Paint Amount Calculator: How Much Paint Do I Need?

This paint amount calculator works out how many liters or gallons of paint you need for a room or surface. You enter the total area to be painted (or the room dimensions), the number of coats, and the paint coverage per liter or gallon, and it returns the required paint volume. It also shows you how many cans to buy based on the can size you choose.

Formula

The core formula is simple: Paint needed (L) = (Area to paint × Number of coats) ÷ Coverage per liter.

• Area to paint (a) – the total surface area you will cover, in square meters (m²) or square feet (ft²).
• Number of coats (c) – how many layers of paint you plan to apply (usually 2 for a color change, 1 for a touch-up).
• Coverage per liter (cov) – how many square meters one liter covers. Typical values: 8–12 m²/L for most wall paints, but check the can.
• Paint needed – the result in liters (or gallons if you use those units consistently).
• Cans to buy – the number of full cans you need, rounded up, based on the can size you enter.

If you work in feet and gallons, use the same formula but with ft² and ft²/gallon. Most paint labels list coverage in both systems.

Worked Example

Using the default values: area = 50 m², coats = 2, coverage = 10 m²/L, can size = 5 L.

1. Multiply area by coats: 50 × 2 = 100 m² (total area to cover).
2. Divide by coverage: 100 ÷ 10 = 10 L of paint needed.
3. Divide by can size: 10 ÷ 5 = 2 cans. Since it’s a whole number, you buy 2 cans.

The calculator gives the same: 10.0 L paint needed, 2 cans.

How to Measure Your Room’s Paintable Area

For walls, measure the length of each wall and multiply by the wall height. Add all wall areas together. Then subtract the area of doors and windows (roughly 1.5–2 m² per door, 1–2 m² per window). For ceilings, just multiply length by width. For trim and baseboards, measure the linear length and multiply by the trim height (often 5–10 cm).

If you don’t want to measure every wall, a rough rule of thumb: for a rectangular room, perimeter × wall height minus openings. For a typical 4m × 5m room with 2.5m ceilings, that’s (4+5+4+5) × 2.5 = 45 m² of wall area, minus about 3 m² for a door and window, giving ~42 m².

Why Coverage Varies So Much

Paint coverage depends on the surface texture and porosity. Smooth, primed drywall soaks up less paint than raw plaster or old, chalky paint. Rough surfaces like brick or textured stucco have more surface area per square meter, so they need more paint. The number on the can is a best-case figure for smooth, primed surfaces. For rough or unprimed surfaces, reduce the coverage by 20–30% in your calculation.

Also, dark colors often need an extra coat to cover a light base, and light colors over dark walls may need two or even three coats. If you’re unsure, add 10% extra to your final amount to be safe.

Common Mistakes and How to Avoid Them

• Mixing units: If you measure in feet but use coverage per liter (m²/L), you’ll get a wildly wrong answer. Always keep the same unit system: m² with m²/L, or ft² with ft²/gallon.
• Forgetting the number of coats: Many people calculate for one coat and then have to make a second trip. Always multiply by the actual number of coats you plan.
• Rounding down: If you get 2.1 L, you can’t buy 2.1 L – you need 3 L (or a 2.5 L can plus a 1 L can). The calculator rounds up to the nearest whole can, which is the safe approach.
• Ignoring waste: Paint gets left on rollers, brushes, and in the tray. A 10% waste factor is realistic for most DIY projects.

What the Result Tells You

The “Paint needed� number is the theoretical volume for the area you entered. The “Cans to buy� is the practical amount to purchase. If the result is, say, 3.2 L, you’ll need to buy a 4 L can (or a 2.5 L plus a 1 L). It’s always better to have a little extra than to run out mid-project, because the same color from a different batch can vary slightly.

If you’re painting a whole room, don’t forget the ceiling and trim. The calculator only handles the area you input, so you may need to run it twice: once for walls, once for ceiling, and once for trim.

Limits of the Formula

This formula assumes a flat, uniform surface. It doesn’t account for:

• Absorption differences between primer and topcoat.
• Paint loss from overspray if using a sprayer (which can waste 20–30% more than a roller).
• Textured surfaces that increase effective area.
• Multiple colors in one room.

For spray application, increase the total by 20% or more. For roller application, the 10% waste factor is usually enough.

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 = 384 ft²). Subtract about 20 ft² for a door and window, giving ~364 ft². With 2 coats and a coverage of 350 ft² per gallon, you’d need 364 × 2 / 350 ≈ 2.08 gallons, so buy 2 gallons (or 3 if you want extra).

Does primer count as a coat?
Primer is a separate product and should be counted separately. If you prime first, you need the same volume of primer as one coat of paint. The calculator doesn’t include primer – just enter the area and 1 coat for primer, then run it again for the paint coats.

What if I’m painting over a dark color?
You’ll likely need an extra coat. Instead of 2 coats, use 3 in the calculator. If the dark color is very saturated (like red or black), you might need a tinted primer first, which also counts as a coat.

How do I know the coverage per liter for my paint?
It’s printed on the can label, usually as “spread rate� or “coverage�. Typical values: 10–12 m²/L for interior wall paint, 8–10 m²/L for exterior masonry, and 12–14 m²/L for ceiling paint. If you don’t have the can, use 10 m²/L as a safe average.

Should I buy extra paint for touch-ups?
Yes, it’s wise to buy 10% more than the calculated amount. Store leftover paint in a sealed container with the date written on it. It can last for years if kept from freezing and direct sunlight.

Can I use this calculator for exterior painting?
Yes, but exterior surfaces (wood, stucco, brick) are more porous and may need a higher coverage factor. Reduce the coverage per liter by 20% or add an extra coat. Also, exterior paint is often applied at a different thickness, so check the label.

Related

• Gravel Tonnage Calculator: How Much Do You Need
• Gravel Usage Calculator: How Much Do You Need?
• Gravel Volume Calculator: How Much Do You Need?

in a | Area to paint | m² | 50 | 0
in c | Number of coats | - | 2 | 1
in cov | Coverage per liter | m²/L | 10 | 1
in can | Can size | L | 5 | 1
out paint | Paint needed | L | (a * c) / cov | 1
out cans | Cans to buy | - | ceil((a * c) / cov / can) | 0