Paint and Tile Quantity Calculator

Paint needed by number of coats

Running out of material halfway through a job costs both time and money, and with tiles it can cost more than that — buy more later and the shade rarely matches the batch you started with. This calculator works from the real room dimensions, deducts doors and windows, and adds the waste allowance that tradespeople actually use.

Calculating paint. Start with the wall area: the room’s perimeter multiplied by the wall height. The perimeter is twice the width plus the length. Then subtract the doors and windows, since those are not being painted.

A standard door is about 1.7 square metres and a typical sliding window about 2 square metres each, so a bedroom with one door and two windows deducts roughly 5 to 6 square metres.

Multiply the remaining area by the number of coats and divide by the coverage per litre, which is printed on the tin. Ordinary acrylic emulsion covers 8 to 12 square metres per litre per coat.

Why two coats. A single coat usually leaves an uneven finish with the old colour showing through, particularly going from a dark colour to a light one. Two coats is the general standard. One can be enough when painting over a very similar existing colour on a wall in good condition.

Paint tin sizes in Thailand. Thai paint retail uses US gallons: one gallon is 3.785 litres, and one bucket is five gallons, about 18.9 litres. The bucket is what tradespeople buy, because it works out cheaper per litre.

Calculating tiles. Divide the floor area by the area of one tile. A 60 by 60 cm tile is 0.36 square metres, so a 20 square metre room needs about 56 tiles exactly.

Why allow for waste. Tiles at the edges have to be cut, and the offcuts are rarely usable. Add breakage in transit and during laying, and 10% is the standard allowance. Rooms with pillars, alcoves, or tiles laid diagonally need closer to 15%.

More importantly, buy spares to keep. Tile batches differ slightly in shade, so when one cracks in a few years’ time, a replacement bought then will usually be visibly different. Spares from the original batch are the only reliable fix.