or stains, and count that as an extra coat in the calculator.
Should I subtract doors and windows?
Yes, for large openings like doors and patio windows, since you are not painting them. The calculator subtracts about 1.9 sq m per door and 1.5 sq m per window by default. Skipping very small windows is fine and leaves a small safety margin.
Does this calculator handle ceilings?
Use the "By wall area" mode and enter the ceiling area, which is simply room length times width. For a 4 m by 5 m ceiling that is 20 sq m, then set your coats and coverage as usual.
Why does the calculator round up to whole cans?
Paint is sold in fixed sizes such as 5 litre tins or 1 gallon cans, and you want a little spare for touch-ups. Rounding up to whole cans makes sure you do not run out partway through the final wall.