Paint Quantity Calculator: How Much Paint Do I Need?

This paint quantity calculator works out how many liters (or gallons) of paint you need for a room or a set of walls. You enter the total wall area, the number of coats, and the paint coverage rate, and it returns the exact volume required, plus a practical suggestion for how many tins to buy. It uses the standard formula: total area × coats ÷ coverage per liter.

Formula

The core formula is simple:

Paint needed (L) = (Wall area (m²) × Number of coats) ÷ Coverage (m²/L)

• Wall area – the total surface you plan to paint, in square meters (or square feet).
• Number of coats – how many layers you will apply. Usually 2 for a color change, 1 for touching up the same color.
• Coverage – how many square meters one liter covers. Most paints list this on the tin; typical values are 8–12 m²/L for emulsion, 10–14 m²/L for primer.

If you work in feet and gallons, the same formula works: coverage is in ft²/gal, and the result is gallons. One US gallon covers roughly 350–400 ft² per coat.

Worked Example

Using the default values from the calculator above:

• Wall area = 50 m²
• Coats = 2
• Coverage = 10 m²/L

Step 1: Multiply area by coats: 50 × 2 = 100 m² (total surface to cover).
Step 2: Divide by coverage: 100 ÷ 10 = 10 L.
Step 3: The calculator rounds to one decimal, so paint needed = 10.0 L. It then suggests buying 4 tins of 2.5 L because 10 ÷ 2.5 = 4 exactly.

If the result had been 10.4 L, you would need 5 tins (ceil(10.4/2.5) = 5). Always round up to the nearest whole tin when buying.

How to Measure Your Wall Area

You don’t need to paint the whole room, only the surfaces you intend to cover. Here’s how to get the wall area in square meters:

1. Measure the length and height of each wall in meters. Multiply them to get that wall’s area.
2. Add all wall areas together.
3. Subtract the area of doors and windows: a standard door is about 1.6–2.0 m², a typical window 1.0–2.5 m². Measure them if you want precision.

For a rectangular room, you can also use: perimeter × wall height − openings. For example, a 4 m × 5 m room with 2.5 m ceiling has perimeter (4+5)×2 = 18 m, so wall area = 18 × 2.5 = 45 m². Subtract a door (2 m²) and a window (1.5 m²) to get 41.5 m².

Ceilings are separate. If you’re painting the ceiling, measure its length × width (e.g., 4 × 5 = 20 m²) and add it to the wall area.

Choosing the Right Coverage Value

The coverage number on a paint tin is the theoretical maximum for a smooth, primed surface. Real-world coverage depends on:

• Surface porosity – new plaster, bare wood, or unprimed drywall absorbs more paint, reducing coverage to 6–8 m²/L.
• Texture – rough or textured walls (e.g., stucco, brick) have more surface area and can cut coverage by 20–30%.
• Application method – rollers and brushes leave more paint than sprayers; spraying can lose 10–20% to overspray.
• Color change – going from dark to light usually needs an extra coat, effectively lowering coverage per liter.

If you’re unsure, use a lower coverage estimate (like 8 m²/L) to build in a safety margin. It’s cheaper to buy one extra liter than to run out mid-project.

Common Mistakes and How to Avoid Them

• Mixing units – Don’t combine square meters with coverage in square feet per gallon. Convert everything to the same system first (1 m² = 10.764 ft², 1 L = 0.264 US gal).
• Forgetting the second coat – If you’re changing color or painting over bare drywall, you almost always need two coats. The calculator multiplies by coats, so set it to 2.
• Ignoring waste – Paint left on rollers, brushes, and tray liners is wasted. Add 10% to your final number to account for this.
• Rounding down – Always round up to the nearest tin size. You can’t buy 3.7 liters; you buy 4 liters (or two 2L tins).

A common misconception: “One coat of expensive paint is enough.� In practice, most paints need two coats for even color and full hide, especially on light colors over dark walls. The coverage figure assumes a single coat.

Limits of the Formula

This formula gives a good estimate, but it can’t account for every variable:

• It assumes you paint the entire area you entered. If you’re doing a feature wall or only patching, adjust the area accordingly.
• It doesn’t know the paint’s actual spreading rate – that’s why you enter the coverage from the tin.
• It doesn’t factor in primer. Primer is a separate product with its own coverage; add it as a separate calculation if needed.
• It doesn’t know the shape of the room or the number of windows – you provide the net area.

For most residential projects, the estimate is within 10–15% of actual usage, which is fine because you buy whole tins anyway.

Cost and Waste Considerations

Paint is sold in standard sizes: 1 L, 2.5 L, 5 L, and 10 L in most countries (1 quart, 1 gallon, 5 gallons in the US). Buying a larger tin is usually cheaper per liter, but you don’t want leftover paint that will dry out. A 5 L tin might be perfect for a small room, but if you only need 3 L, buying two 1.5 L tins (if available) or a 2.5 L and a 1 L is better than a 5 L.

If you have leftover paint, store it in a cool, dry place with the lid sealed. It can last 2–5 years if kept from freezing. For disposal, check local regulations – never pour paint down drains.

Frequently Asked Questions

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

Assuming 8-foot ceilings, a 12×12 room has four walls, each 12 ft × 8 ft = 96 ft², total 384 ft². Subtract a door (20 ft²) and two windows (30 ft²) to get about 334 ft². With typical coverage of 350 ft²/gal per coat and two coats, you need 334 × 2 ÷ 350 = 1.9 gallons, so buy 2 gallons.

Do I need primer before painting?

Primer is recommended on new drywall, bare wood, or when covering a dark color with a light one. It seals the surface and improves adhesion, which can reduce the number of topcoats needed. If you use primer, calculate it separately using the same formula.

What if the paint coverage on the tin is in square feet per gallon?

That’s fine – just use feet and gallons in the formula. If you measured in meters, convert: 1 m² = 10.764 ft², and 1 gallon = 3.785 L. For example, 10 m²/L is about 400 ft²/gal.

How do I account for textured walls?

Textured surfaces (popcorn ceilings, stucco, brick) have more surface area. Reduce the coverage by 20–30% or add 20–30% to your area. For example, if coverage is 10 m²/L, use 7–8 m²/L instead.

Why does the calculator say I need 10 L but the store only sells 2.5 L tins?

You buy 4 tins of 2.5 L to get exactly 10 L. If the calculation gives 10.4 L, you’d need 5 tins (12.5 L) because you can’t buy a fraction of a tin. Always round up.

Can I use this for exterior painting?

Yes, the formula is the same, but exterior surfaces (wood siding, brick, stucco) are more porous and textured, so use a lower coverage estimate. Also, exterior paint may require a primer and a topcoat, each with its own coverage.

Related

• Drywall Paint Calculator: How Much Paint You Need
• Drywall Water Damage Repair Calculator
• Dulux Paint Calculator: How Much Paint You Need

in a | Wall area | m² | 50 | 0
in c | Coats | - | 2 | 1
in cov | Coverage | m²/L | 10 | 1
out paint | Paint needed | L | a * c / cov | 1
out tins | 2.5L tins to buy | - | ceil((a * c / cov) / 2.5) | 0