Wall Paint Calculator: How Many Litres Do You Need?
Calculate how many litres of paint a room needs, with a 5% allowance for touch-ups and an optional materials cost estimate.
A 3.5 m × 4 m bedroom with a 2.5 m ceiling needs about 6.6 litres of interior paint for two coats, assuming a coverage of 12 m²/L and a 5% allowance for touch-ups. That is usually one 5 litre tin and one 2 litre tin. The same calculation for a 4 m × 5 m living room gives about 7.9 litres. The two values that move the answer most are the coverage printed on the tin and the number of coats, so check both before you go to the shop.
The calculation starts with the wall area. For a rectangular wall, multiply the length by the height and add the walls together. The 3.5 m × 4 m bedroom has four walls with a combined area of 37.5 m², which is 2 × (3.5 + 4) × 2.5. The calculator accepts up to 20 walls, so rooms with alcoves or uneven shapes can be entered one wall at a time. In a small room, doors and windows can usually be left out, because the normal waste in painting, such as paint left in the tray and drips, makes up for them. In larger rooms, or where a door is particularly wide, subtract those openings before entering the figures.
Coverage is the figure that matters most. It is the area one litre covers in one coat on a properly prepared surface, and it is printed on the tin in m²/L. The figure on the tin describes that ideal case, so treat it as a starting point rather than a guarantee. Porous plaster, textured finishes and deep colours all take more paint in practice. Use the value from the product you intend to buy, and expect to need a little more on a rough or unsealed surface.
The number of coats follows a simple pattern. One coat is only for touch-ups over the same colour. Two coats are the normal standard and cover most light-over-light jobs. Three coats are needed when moving from a dark colour to a light one. New plasterboard or repaired plaster should be sealed or primed first, then given two topcoats. The calculator takes the number of coats as an input, so change it to suit the job.
The 5% allowance covers paint left in the tray, spatter and a small reserve for touch-ups later. Keep the leftover paint, label it with the room, and avoid buying a tin much larger than you need, since opened paint loses quality over time. Enter the price per litre in the cost field to get a materials estimate, and treat the result as a planning figure rather than a quote.
The result updates automatically.
Check the paint tin. Typical values: 10–14 m²/L (about 108–151 sq ft/L).
Litres needed
for an area of 40.0 with 2 coats
- Total area to paint
- 40.0 m²
- Safety margin
- 5% included for touch-ups
How it works
Formula
How the calculation works
How to work out the paint a room needs
Working out the right amount of paint for a room is simpler than it looks, but three variables are often estimated badly: the real wall area, the coverage stated on the tin, and the number of coats. This calculator covers all three and adds a fixed 5% allowance for touch-ups. That allowance is a planning convention built into the calculator, not a figure taken from a published standard, so treat it as a practical buffer rather than an exact requirement.
Calculating the wall area
The area of a rectangular wall is its length multiplied by its height. A typical bedroom with a 2.5 m ceiling and four walls, two of 3.5 m and two of 4 m, has a total wall area of 2 × (3.5 + 4) × 2.5 = 37.5 m². The calculator accepts up to 20 walls, so rooms with alcoves, recesses or uneven shapes can be entered one wall at a time rather than estimated as a single rectangle.
Doors and windows need some thought. In a small room you can usually leave them out. The waste that comes with painting, such as paint left in the tray, drips on the floor and the edges cut in around the frame, tends to cover the openings. In a larger space, or one with a wide door or a large glazed area, subtract those openings yourself before you enter the figures. The difference becomes more significant as the openings get bigger.
Measure the height from the floor to the ceiling line, and measure to the highest point of any sloping or gabled wall. Where a wall has a sloping section, divide it into simple rectangles and add them together. Measuring each wall separately and then checking that the walls add up to a sensible total is a good habit, because it catches mistakes before you buy anything.
Coverage: the figure that moves the answer most
Coverage describes 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 a side label in small type, and it refers to one coat on a surface that has been prepared properly. The figure is a useful starting point, but it may not match the surface in your room. In practice, a porous plaster surface, a textured finish or a deep colour will take more paint than the label suggests.
Coverage also varies by product type. Matt, low-sheen and premium interior paints can differ noticeably in how much area one litre covers. Primers and sealers are often rated differently again, so enter the value for the product that is actually going on the wall. If you use more than one product on the same job, run the calculation separately for each one and add the results together.
Comparing the label with the manufacturer's technical data sheet is a sensible check before you buy. The data sheet usually gives coverage in more detail and is generally available from the manufacturer's website, so it tells you more about how much paint you will need than the label alone.
How many coats are really needed?
The general rule for interior painting is as follows.
- One coat: only for touch-ups over the same colour and sheen.
- Two coats: the normal standard. This covers most light colours over light colours and gives an even finish.
- Three coats: when moving from a dark colour to a light one, or when using a thin-bodied paint that needs more layers to build up cover.
- Sealer or primer plus two coats: for new plasterboard wall linings, repaired plaster, or a dramatic colour change. Bare plasterboard absorbs paint unevenly, so a sealer evens out the surface before the topcoats go on.
Skipping a coat to save paint is rarely worth it. The result shows uneven sheen and visible patches, particularly under side lighting from a window. This calculator uses two coats as its default, and the coats input lets you change that to suit the job.
Why the 5% allowance?
