Estimating paint quantities correctly saves money and avoids delays. Buy too little and the job stops mid-way. Buy too much and you pay for tins you will never use. The most reliable approach is to estimate using surface area and then adjust for coats and surface condition.
Start by measuring the walls. Measure the width of each wall and multiply by the wall height to get wall area. Add all wall areas together to get a total. If you want a tighter estimate, subtract large openings such as doors and big windows, but do not overdo it, because cutting in and edges still consume paint even when openings exist.
If you are painting ceilings, the ceiling area is usually close to the floor area of the room. Measure the room length and width and multiply them. Sloped ceilings and features will require more careful measuring, but for most standard rooms this is sufficient.
Once you have an area, you apply paint coverage. Coverage varies, but many wall paints state a coverage figure on the tin. Real-world coverage is often lower on porous surfaces, fresh plaster, or walls with heavy colour changes. Textured walls can also reduce coverage. A safe way to avoid under-buying is to assume slightly less coverage than the maximum stated on the tin.
Then consider coats. Two coats is common. If you are changing from a strong colour, or painting over stains, you may need additional coats and a primer. Primer is not a luxury, it is a tool that can improve adhesion, block stains, and reduce the number of finish coats required.
Finally, consider wastage. Cutting in, roller loading, and small touch-ups all consume paint. If your estimate is right on the margin, it is usually better to buy a small additional tin rather than risk colour mismatch later. Even within the same brand, different batches can vary slightly.
If you want help turning your measurements into a sensible purchase list for walls, ceilings, and woodwork, contact P&D Online.