Paint Calculator

When to use this paint calculator

Painting a wall 4.5 m wide by 2.8 m high takes about 1.3 litres of paint per coat. Whether you are refreshing a bedroom, renovating a rental, or simply changing yellowed living-room walls to a new colour, the hardest question is often: “How much paint should I buy?” Buy too little and you run out; buy too much and you waste money and storage space. This calculator accounts for wall area, doors and windows, coverage rate, and coats to tell you exactly how many litres and cans of paint to buy.

The calculation is straightforward: add the area of every wall you plan to paint, subtract doors and windows, divide by the paint coverage per litre, then multiply by the number of coats. No need to dig out your old geometry notes—enter your measurements and the calculator does the rest.

How to use this calculator

On the Paint Calculator page, you will find these fields:

1. Room length, width, and height: Enter the internal room dimensions in metres. For example, a main bedroom may be 4.5 m long, 3.2 m wide, and 2.8 m high.
2. Door and window area: Add up the area of every door and window in the room and enter it here. Enter 0 if you do not need a precise deduction, but your estimate will be higher.
3. Paint coverage rate: This is usually printed on the paint can, such as “10 m²/L per coat”. If you cannot find it, the calculator defaults to 12; adjust it to suit your paint.
4. Number of coats: A typical job uses one coat of primer plus two finish coats, or simply two finish coats. Enter the number of coats you plan to apply, such as 2.
5. Can size (optional): If you are buying by the can, enter its volume in litres—for example, 5 L. The “Cans needed” card will show how many to buy.

Select “Calculate” and the result cards will immediately show “Total paint required (L)” and “Recommended cans to buy”. Leave the can size blank to see litres only.

Worked example: How much paint for a bedroom?

Suppose your bedroom is 4.5 m long, 3.2 m wide, and 2.8 m high. It has a 1.5 m × 2.1 m door and a 1.8 m × 1.5 m window. You chose interior emulsion paint with a theoretical coverage of 12 m² per litre per coat and plan to apply two coats. The paint comes in 5 L cans.

Step 1: Calculate total wall area
Total wall area = (length + width) × 2 × height = (4.5 + 3.2) × 2 × 2.8 = 7.7 × 2 × 2.8 = 43.12 m²

Step 2: Subtract doors and windows
Door area = 1.5 × 2.1 = 3.15 m²
Window area = 1.8 × 1.5 = 2.7 m²
Paintable area = 43.12 - 3.15 - 2.7 = 37.27 m²

Step 3: Calculate paint needed for one coat
Paint per coat = paintable area ÷ coverage rate = 37.27 ÷ 12 ≈ 3.11 L

Step 4: Multiply by the number of coats
Total paint required = 3.11 × 2 = 6.22 L

Step 5: Convert litres to cans
Cans needed = 6.22 ÷ 5 = 1.24 → round up to 2 cans (5 L each).

In this calculator, enter 4.5 for “Room length”, 3.2 for “Width”, 2.8 for “Height”, 5.85 for “Door and window area” (3.15 + 2.7), 12 for “Coverage rate”, 2 for “Coats”, and 5 for “Can size”. Select Calculate: “Total paint required” will show 6.22 L and “Recommended cans to buy” will show 2 cans.

More examples: How different inputs affect paint usage

Example 1: Ignoring door and window area
Using the same bedroom but entering 0 for “Door and window area”, the calculator uses 43.12 m². Paint per coat = 43.12 ÷ 12 ≈ 3.59 L; two coats require 7.19 L, or 2 cans (7.19 ÷ 5 = 1.44, rounded up). The number of cans stays the same, but the estimate increases by nearly 1 litre—roughly the cost of half a small can.

Example 2: High-coverage vs. low-coverage paint
One premium paint covers 15 m²/L per coat, while a budget paint covers only 8 m²/L per coat. For two coats over 37.27 m², the first needs 4.97 L (about one 5 L can) and the second needs 9.32 L (two 5 L cans). Adjust the “Coverage rate” field to see how coverage affects the number of cans and your budget.

Edge case: Painting one wall only
If you are painting one wall that is 3 m wide and 2.8 m high, its area is 8.4 m². At a coverage rate of 12 and two coats, total paint required = 8.4 ÷ 12 × 2 = 1.4 L. If you buy a 5 L can, 3.6 L will remain. The calculator will show that you need one can, helping you decide whether to buy 5 L or look for a smaller size.