The calculator adds 5% to the total, and the reason is practical. Paint does not all end up on the wall. Some stays in the tray, some is lost to spatter and drips, and some is needed later for touch-ups around corners, marks or areas that are hard to cover. The 5% figure is a fixed planning allowance, not a value taken from a standard, so the result is an estimate. If the room has a lot of detail, plan to buy slightly more than the result.
The margin also protects against a mid-job trip to the shop. A tin may come from a different production batch than the one you started with, and even a white can vary slightly between batches. Where possible, buy all the paint for the job at the same time, and keep a small amount in a sealed tin for later repairs.
Choosing paint by quality and use
The right paint depends on the surface, the room and how much wear it will take. Interior paints are commonly sold in three broad tiers: entry-level, mid-range and premium. Entry-level paints are cheaper and often have lower coverage and less scrub resistance. Mid-range paints are a usual choice for bedrooms and living areas. Premium paints generally offer better hiding power, washability and a more consistent finish, which can mean fewer coats in practice.
For wet areas such as bathrooms and laundries, choose a paint or coating designed for that purpose and follow the manufacturer's preparation instructions. Ceilings and walls are often specified differently, so a ceiling-specific paint is a reasonable choice for that surface. Exterior paints are a separate category, and this calculator is designed for interior walls.
If you want to compare products, the European standard EN 13300 classifies water-based interior paints by performance properties such as hiding power and wet scrub resistance. The standard does not tell you the coverage of a particular product, so the figure on the tin or in its data sheet remains the number to use in the calculation.
Common mistakes to avoid
- Buying a very large tin for a small job. Opened paint deteriorates, especially if the lid does not seal properly. For a small room, a 5 litre tin is usually a better fit than a 15 litre one.
- Mixing different batches of the same colour. Even a white can vary between production runs. If you need more paint partway through, take a sample of the paint you already have to the shop so the colour can be matched.
- Trusting coverage without reading the label. Check the coverage on the tin you intend to buy, not on a generic chart or an old tin at home.
- Forgetting primer or sealer on new surfaces. Without it, new plaster absorbs paint unevenly, and the finish can show patches for years.
Safety and older homes
The Health and Safety at Work Act 2015 requires the risks of work at height to be managed, so use proper ladders, scaffolding or a platform rather than improvised support. Older homes may have lead-based paint in the layers beneath the current finish. Before you sand, scrape or strip an older surface, check for lead and follow the guidance from WorkSafe New Zealand. Disturbed lead paint produces dust that should not be breathed in or left around the house. This calculator does not assess these risks, but they should be checked before any work begins.
When not to rely on this calculator
The calculator estimates paint quantity for simple rectangular walls. It does not account for unusual surfaces such as heavily textured plaster, timber linings with deep grooves, or surfaces that need stripping before painting. In those cases, treat the result as a minimum and buy extra. It is also not a substitute for a professional quote when the job involves preparation, repairs or access equipment.
If you have a question about a specific situation, ask the staff at the shop where you buy the paint. Many large DIY retailers have staff trained by paint manufacturers, and they can recommend a product and advise on coverage for your particular surface.
Frequently asked questions
How many litres of paint do I need for a bedroom?
A bedroom of 3.5 m × 4 m with a 2.5 m ceiling has about 37.5 m² of wall. With two coats of paint rated at 12 m²/L, you need about 6.6 litres, which is usually one 5 litre tin and one 2 litre tin. The calculator gives the exact figure for your own room, based on your measurements, the number of coats and the coverage printed on the tin you plan to buy.
Should I subtract doors and windows from the wall area?
In a small room you can usually leave them out, because the normal waste in painting, such as paint left in the tray and drips, tends to cover the openings. In a larger room, or one with wide doors or large glazed areas, it is worth measuring the openings and subtracting them before you enter the figures. The difference grows as the openings get bigger, so the more glazing a room has, the more it matters.
How much does it cost to paint a wall in New Zealand?
The cost depends on the quality of paint, the number of coats and whether you do the work yourself or hire a painter. For materials only, enter the price per litre of the paint you plan to use in the cost field, and the total appears automatically. If you hire a painter, ask for a written quote that names the paint product, the number of coats and whether preparation, such as filling, sanding and priming, is included. Prices vary by region and by job size, so get more than one quote before you agree.
What does paint coverage mean?
Coverage is the area one litre of paint covers in one coat on a properly prepared surface. It is printed on the tin, usually in square metres per litre (m²/L). Label figures apply to one coat on that kind of surface, so rough, porous or dark surfaces will usually need more paint than the label suggests. The manufacturer's technical data sheet usually gives more detail, and comparing it with the label is a good way to check a figure before you rely on it.
Can I use this calculator for a ceiling?
Yes, with a small adjustment. Enter the ceiling length and width as a single surface, so the area is length × width × 1, and set the number of coats as usual. Ceiling paints can have a different coverage from wall paints, so check the figure on the tin you are buying. Ceilings are also harder to paint evenly, so many people allow a little extra paint for them.
How many coats do I need to change from a dark colour to a light one?
Usually three coats, or a coat of white primer followed by two coats of the final colour. Painting a light colour straight over a dark one in only two coats often leaves uneven coverage, particularly when the wall is lit from the side by a window. If you change colours often, a high-hiding primer is a worthwhile investment, because it reduces the number of topcoats you need.
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By RivoCalc · Reviewed under our editorial methodology · Updated 4 May 2026
This calculator gives an estimate of paint quantity based on the measurements and coverage you enter. Actual use varies with the surface, the application method, the paint product and the skill of the person painting, so check the label of the tin you buy and allow a little extra for the job.