Paint Litre Calculator: How Much Paint You Need

This paint litre calculator works out the volume of paint you need for a decorating job. You enter the total area to be painted (in square metres), the number of coats, and the paint’s coverage rate (in square metres per litre), and it returns the required litres. It also gives you the amount in gallons for those who work in imperial units.

Formula

The basic formula is simple:

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

Where:

• Area to paint is the total surface area you will cover, in square metres (m²).
• Number of coats is how many layers of paint you plan to apply (usually 2 for a colour change).
• Coverage is the spreading rate of the paint, in square metres per litre (m²/L). This is often printed on the tin.

The result is the minimum litres you need. In practice, you should buy a little extra (about 10–20%) to account for waste, uneven surfaces, and touch-ups.

Worked Example

Using the default values from the calculator above:

• Area to paint = 30 m²
• Number of coats = 2
• Coverage = 10 m²/L

Step 1: Multiply the area by the number of coats to get the total area to be covered:

30 m² × 2 = 60 m²

Step 2: Divide by the coverage rate:

60 m² ÷ 10 m²/L = 6 L

So you need 6 litres of paint. In US gallons, that is 6 × 0.264172 ≈ 1.58 gallons. The calculator shows 6.0 L and 1.58 gal.

How to Measure the Area to Paint

Getting the area right is the most important part of the calculation. Here’s how to do it for common situations:

• Walls: Measure the length of each wall and its height (floor to ceiling). Multiply length × height for each wall, then add them together. Subtract the area of windows and doors (roughly 1.5–2 m² for a door, 1–2 m² for a window).
• Ceilings: Measure the length and width of the room and multiply them. That’s the ceiling area.
• Woodwork (doors, skirting, frames): Measure the width and height of each item. For skirting boards, measure the length and multiply by the height (usually about 0.1–0.15 m).

For irregular surfaces, break them into rectangles and add the areas. Don’t try to be too precise – a small overestimate is fine.

What Affects Coverage?

The coverage rate on the tin is a theoretical maximum under ideal conditions. In reality, several factors reduce it:

• Surface porosity: New plaster, bare wood, and unpainted drywall soak up more paint, so you may get only half the stated coverage on the first coat.
• Texture: Rough surfaces (e.g., brick, textured plaster) have more surface area than smooth ones, so they use more paint.
• Colour change: Painting a dark colour over a light one (or vice versa) may require an extra coat to get full opacity.
• Application method: Rollers and brushes leave different film thicknesses. Spraying can waste more paint due to overspray.
• Quality of paint: Cheaper paints often have lower solids content and may need more coats to achieve the same finish.

If you are painting a porous surface, consider reducing the coverage rate by 20–30% in the calculator. For example, use 7 m²/L instead of 10 m²/L.

Common Mistakes and How to Avoid Them

• Mixing units: The formula only works if all measurements are in the same unit. If you measure the room in feet, convert to metres (1 ft = 0.3048 m) or use a coverage rate in ft²/gallon. The calculator above uses metric inputs.
• Forgetting the number of coats: Many people calculate for one coat and then are surprised they need twice as much. Always multiply by the number of coats.
• Ignoring waste: Paint gets left in the tray, on the roller, and in the tin. Add 10–20% to your final figure to be safe.
• Rounding too early: If you round the area to the nearest whole number, you might underestimate. Keep one decimal place until the end.
• Not accounting for future touch-ups: It’s wise to keep a small amount of leftover paint for repairs. Buy an extra 0.5–1 L for this purpose.

How to Read the Result

The result is the theoretical minimum litres required for the area and coats you entered. It assumes you apply the paint at the manufacturer’s recommended film thickness. In practice, you should buy the next standard can size up. Paint is usually sold in 0.5 L, 1 L, 2.5 L, 5 L, and 10 L containers (in metric countries). For example, if the calculator says 6 L, you would buy a 5 L tin and a 1 L tin, or two 2.5 L tins plus a 1 L tin.

If you are painting multiple rooms, calculate each room separately and add the totals. Different surfaces (walls vs. ceiling) often require different paint types, so you may need separate calculations.

Limitations of the Formula

This calculator is a planning tool, not a guarantee. It cannot account for:

• Primer or undercoat – these are separate products and may be needed first.
• Special finishes (e.g., textured paint, masonry paint) that have different coverage rates.
• Wastage due to spills, drips, or over-application.
• Surface repairs that absorb extra paint.

Always read the paint tin for the manufacturer’s specific coverage and follow their recommendations. When in doubt, buy slightly more than the calculator suggests – unused paint can often be returned or saved for touch-ups.

Frequently Asked Questions

How many square metres does 1 litre of paint cover?

Typically, 1 litre of emulsion paint covers about 10–12 m² per coat on a smooth, primed surface. However, this varies by brand and type. Gloss and satin paints often cover slightly less (around 8–10 m²/L) because they are thicker. Check the tin for the exact figure.

Do I need to use a primer first?

Primer is recommended on new plaster, bare wood, or metal. It seals the surface and improves adhesion, which can reduce the amount of topcoat needed. On previously painted surfaces in good condition, a primer is usually not necessary.

How do I calculate paint for a room with windows and doors?

Measure the total wall area (perimeter × height) and subtract the area of windows and doors. For a standard door (about 2 m²) and a typical window (about 1.5 m²), you can simply deduct 3–4 m² from the total. Or measure each opening precisely.

Is it better to buy more paint than the calculator says?

Yes, it is wise to buy 10–20% extra. This covers waste, uneven surfaces, and future touch-ups. It’s also helpful if you need to do a second coat because the first one didn’t cover well. Leftover paint can be stored for years if sealed properly.

Can I use this calculator for exterior paint?

Yes, the same formula works for exterior walls, fences, and other surfaces. However, exterior surfaces are often more porous or textured, so reduce the coverage rate by 20–30% or use the manufacturer’s figure for exterior paint, which is usually lower.

What if my paint coverage is listed in square feet per gallon?

Convert to metric: 1 gallon = 3.785 litres, and 1 m² = 10.764 ft². So if the coverage is 350 ft²/gallon, that’s about 350 ÷ 3.785 ÷ 10.764 ≈ 8.6 m²/L. You can enter that number into the calculator.

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in area | Area to paint | m² | 30 | 0
in coats | Number of coats | - | 2 | 1
in cov | Coverage | m²/L | 10 | 1
out litres | Paint needed | L | (area * coats) / cov | 1
out gallons | Paint needed | US gal | ((area * coats) / cov) * 0.264172 | 2