How much paint do you need,
down to the last litre.

Field guide

How to calculate paint for a room.

Ordering too little paint is one of the most common and costly mistakes in a DIY painting project. Buying a second can from a different batch can result in a visible colour difference between the two halves of a wall. Buying too much is wasteful but manageable: most retailers allow unopened cans to be returned. This calculator gives you the right number the first time.

The wall area formula

A rectangular room has four walls. The total wall area is:

This calculator covers the four walls only. If you also want to paint the ceiling, measure it separately (Length × Width) and add it as additional area, or run the calculator a second time.

Subtracting doors and windows

Doors and windows are not painted, so their area is deducted from the total. The default sizes used are:

• Door: 2.0 m² (21.5 sq ft). This covers a standard interior door roughly 0.9 m wide by 2.1 m tall.
• Window: 1.5 m² (16.1 sq ft). This covers a typical window approximately 1.0 m wide by 1.5 m tall.

If your doors or windows are unusually large or small, you can adjust by entering fewer or more of them to approximate the actual area excluded.

Coverage rates explained

Coverage rate (or spreading rate) tells you how much surface area a litre or gallon of paint covers in a single coat. It varies by product type, paint quality, and surface texture. Common ranges:

• Standard interior emulsion (metric): 10 to 12 m² per litre on smooth plaster. Rough surfaces or porous walls absorb more, dropping coverage to 8 or 9 m²/L.
• High-hide or thick paint: 6 to 9 m²/L. Better colour coverage in fewer coats, but costs more per litre.
• Primer and undercoat: 8 to 10 m²/L. Always prime bare walls, patched areas, or when making a dramatic colour change.
• Exterior paint: 8 to 10 m²/L on smooth render, less on rough surfaces like exposed brick or textured cladding.

The calculator defaults to 10 m²/L (metric) or 350 sq ft/gal (imperial). Edit this field if your paint tin shows a different spreading rate.

How many coats do you need?

• 1 coat: Only appropriate when repainting with exactly the same colour over a clean, sound, previously painted surface.
• 2 coats: The standard recommendation for most interior painting projects. Gives consistent, opaque coverage and is the basis for most manufacturer coverage claims.
• 3 coats: Needed when making a dramatic colour change (for example white over dark navy), painting over bare plaster, or when the surface is uneven. Applying a primer first is often more efficient than a third coat of topcoat.

Why buy 10% extra?

Even with accurate measurements, paint is lost to:

• Roller nap and brush absorption on the first dip
• Touch-up work during and after the project
• Drips, spills, and overspray
• Future scuffs and repairs where you need to match the colour exactly

A 10% buffer is the industry standard advice from professional decorators. If the paint is a custom-mixed colour, consider keeping an unopened tin long-term: reformulating an exact match months or years later is difficult.

Choosing the right finish

Paint finish affects both the look and the practicality of a painted surface. From least to most sheen:

• Matt / flat: No sheen. Hides surface imperfections well. Not washable. Best for low-traffic areas such as living room ceilings.
• Eggshell: Very low sheen. Slightly wipeable. A good all-round choice for most interior walls.
• Satin: Soft mid-sheen. Durable and cleanable. Works well in hallways, children's rooms, and kitchens.
• Semi-gloss: Noticeable sheen. Highly washable. Standard choice for woodwork, skirting boards, and doors.
• Gloss: High sheen. Very durable and waterproof. Common on exterior woodwork and metalwork.

Higher-sheen finishes tend to show surface imperfections more clearly. For walls with uneven plaster, a matt or eggshell finish will look better even if it requires more careful maintenance.

Practical tips before you buy

• Measure each wall individually if the room is not a simple rectangle (L-shaped rooms, alcoves, bay windows).
• Buy all tins from the same batch code where possible. The batch number is printed on the tin lid or base. Even the same colour from the same manufacturer can vary slightly between batches.
• When in doubt, round up to the next full tin size. A half- used tin of paint is far less of a problem than running short mid-wall.
• If painting textured or rough surfaces (artex ceilings, rendered external walls, timber shiplap), increase the estimated coverage by 20 to 30% by entering a lower coverage rate in the calculator.

Disclaimer

Results are estimates based on the dimensions and coverage rate you provide. Actual paint consumption varies with surface porosity, application method, and painter technique. Always check the coverage rate on your specific tin before ordering.