Paint Needed Calculator: How Much Paint Do I Need

This calculator works out how much 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. It also shows the result in both metric and US customary units so you can buy the right amount on your first trip to the store.

Formula

The basic formula for paint quantity is:

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

Where:

• Area to paint – the total surface area in square metres (m²) or square feet (ft²). For walls, this is the sum of each wall’s height × width, minus windows and doors.
• Number of coats – how many layers you plan to apply. Most quality paints need two coats for full hide and durability.
• Coverage per litre – the area one litre of paint covers in one coat, typically 8–12 m²/L for interior wall paint. In the US, coverage is often given as square feet per gallon (usually 350–400 ft²/gal).

The result is in litres. To convert to US gallons, divide by 3.78541. To convert square feet to square metres, multiply by 0.092903.

Worked Example

Using the default values from the calculator: area = 30 m², coats = 2, coverage = 10 m²/L.

1. Multiply area by coats: 30 × 2 = 60 m² of total surface to cover.
2. Divide by coverage: 60 ÷ 10 = 6 litres.
3. Convert to US gallons: 6 ÷ 3.78541 ≈ 1.59 gallons.

The calculator shows 6.0 L and 1.6 US gal as the headline answers.

How to Measure the Area Correctly

The most common mistake is measuring the floor area instead of the wall area. For a rectangular room, measure each wall’s height and width, then add them up. For example, a 4 m × 5 m room with a 2.5 m ceiling has walls of:

• Two walls 4 m × 2.5 m = 10 m² each (20 m² total)
• Two walls 5 m × 2.5 m = 12.5 m² each (25 m² total)
• Sum = 45 m² of wall area

Subtract windows (roughly 1–2 m² each) and doors (about 1.5–2 m² each). If painting the ceiling, measure the floor length × width. For trim, measure the linear length and multiply by the trim width (usually 0.1 m or 4 inches).

Coverage: Why It Varies

Coverage per litre depends on the paint type and surface porosity. Typical values:

• Interior wall paint (matte/eggshell): 9–12 m²/L (350–400 ft²/gal)
• Primer: 8–10 m²/L (300–350 ft²/gal)
• Exterior masonry paint: 6–8 m²/L (250–300 ft²/gal)
• High-gloss or enamel: 12–14 m²/L (450–500 ft²/gal)

Rough, unsealed surfaces (bare plaster, brick, unpainted drywall) absorb more paint, so reduce coverage by 20–30%. Smooth, previously painted surfaces give the best coverage. The same paint can cover 10 m²/L on a smooth wall but only 7 m²/L on a textured one.

Common Mistakes and How to Avoid Them

• Mixing units: If your area is in square feet and your coverage is in m²/L, convert first. 1 ft² = 0.0929 m². 1 gallon = 3.785 litres.
• Forgetting coats: One coat rarely covers well, especially over dark colours. Always multiply by the number of coats you plan.
• Ignoring waste: Paint gets spilled, left on rollers, or used for touch-ups. Add 10–15% to the final number.
• Rounding down: Paint is sold in fixed container sizes (e.g., 1 L, 2.5 L, 5 L, or 1 gallon, 5 gallons). Always round up to the next full container.
• Not subtracting openings: Windows and doors can reduce area by 10–20%. Measure them and subtract.

How to Read the Result and Buy Paint

The calculator gives a theoretical minimum. In practice, buy slightly more. For the worked example (6 L), you would need one 5 L tin plus one 1 L tin (total 6 L). If you add 10% waste, that becomes 6.6 L, so you might buy two 5 L tins and keep the extra for touch-ups. Many stores accept returns of unopened tins, but check the policy before buying.

If you are painting a dark colour over light, or light over dark, you may need an extra coat. The calculator lets you adjust the coats input accordingly. For primer, use the same formula but with primer’s coverage rate.

Alternatives and When Not to Use This Calculator

This calculator works for flat surfaces like walls, ceilings, and fences. It does not account for:

• Textured surfaces (e.g., stucco, brick) – use a lower coverage value.
• Spray application – overspray can waste 20–30% more paint.
• Specialty paints – some have their own coverage rates printed on the tin.

For multi-colour schemes, calculate each colour separately. For exterior walls, account for the fact that you may need a separate primer and a top coat. If you are unsure about coverage, check the paint tin – every reputable brand prints the coverage per litre or gallon.

Frequently Asked Questions

How many square feet does a gallon of paint cover?

A typical gallon of interior paint covers about 350–400 square feet with one coat. That’s roughly 13–15 m² per litre. For two coats, divide the total area by 175–200 square feet per gallon.

Do I need to prime before painting?

Primer is recommended on bare drywall, plaster, wood, or when covering a dark colour with a light one. It seals the surface and improves adhesion, but it also adds a coat to your calculation. Use the same formula with primer’s coverage rate.

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

A 12×12 foot room (3.66 m × 3.66 m) with 8-foot (2.44 m) ceilings has about 384 square feet of wall area (subtracting one door and one window). At 350 ft²/gal per coat, you need about 1.1 gallons per coat, so for two coats, about 2.2 gallons – buy 2.5 gallons or three 1-gallon cans.

Is it better to buy extra paint?

Yes, buying 10–15% extra is wise for touch-ups and mistakes. Unopened cans can often be returned, and you can store leftover paint for future repairs. Just label the can with the room and date.

Does paint coverage change if I use a roller vs. a brush?

Rollers generally give more even coverage and use slightly less paint than brushes, but both are close. Spraying uses the most due to overspray. The coverage on the tin assumes roller application.

What if my paint says coverage in square feet per gallon?

Use the same formula but with consistent units. If your area is in square feet and coverage in ft²/gal, the result will be in gallons. If you want litres, multiply the gallons by 3.78541.

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in area | Area to paint | m² | 30 | 0
in coats | Number of coats | - | 2 | 1
in coverage | Coverage per litre | m²/L | 10 | 1
out paint | Paint needed | L | area * coats / coverage | 1
out paint_gal | Paint needed (US gal) | gal | (area * coats / coverage) / 3.78541 | 1