How many square metres does 1 gallon of paint cover? Work out your house paint in 5 steps, with real examples

How many square metres does 1 gallon cover, and what does a pail of paint cover?

1 gallon (3.785 litres) of ordinary acrylic house paint covers around 30–40 square metres per coat on a smooth, primed plaster wall. Think of it like a car's fuel economy: you go further on a smooth road and less far on a dirt track. Rough walls, new plaster that soaks up paint, and porous exterior walls all use more.

As for a pail of paint, check the litres on the label first, because sizes vary by brand. The most common sizes are a small pail of about 9 litres and a large pail of about 18 litres, which work out at about 2.4 and 4.75 gallons respectively.

• Small tin, 0.946 litres (1/4 gallon): about 7.5–10 sq m per coat
• 1 gallon, 3.785 litres: about 30–40 sq m per coat
• 9-litre pail: about 71–95 sq m per coat
• 18-litre pail: about 143–190 sq m per coat
• With the standard 2 coats: the wall area you can actually finish is half the figures above

How to calculate house paint in 5 steps, with just a tape measure and a calculator

The standard paint system is 1 coat of primer and 2 coats of the paint itself. Calculate the paint and the primer separately, because you apply different amounts, and the primer is usually a different product from the paint.

The bonus of doing the maths yourself is that you'll understand a painter's quote too. If a painter tells you a small bedroom needs six gallons of paint, you'll know straight away to ask more questions.

• 1. Measure each wall: width × height, then add all the walls together (or use room perimeter × height)
• 2. Subtract openings: a standard door is about 1.8–2 sq m; measure each window, width × height
• 3. Multiply by the number of coats: 2 coats of paint, 1 coat of primer
• 4. Divide by the coverage rate: use the figure on the label, for example 30–40 sq m per gallon per coat
• 5. Add 10% extra: for spills, touch-ups, and keeping some for future repairs
• Ceiling: room width × length, calculated separately from the walls

Example 1: how many gallons for a 3 × 4 m bedroom?

A 3 × 4 m bedroom with a 2.6 m ceiling height, 1 door (0.9 × 2.0 m) and 1 window (1.2 × 1.2 m). The room perimeter is 3 + 4 + 3 + 4 = 14 m. Multiply by the 2.6 m height to get 36.4 sq m. Subtract 1.8 for the door and 1.44 for the window, leaving about 33 sq m of wall.

Paint, 2 coats: 33 × 2 = 66 sq m. Divide by 30–40 to get 1.65–2.2 gallons. Add 10% to get 1.8–2.4 gallons. For smooth walls, 2 gallons is enough. If the walls are rough, or you're going from a dark colour to a light one, allow 1 extra small tin.

Primer, 1 coat: 33 sq m divided by the rate on the primer label. If it's close to 30–40 sq m per gallon, you'll use about 1 gallon. The ceiling is 3 × 4 = 12 sq m; 2 coats makes 24 sq m, which uses less than 1 gallon. Buy 1 gallon and keep the rest for touch-ups.

• Walls: about 33 sq m, 2 gallons of paint (+ a small tin if the walls are rough)
• Primer: about 1 gallon (check the rate on the label)
• Ceiling: 12 sq m, 1 gallon of ceiling paint

Examples 2 and 3: a living room, and the front of a townhouse

A 4 × 5 m living room with a 2.7 m ceiling height, a 2.0 × 2.2 m sliding door and a 1.5 × 1.2 m window. The room perimeter is 18 m × 2.7 = 48.6 sq m. Subtract openings of 4.4 + 1.8 = 6.2, leaving 42.4 sq m. 2 coats makes 84.8 sq m. Divide by 30–40 to get 2.1–2.8 gallons, and with the extra that's 2.3–3.1 gallons. Buying 3 gallons is about right.

The front of a 2-storey townhouse, 5 m wide and 7 m high = 35 sq m. Subtract the 2.2 sq m door and 3 windows (1.5 × 1.2 = 1.8 each), 7.6 sq m in total, leaving 27.4 sq m. 2 coats makes 54.8 sq m, which needs 1.4–1.8 gallons, or 1.5–2.0 gallons with the extra. Exterior walls are usually rougher, so buy 2 gallons. If you're painting a back wall of a similar size too, that's about 4 gallons in total.

This is where pail size can save you money. For example, Voqe Wall Paint covers 35–40 sq m per gallon per coat. The 1-gallon size costs ฿1,299 and the 18-litre size (about 4.75 gallons) costs ฿3,990. If a job uses 4 gallons or more of a single shade, the large pail works out a good deal cheaper than buying separate gallons.

• 4 × 5 m living room, 2.7 m high: about 3 gallons
• 5 × 7 m townhouse front: about 2 gallons
• Townhouse front + back: about 4 gallons, or one 18-litre pail if it's all one colour
• Two-tone house: work out the area for each colour separately, and buy each colour separately

Quick chart: room size vs gallons of paint

If you don't feel like doing the maths, just use this quick chart. It assumes a 2.6 m ceiling height, 1 door and 1 window (about 3 sq m subtracted), 2 coats of paint, 10% extra, and a coverage rate of 30–40 sq m per gallon per coat. It counts the walls only, not the ceiling or primer.

If your room has a higher ceiling, large windows or sloping walls, the chart may be off. Go back to the 5 steps above. It takes less than five minutes.

• 3 × 3 m room: about 2 gallons
• 3 × 4 m room: about 2–2.5 gallons
• 4 × 4 m room: about 2–3 gallons
• 4 × 5 m room: about 3 gallons
• 5 × 6 m room: about 3–4 gallons

Calculating mineral paint in litres: one feature wall

Textured paint applied with a brush is calculated differently from house paint, because it's thicker and applied in criss-cross layers. Voqe Mineral Finish covers 8–12 sq m per litre per coat, depending on the wall surface and the hand applying it. The system is 1 coat of primer and 2 coats of Mineral Finish.

A headboard wall 3 m wide and 2.6 m high = 7.8 sq m. 2 coats makes 15.6 sq m. Divide by 8–12 to get 1.3–2.0 litres. Set 2QT (two tins, Light Shade and Shadow Shade) at ฿1,180 suits a wall this size if the wall is smooth. If the wall is 4 m wide or more, or you're doing more than one wall, move up to Set 2GL at ฿2,998.

• Formula: wall area × 2 coats ÷ 8–12 = litres needed
• 3 × 2.6 m wall: about 1.3–2.0 litres
• 4 × 2.7 m wall: about 1.8–2.7 litres
• Several walls in one room: add them up and choose the larger set, so the colour is more consistent