How to interpret the results

Once you have “Total paint required” and “Recommended cans to buy”, use these guidelines:

• The result is exactly a whole number of cans: For example, if the result is 1.0 can, one can should generally be enough. Consider buying a little extra for touch-ups and edges.
• Rounding up leaves a substantial amount over: For example, 4.1 L with a 5 L can means one can is enough and about 18% remains for future touch-ups.
• The number of cans is much higher than expected: If the estimate is 7.2 L and you need two 5 L cans but thought one would be enough, you may have missed the number of coats or entered an incorrect coverage rate. Check the paint label and confirm whether two coats are needed.
• Total paint exceeds 10 L: For a small three-bedroom home, primer and finish paint can easily exceed 20 L. If one bedroom alone exceeds 10 L, the room dimensions may be wrong or the coverage rate may have been entered at an unusually low value such as 4–5.

Common mistakes to avoid

1. Using floor area instead of wall area: Some people enter a room’s floor area—for example, 15 m²—as its wall area. These are entirely different. Wall area is the combined area of all surrounding walls; enter the room length, width, and height so the calculator can work it out.
2. Using the wrong coverage-rate units: Coverage is usually stated as “square metres per litre per coat”. If the can says “10 m²/L”, it normally refers to one coat. If it says “5 m²/kg”, convert it first: paint density varies, but 1 L is roughly 1.4–1.6 kg, so do not enter 5 directly.
3. Forgetting the number of coats: Going from a dark colour to a light colour usually requires at least two finish coats, sometimes three. If you enter only one, the calculated amount may not cover the original colour and you will need another trip to the paint store.
4. Subtracting doors and windows twice: Deducting the same door or window area twice makes the paintable area too small, leaving you short of paint.
5. Combining primer and finish paint: Primer usually has a higher coverage rate and is typically applied in one coat. This calculator uses finish-paint coverage by default. Calculate primer separately using its own coverage rate instead of combining both products in one field.

Frequently asked questions

Q: My paint says 8–10 m²/L. What should I enter?
A: Use the midpoint or a slightly lower number, such as 9. If the wall is rough, highly absorbent, or you plan a heavier application, use 8. A conservative estimate makes running short less likely.

Q: Can I mix a half-used can of old paint with new paint?
A: Yes, if the colour, product line, and batch are the same, and the old paint has not deteriorated—such as forming a skin or developing an odour. This calculator estimates new paint only, so estimate how many litres remain and subtract that from the total yourself.

Q: The calculator says I need 1.2 cans. Should I buy one or two?
A: Buy two. Opened paint cannot usually be returned, but leftovers can be sealed and saved for touch-ups. If you buy only one and run short, a new can in the same colour may still have a slight colour difference.

Q: Should I include the ceiling and skirting boards?
A: This calculator estimates the four walls by default. To paint the ceiling, calculate length × width and add it to the wall area before subtracting doors and windows. Skirting boards have a small area and can usually be ignored.

Q: Can I use this calculator for exterior paint?
A: Yes, but exterior paint usually covers less than interior paint, and textured coatings may have special application rates. Enter the coverage rate listed for your exterior paint; the calculation works the same way.

Limits and considerations

This calculator assumes flat, smooth walls and standard application techniques. Allow extra paint in the following situations:

• Uneven older walls: If walls are cracked or peeling and need skim coating first, the surface may absorb more paint. Actual use can be 10%–20% above the estimate.
• Highly absorbent surfaces: Unprimed drywall or brick can absorb a large amount of the first coat, reducing real-world coverage well below the stated rate.
• Painting dark colours light, or using multiple accent colours: Greater opacity may require three or more coats. Increase the coat count accordingly.
• Spray application: Spraying typically wastes about 10%–15% more paint than rolling. The calculator does not include application loss, so multiply the total by 1.1–1.15 as a guide.
• Specialty textured coatings: Products such as stone-effect paint or diatom mud differ greatly from standard emulsion in coverage and application. This tool is not suitable for them; follow the product instructions.

Results are estimates only and should not be your sole basis for purchase. When buying paint, allow an extra 5%–10% for colour variation, touch-ups, and unexpected needs. Try your own room measurements in the calculator above to see how many cans you need.