How much paint do I need for a room?
Calculate the wall area — perimeter times height — then subtract large openings such as doors and windows, and divide by the coverage stated on the tin. Typical coverage is 10–14 square metres per litre per coat, or 350–400 square feet per US gallon. Then multiply by the number of coats. Most jobs need two, and paint calculators that quietly assume one are the main reason people run short.
Should I subtract doors and windows?
Yes for substantial openings — a standard door is about 1.9 square metres and a large window similar. Ignoring several of them can overstate paint by a litre or more. Do not bother deducting small features, and do not deduct anything you intend to paint, such as door frames and window reveals, which need their own allowance in a different finish.
Why does the first coat cover so much less?
Because bare or previously dark surfaces absorb paint unevenly and need more to build an even film. New plaster is especially thirsty and normally wants a mist coat — paint thinned with water — before the finish coats. A strong colour change, particularly covering red or dark blue with white, can need three coats or a tinted primer. Budget more for the first coat than the second.
Does the finish change how much I need?
Somewhat. Matt emulsions generally cover slightly further than silk or gloss, and ceiling paint tends to be formulated for one-coat opacity. Sheen levels matter more for practicality than quantity: bathrooms and kitchens want a wipeable finish, ceilings usually matt to hide unevenness. Always take coverage from the specific tin rather than a general figure, since it varies by brand and product.
What about primer and undercoat?
They are separate quantities, not part of your topcoat total. Primer is needed on bare wood, metal, new plaster and any glossy surface being overpainted. Undercoat builds opacity under gloss. Skipping primer on new plaster is a false economy: the finish coat sinks in, dries patchy, and you end up using more paint fixing it than the primer would have cost.
How much extra should I buy?
Around 10% over the calculation, and buy it in one go. Paint is batch-mixed and colours can vary subtly between batches, so a top-up tin bought later may not match — particularly noticeable on a large wall in daylight. Keeping a small amount for touch-ups afterwards is also worth it, and the tin records the exact colour code for the future.