Paint Calculator: How Much Paint Do I Need?

A paint calculator works out how many cans of paint you need for a room or surface. You enter the total area to be painted, the number of coats, and the paint coverage per litre (or gallon), and it returns the exact litres or gallons required, plus the number of cans to buy. This page explains the simple formula behind every paint calculator, shows a worked example, and covers the common mistakes that lead to wasted paint or a second trip to the store.

Formula

The core formula is simple:

Paint needed (L) = (Area to paint (m²) × Number of coats) ÷ Coverage per litre (m²/L)

Then, to find how many cans to buy:

Cans to buy = ceil( Paint needed (L) ÷ Can size (L) )

Where:

• Area to paint (a) – the total surface area you will cover, in square metres (m²) or square feet (ft²).
• Number of coats (c) – how many layers of paint you plan to apply. Usually 2 for a colour change, 1 for touching up.
• Coverage per litre (cov) – how many square metres one litre of paint covers. Most paints list this on the tin, typically 8–12 m²/L for emulsion, 10–14 m²/L for gloss.
• Paint can size (p) – the volume of the can you intend to buy, in litres (or gallons).

If you are working in feet and gallons, the formula is identical: Gallons = (Area (ft²) × Coats) ÷ Coverage (ft²/gal). Just keep all units consistent – do not mix metres with gallons.

Worked Example

Using the default values from the calculator above:

• Area = 50 m²
• Coats = 2
• Coverage = 10 m²/L
• Can size = 2.5 L

Step 1: Multiply area by coats: 50 × 2 = 100 m² of total surface to cover.

Step 2: Divide by coverage per litre: 100 ÷ 10 = 10 L of paint needed.

Step 3: Divide by can size: 10 ÷ 2.5 = 4 cans exactly.

So you need 10 litres of paint, which means buying 4 × 2.5 L cans. The calculator rounds up to the next whole can because you cannot buy a fraction of a tin.

How to Measure the Area to Paint

The most common error is measuring the floor instead of the walls. For walls, measure the length of each wall and multiply by the ceiling height. Add all walls together. For a rectangular room, that is:

Wall area = 2 × (length + width) × height

Subtract the areas of windows and doors (roughly 1.5–2 m² for a door, 1–2 m² for a window). For ceilings, simply multiply length × width. For trim, baseboards, and doors, measure their length and multiply by the width of the trim (or use a rough estimate of 20% extra).

When to Use a Paint Calculator

Use a paint calculator whenever you are buying paint for a room, a wall, a piece of furniture, or an exterior surface. It is especially useful when:

• You are changing from a light to a dark colour (or vice versa) and need extra coats.
• You are painting over bare drywall or plaster, which soaks up more paint.
• You are buying paint online and cannot ask a store assistant.
• You want to compare costs between different can sizes.

For exterior walls, measure the perimeter of the house and multiply by the height, subtract large windows and doors. Rough surfaces like brick or stucco absorb more paint, so reduce the coverage per litre by 20–30%.

Common Mistakes and Misconceptions

Mistake 1: Ignoring the number of coats. Many people calculate for one coat and end up short. Always multiply by the number of coats you plan to apply.

Mistake 2: Using the wrong coverage value. Paint coverage varies by brand, type, and surface. A cheap paint might cover only 8 m²/L, while a premium one covers 12 m²/L. Always check the label on the specific tin.

Mistake 3: Forgetting to subtract windows and doors. If you include them, you will overestimate by 10–20%, which is not terrible, but you might buy an unnecessary can.

Mistake 4: Mixing units. If you measure area in square feet, use coverage in square feet per gallon. Do not mix metres with gallons.

Misconception: “One coat is enough.� In most cases, two coats are needed for even colour and durability, especially over a contrasting colour or bare surface. Some high-quality paints claim one-coat coverage, but that usually only works when painting a similar colour over a well-prepared surface.

Limits of the Formula

The paint calculator gives a theoretical estimate. Real-world usage varies because of:

• Surface porosity: Bare drywall, wood, or masonry absorb more paint than previously painted surfaces.
• Application method: Spraying wastes more paint than rolling, and rolling wastes more than brushing.
• Texture: Textured walls (e.g., stucco, orange peel) require significantly more paint.
• Colour change: Going from dark to light or light to dark may require an extra coat.

As a rule of thumb, add 10–15% extra to the calculated amount for touch-ups and waste. It is better to have a little leftover than to run out mid-project.

Frequently Asked Questions

How much paint do I need for a 12×12 room?

A 12×12 ft room with 8 ft ceilings has about 384 ft² of wall area (perimeter 48 ft × 8 ft height). Subtract 21 ft² for one door and one window, giving ~363 ft². With two coats and a typical latex paint coverage of 350 ft² per gallon, you need 363 × 2 ÷ 350 = 2.07 gallons, so buy 3 gallons (or 2 gallons if you are okay with touch-up later).

Does primer count as a coat?

No, primer is not the same as paint. Primer is designed to seal the surface and provide adhesion, not to provide the final colour. You still need the full number of paint coats over the primer. The calculator does not include primer; enter only the paint coats.

What is the average coverage of a litre of paint?

Most interior emulsion paints cover about 10–12 m² per litre per coat. Gloss and satin paints cover slightly less, around 8–10 m²/L. Exterior masonry paints may cover only 6–8 m²/L due to rough surfaces. Always check the tin.

Should I buy all the paint at once?

Yes, if possible. Paint batches can vary slightly in colour, so buying all cans from the same batch ensures consistency. If you need to buy more later, the colour may not match perfectly, especially for tinted colours.

How do I calculate paint for a ceiling?

Measure the length and width of the ceiling and multiply to get the area. For a 4 m × 3 m ceiling, that is 12 m². With one coat of ceiling paint at 10 m²/L, you need 1.2 L, so a 2.5 L can is more than enough.

Why does the calculator round up the cans?

Because you cannot buy a fraction of a can. If you need 9.5 litres and the can size is 2.5 L, you need 4 cans (10 L total). Rounding up ensures you have enough paint for the job, and the leftover can be saved for touch-ups.

Related

• Ez Asphalt Calculator: How to Estimate Asphalt Needs
• Ez Stair Calculator: How to Use It
• Ez Stair Calculator Free? Yes, Here’s How

in a | Area to paint | m² | 50 | 0
in c | Number of coats | - | 2 | 1
in cov | Coverage per litre | m²/L | 10 | 0
in p | Paint can size | L | 2.5 | 0
out need | Paint needed | L | (a * c) / cov | 2
out cans | Cans to buy | cans | ceil(((a * c) / cov) / p) | 0