Paint Calculator

What this calculator does. It works out how much paint to buy for a room by adding up the wall area, subtracting the obvious openings, then multiplying by the number of coats and dividing by the manufacturer's coverage figure.

The formula in plain English. Wall area = 2 × (width + length) × height. We subtract roughly 1.5 m² for each door and window, then calculate litres needed = (paintable area × coats) ÷ 10 m² per litre. Ten square metres per litre is the standard figure on UK emulsion tins; check the tin for the exact number if you've already picked a paint.

A worked example. A typical 4 m × 5 m bedroom with 2.4 m ceilings has a wall area of 2 × (4 + 5) × 2.4 = 43.2 m². Take off 1.5 m² each for one door and one window (3 m² total) and you're painting 40.2 m². Two coats × 40.2 m² ÷ 10 m² per litre = roughly 8 litres. In practice that means a 5 L tin and a 2.5 L tin, or just one 10 L tub if you want spare paint for touch-ups.

Why this matters. Buying too little paint is the single most common DIY mistake — running out halfway through the second coat means a second shop trip, a possibly different dye lot, and a visible join line that you'll see every time you walk into the room. Buying way too much is wasteful and expensive; a 10 L tub of mid-range UK emulsion is £40–£70. Getting the quantity right means one clean shopping list, one weekend of decorating, and a sensible amount of paint left over for future touch-ups.

When to use it. Planning any room repaint, comparing tin sizes before buying, working out whether a 10 L tub is cheaper than two 5 L tins for your room, or pricing up a multi-room project where coverage adds up quickly.

When not to rely on it alone. Fresh plaster needs a thinned mist coat first which uses extra paint, and textured surfaces (Artex, woodchip, heavily abraded walls) can soak up 30–50% more than smooth walls. Going from a dark colour to a light one almost always needs a tinted primer plus two top coats — treat the calculator's figure as the top-coat quantity and budget for primer separately.

Common mistakes. Forgetting to multiply by the number of coats halves the estimate. Using the room's floor area instead of wall area is the second classic error. And buying just enough — to the litre — almost always backfires: rollers and trays retain paint, and most decorators keep a small stash for inevitable scuffs and touch-ups. Round up to the next tin size.