Paint Calculator — Litres & Coverage

Paint coverage depends on the product type and on the surface underneath it. Emulsion for walls typically covers 10 to 12 m² per litre on smooth plaster, while gloss and satin for woodwork stretch to 12 to 16 m² per litre. Textured, bare or porous surfaces can swallow 20 to 30% more paint, because the first coat soaks into the wall rather than sitting on top of it.

This calculator measures each wall as height by width, subtracts the doors and windows, then divides the remaining area by the coverage rate for the product you pick. Two coats are standard for a colour change, while a single coat often does for maintenance, and the result adjusts to the number of coats you choose. Enter the room dimensions, the number of doors and windows, the paint type and the coats, and you get total litres plus a suggested tin size. Most British paints come in 2.5 L and 5 L tins, and buying the next size up usually beats adding a small tin later.

Working it through by hand shows what the arithmetic is doing. A room of 4 m by 3 m with a 2.4 m ceiling has 33.6 m² of wall, from which you take off about 1.8 m² for each door and 1.5 m² for each window. That leaves roughly 30 m² of wall, and over two coats it becomes 60 m² of painted surface, which at 12 m² per litre comes to 5 litres. Add 10 to 15% on top for touch-ups, because matching a colour from a fresh tin months later rarely works.

Matt emulsion suits living rooms and bedrooms, since it hides imperfections and carries no sheen. Silk and satin belong in kitchens, bathrooms and high-traffic rooms where a washable surface with a slight sheen earns its keep. Eggshell is the usual British choice for woodwork, doors and skirting. Traditional gloss still gives the hardest-wearing trim, though it yellows over time, so water-based satinwood or eggshell is now generally preferred. Specialist products cover the rest: anti-mould for bathrooms, masonry paint for exteriors, with Dulux Weathershield carrying a 15-year guarantee, and heat-resistant paint for radiators.

Tin prices vary more than most people expect. Wickes and B&Q own brands sit at £15 to £25 for 2.5 L, Dulux at £25 to £40, and premium ranges such as Farrow & Ball at £67 to £86, with Little Greene at £55 to £75. Trade tins are the quiet bargain, since Dulux Trade Vinyl Matt costs around £35 for 5 L and one 5 L tin goes much further than two 2.5 L retail tins. Cheap paint often needs three coats where premium paint covers in one or two, so the cheaper tin is not always the cheaper wall. Test pots at £5 to £8 are worth buying before you commit to £200 or more of paint for a room.

Preparation is half the job. Wash the walls with sugar soap to lift grease and dust, fill cracks with Polyfilla or skim-coat larger damaged areas, then sand smooth with P120 grit followed by P180 to finish. New plaster needs a mist coat, meaning paint diluted 50/50 with water, to seal the porous surface before the full coats go on. Mask edges with a low-tack tape such as FrogTape, put down drop sheets, and work with a 9-inch microfibre roller for the walls, a 2-inch brush for cutting in and a paint kettle for dipping.

Materials alone for a DIY interior repaint of a three-bed semi come to roughly £200 to £400, covering 8 to 12 litres of premium paint plus brushes, rollers, masking tape and drop sheets. A decorator charges £150 to £300 a day and a full interior on that size of house takes 5 to 7 days, so £750 to £2,100 in labour. An exterior masonry repaint on a typical British home runs £1,500 to £3,500. Check whether a quote includes preparation, and allow drying times of 4 to 6 hours between emulsion coats and 16 hours or more for oil-based paint.