Wall Paint Calculator: How Many Litres Do You Need?

How the calculation works

How to work out the paint for a room

Working out how much paint a room needs is simpler than it looks, but three variables are commonly misestimated: the real wall area, the coverage printed on the tin, and the number of coats. This calculator covers all three and adds a 5% margin for touch-ups, a small allowance that is sensible to plan for and that many generic calculators leave out.

It helps to understand what the calculator is doing before you rely on the result. The paint volume is calculated as wall area × number of coats × 1.05 ÷ coverage. Each step has a physical meaning, and each one is a place where errors creep in. The sections below go through them in order, then cover a worked example, when to use the calculator and when to choose a different method.

Calculating the wall area

The area of a rectangular wall is its length multiplied by its height. In a typical square room of 4 m per side with a 2.5 m ceiling, the four walls are each 4 m long, so the total wall area is 4 × 4 × 2.5 = 40 m². Irregular rooms are easier to handle wall by wall, which is why the calculator accepts up to 20 walls. Measure each one separately and enter it, rather than estimating the room as a single rectangle.

Ceiling height matters more than people expect. Raising the same room to a 2.7 m ceiling adds 3.2 m² of wall, which needs about half a litre more paint for two coats at 12 m²/L. Measure from the floor to the ceiling line at each wall, not to the top of a cornice or a picture rail.

Openings are the other variable. In small rooms, the paint lost to the tray, roller and edges usually covers doors and windows, so the full wall area can be entered. In larger rooms, hallways, or spaces with several large doors, subtract the actual openings before you enter the dimensions. Measure them rather than relying on a standard size, since door and window sizes vary between homes.

Coverage: where most estimates go wrong

Coverage is the relationship between the paint used and the area covered, expressed in square metres per litre (m²/L). It is printed on the tin, usually on the side label in small type, and it is the figure this calculator relies on most. Coverage differs between products, so the number you enter should come from the paint you intend to buy, not from a general rule.

Treat the printed figure as a starting point. Dark colours, new plasterboard, patched plaster and rough or textured surfaces all absorb more paint than a smooth, sealed wall, and the quantity you actually use will reflect that. The technical data sheet for the product usually gives more detail on coverage and surface preparation than the label does, so it is the better reference when you have one.

How many coats do you need?

The general rule for interior painting is as follows:

• 1 coat: only for touch-ups over the same colour
• 2 coats: the standard. This covers almost every light-on-light situation.
• 3 coats: when moving from dark to light colours, or when using thin, low-coverage paints such as some eco-friendly formulations
• Primer plus 2 coats: new plasterboard, repaired cracks or a drastic change of colour

Skipping a coat to save paint rarely works. The finish tends to show uneven sheen, especially under raking light from a window. Two coats is the sensible minimum on most jobs, even when the surface looks easy.

Why the 5% margin?

The calculator adds 5% to the total because paint is never all used on the wall. Some stays in the tray, some is lost in spatter, and some is needed later for touch-ups in corners, marks or hard-to-cover areas. The margin also protects you from a second trip to the shop halfway through the job. That matters because paint from a different batch can vary slightly in colour even when the label reads the same, so it is best to buy enough for the whole job at once. Keep any leftover tin sealed and labelled for future repairs.

The 5% is a planning allowance chosen for this calculator, not a measured loss for every job. A very rough surface, a first coat over bare plaster or a large job may need more, so use the margin as a minimum.

Choosing paint by tier and use

Interior paints are sold in a range of tiers. The tier does not change the formula, only the coverage figure you enter. Coverage should be checked on the data sheet of each specific product rather than assumed from its price or its tier.

Exterior paints are formulated differently and are outside the scope of this calculator. Masonry, rendered walls and bare brick absorb paint differently from plasterboard, so their coverage should be checked separately.

A worked example

Take a 3.5 m × 4 m bedroom with a 2.5 m ceiling. The four walls have a combined length of 15 m, so the wall area is 15 × 2.5 = 37.5 m². Two coats cover 75 m² in total. With an assumed coverage of 12 m²/L, that is 75 ÷ 12 = 6.25 L. Adding the 5% margin gives 6.25 × 1.05 = 6.56 L, which rounds to about 6.6 L.

The same steps apply to a 4 m × 5 m living room. The combined wall length is 18 m, so the wall area is 18 × 2.5 = 45 m². Two coats cover 90 m², which needs 90 ÷ 12 = 7.5 L before the margin, or about 7.9 L with it. Doing the sum by hand is a useful check on the calculator's result before you buy.

Common mistakes to avoid

• Buying a very large tin for a small area. Opened paint can lose quality over time, so for a small room a smaller tin is usually more sensible.
• Mixing batches badly. Even a standard white can vary between production batches. If you must buy more partway through, bring a sample of the paint you already have and check the match before you start. Pouring all the tins into one bucket before you begin is a common way to avoid visible differences.
• Trusting the coverage without reading the label. If the printed coverage looks high for the surface you are painting, assume you will need more paint.
• Skipping primer on new surfaces. Bare plasterboard and patched plaster absorb paint unevenly, so the finish can look patchy even after two coats.

When to use this calculator, and when not to

Use this calculator for interior walls with water-based wall paint on a sound surface. It is not suited to:

• Ceilings, which may have a different coverage figure from wall paint. Calculate them separately by entering the ceiling's length as the wall length and its width as the wall height, counted as one surface.
• Exterior walls, including rendered masonry and bare brick.
• Timber, trim, doors and other woodwork, which use different products and application methods.
• Textured finishes, where the coverage printed on the tin may not be reached in practice.

Where to check the figures

Coverage is a figure supplied by the manufacturer, and the most reliable place to read it is the product's technical data sheet, which is usually available from the paint supplier or the manufacturer's website. Staff at paint suppliers can usually advise on the right product for a surface and the number of coats. If you are unsure about the condition of a wall, an experienced painter can inspect it and advise on preparation before you start, which is especially worth doing on an older or damaged surface.