How Much Paint Do I Need?
Measure the area, multiply by the number of coats and divide by the coverage printed on the tin to estimate the litres required.
Measure the wall area
For each wall, multiply its width by its height. Add the four results to get the total wall area in square metres. In a simple rectangular room, a quicker method is:
Wall area = room perimeter × wall height
A room 4 m long and 3 m wide has a perimeter of 14 m: 4 + 3 + 4 + 3. With a 2.4 m wall height, its gross wall area is 14 × 2.4 = 33.6 m².
Measure in the same unit throughout. If dimensions are in centimetres, divide by 100 to convert each length to metres before multiplying. Measure alcoves, chimney breasts or extensions separately when they materially change the area.
Decide whether to subtract doors and windows
For a closer estimate, subtract the area of large openings. Measure width × height for each door or window and deduct the total from the gross wall area.
An ordinary doorway about 0.9 m × 2 m occupies roughly 1.8 m². A window 1.5 m × 1.2 m also occupies 1.8 m². Deducting both from the 33.6 m² example leaves 30 m² of paintable wall.
You can leave small openings in the calculation and treat them as part of the allowance for paint left in rollers, trays and tins. Do not subtract an opening if its frame, reveal or surrounding detail will use the same paint.
Multiply by the number of coats
The measured area is for one coat. If the wall needs two coats, multiply it by two. The 30 m² room therefore represents 60 m² of coverage for two finish coats.
Two coats are common, but the actual specification comes from the paint manufacturer. A strong colour change, patchy repairs or a porous new surface may need primer or an additional coat. Primer is a separate product with its own coverage rate; do not simply include it as another litre of finish paint.
When repainting a sound wall with a similar colour, the result may cover more easily. Still plan around the instructions on the selected tin rather than assuming one coat will be enough.
Use the manufacturer’s coverage rate
Coverage is normally shown as square metres per litre. It varies by product and may be presented as a range because application method and surface condition affect the result.
Litres required = total coated area ÷ coverage per litre
If the paint states 12 m² per litre and the job requires 60 m² of coated area, the calculation is 60 ÷ 12 = 5 litres. If the label gives a range, using the lower coverage figure produces a more cautious estimate.
Do not transfer the coverage figure from a different brand or product. Matt emulsion, masonry paint, primer, wood paint and specialist coatings can perform differently.
Worked example for a bedroom
- Room dimensions: 4 m × 3 m.
- Perimeter: 4 + 3 + 4 + 3 = 14 m.
- Wall height: 2.4 m.
- Gross wall area: 14 × 2.4 = 33.6 m².
- Door and window deduction: 3.6 m².
- Paintable area: 30 m².
- Two coats: 30 × 2 = 60 m².
- Coverage: 12 m² per litre.
- Base requirement: 60 ÷ 12 = 5 litres.
A modest allowance may push the purchase above the exact five-litre result, particularly if the wall is textured or absorbent. Compare available tin sizes and the cost of an extra small tin with the risk of running out mid-wall.
Adjust for surface condition and colour
New plaster, bare masonry and other porous surfaces may absorb more coating than a previously painted smooth wall. Cracks, filler and stains need suitable preparation. Follow product instructions for sealing, priming and drying times.
Deep texture increases the true surface area and can leave more paint in the roller. Spraying, rolling and brushing also produce different losses. A dramatic change from dark to light—or the reverse—may need a suitable base coat or more finish coats.
Test a prepared patch before buying a very large quantity. It helps confirm colour, adhesion and whether the assumed number of coats is realistic.
Round up to available tin sizes
Paint is sold in fixed container sizes, so the purchase rarely matches the mathematical result exactly. Round up rather than down, especially when a wall must be completed wet-edge to wet-edge. Keep enough correctly stored paint for final touch-ups.
If several tins of the same colour will be used, check that they match and consider mixing them together in a larger clean container before application. Record the brand, product, colour name and batch information for future repairs.
The calculator gives a planning estimate, not a guarantee. The selected manufacturer’s label and preparation instructions take priority.
Enter the room dimensions, coats and coverage rate to calculate a practical starting quantity.
Open Paint Calculator →Frequently asked questions
How do I calculate wall area?
Multiply the room perimeter by wall height, then deduct large openings if useful.
Should I subtract doors and windows?
Large openings can be subtracted; leaving small ones can provide a modest practical allowance.
How many coats should I allow?
Two are common, but always follow the selected product’s instructions and consider the surface and colour change.
Why might I use more paint?
Texture, porosity, preparation, application and normal waste can all reduce practical coverage.