Paint Calculator

Calculate exactly how much paint you need

Buying too little paint means a frustrating mid-project trip to the hardware store, and there is no guarantee the new batch will match perfectly in colour. Buying too much wastes money and creates disposal problems. This paint calculator gives you the right amount to buy for any painting project by taking into account wall size, number of coats, and your paint's coverage rate.

How the calculation works

The formula is straightforward: total area = width × height × number of walls. Total paint needed = total area × number of coats ÷ coverage rate per litre. A standard interior paint covers about 10–12 m² per litre per coat. Most rooms need two coats for a professional-looking finish.

Example: A room with 4 walls, each 4 metres wide and 2.5 metres high, needing 2 coats of paint covering 10 m² per litre:

• Total area = 4 × (4 × 2.5) = 40 m²
• Paint needed = 40 × 2 ÷ 10 = 8 litres

A worked example, extended

Take the same room from the earlier example — four walls, each 4 metres wide and 2.5 metres high, needing two coats — but now switch from a standard emulsion covering 10 square metres per litre to a premium one covering 14 square metres per litre. The total wall area stays at 40 square metres either way, but the paint needed drops from 40 × 2 ÷ 10 = 8 litres to 40 × 2 ÷ 14 ≈ 5.7 litres, a saving of more than two litres purely from choosing a better-covering product. Whether that saving justifies the higher price per litre of the premium paint is a calculation this tool makes easy to check directly, rather than guessing.

Coverage rates by paint type

Coverage varies significantly by paint type and quality:

• Budget interior emulsion: 8–10 m² per litre
• Standard interior emulsion: 10–12 m² per litre
• Premium interior emulsion: 12–14 m² per litre
• Exterior paint: 8–12 m² per litre (lower due to surfaces being less smooth)
• Undercoat/primer: 10–12 m² per litre
• Gloss/satinwood (woodwork): 16–20 m² per litre (smooth surface absorbs less)

Always check the manufacturer's stated coverage on the paint tin, as this is the most reliable figure for that specific product.

Number of coats

For most wall painting projects, two coats give the best result. One coat may not provide full, even coverage — you may see the original colour or primer through thin spots. Three coats are sometimes needed when going from a very dark colour to a very light one, or when applying a red or yellow (colours with notoriously low opacity).

Ceilings typically need one coat with ceiling paint (which is formulated for high coverage and drip resistance) or two coats for a very fresh, bright appearance.

Accounting for doors, windows, and waste

The calculator computes the gross wall area without subtracting openings. For planning purposes, it is usually acceptable to use the gross area and treat the extra paint as buffer for:

• Openings (doors and windows) that will not be painted with wall paint
• Waste from loading the roller or brush
• Spills and drips
• Touch-up work

If you want a more precise calculation, measure your door and window areas and subtract them before entering the wall dimensions.

Choosing paint quantities

Paint is typically sold in 1-litre, 2.5-litre, 5-litre, and 10-litre tins. Always round up to the next available tin size. For example, if the calculation shows 8.2 litres, buy two 5-litre tins (10 litres total) rather than a 5-litre tin and four 1-litre tins, which is more expensive per litre. Leftover paint can be stored for touch-ups if sealed properly.

How to use the calculator

Enter the width and height of each wall, or an average wall size and the number of walls, along with how many coats you plan to apply and your paint's stated coverage rate. The total litres needed appears immediately, and adjusting any value recalculates it instantly, which makes it easy to compare how switching from a budget paint to a premium one with better coverage would change how much you need to buy.

Why rounding up to the next tin size matters

Because paint is sold in fixed tin sizes rather than by the exact litre, the practical answer to "how much do I need" is always the next tin size up from the calculated figure, not the figure itself. Buying a 5-litre tin and a 1-litre tin to cover an 8.2-litre requirement, for instance, is usually both more expensive per litre and more wasteful than simply buying two 5-litre tins, since larger tins almost always offer a better price per litre and leftover paint is genuinely useful for future touch-ups if stored properly. Treating the calculator's output as a minimum rather than an exact target, and rounding generously, avoids the single most common paint-buying mistake: running out with one wall left to go and being unable to match the exact tint of a already-opened, partially-used batch.

Private and instant

All calculations run entirely in your browser, so the result updates instantly as you adjust any input and no measurements you enter are ever sent to a server, logged or shared, and it works offline once the page has loaded, ready whenever you are standing in the paint aisle trying to decide how many tins to grab.