How Much Paint Do I Need? A Room-by-Room Guide
You're mid-roll on the second wall and the tray's nearly dry. The tin is empty. The shop is 20 minutes away. Sound familiar? Here's how to get the quantity right before you start.
By Krishna Chaitanya, Software Engineer
You're mid-roll on the second wall and the tray's nearly dry. The tin is empty. The shop is 20 minutes away. Your partner is holding a wet brush and looking at you. The first wall is done, the second wall is half done, and now there's a very visible join forming at the point where you stopped. You drive to the shop, buy another tin, open it, and realise the new batch is a slightly different shade from the first.
That whole scenario is avoidable. Getting paint quantities right before you start isn't difficult -- it takes five minutes and a tape measure. The formula is straightforward and the maths is basic. The problem is that most people either guess, or they go by vague advice like "a litre per wall" that falls apart the moment you have high ceilings or an awkward room shape.
Here is how to do it properly.
The Real Problem With Getting Paint Quantity Wrong
Under-buying is the obvious failure: you run out mid-job, make an emergency trip, and risk a colour mismatch between batches. Paint manufacturers adjust their formulas periodically, and even the same product code can vary slightly between production runs. A join in an emulsion finish between two slightly different batches is often only visible once the paint dries, by which point it's too late.
Over-buying is less dramatic but still wasteful. A 5-litre tin of decent trade emulsion costs £25 to £40. Buy one tin too many on a three-room refresh and you've wasted £30 that will sit on a shelf in your garage for five years before you throw it away. Multiply that across a full house repaint and it adds up fast.
There's also the mid-project decision fatigue. Standing in the paint aisle trying to remember whether your ceilings are 2.4m or 2.6m, estimating wall counts in your head, second-guessing whether you measured the bay window correctly -- that is not where you want to be. Calculate it once, at home, before you go.
What You Need to Know: The Formula
The maths behind paint quantity is simple. Here it is, step by step.
Step 1: Calculate total wall area.
Wall area = (room length + room width) x 2 x ceiling height
This gives you the total surface area of the four walls. You're not calculating each wall individually -- that leads to errors when rooms aren't perfectly rectangular.
Step 2: Subtract doors and windows.
A standard UK door opening is roughly 2 m². A standard window is roughly 1.5 m². Subtract one figure for each door and window in the room. You don't paint those areas, so there's no point buying paint to cover them.
Step 3: Divide by coverage rate.
Every tin of paint states its coverage rate on the label -- typically 10 to 12 m² per litre for standard emulsions on smooth, previously painted walls. Use 10 m² per litre if your walls are textured, bare plaster, or if you're painting a light colour over a dark one. Use 12 m² per litre for smooth, sealed surfaces.
Divide your adjusted wall area by the coverage rate to get litres needed per coat.
Step 4: Multiply by number of coats.
For most interior walls, two coats is standard. Multiply your per-coat figure by two.
The result is the total litres of wall paint required for the room. In my experience, two coats is almost always necessary -- one coat almost never gives you the depth of colour the tin shows on the swatch, even with good quality paint.
Use the Paint Calculator to skip the manual maths and get an instant result for any room.
How the Paint Calculator Solves This
Rather than working through the formula by hand, the Paint Calculator does it for you in a few seconds. You enter:
- Room dimensions (length, width, ceiling height)
- Number of doors and windows
- Number of coats
- Your paint's coverage rate (or leave it at the default)
The calculator outputs the litres required and suggests the tin sizes to buy -- for example, whether it's better to buy one 5-litre tin or two 2.5-litre tins for a given quantity.
It also handles rooms that aren't simple rectangles, and lets you calculate for walls only, ceiling only, or both. If you're doing a full room including ceiling, that's a separate calculation -- ceiling area is simply length x width -- and the tool covers that too.
The whole thing takes under a minute, saves you a trip back to the shop, and is free to use.
Worked Example: A Standard Double Bedroom
Let's run through the formula for a typical UK double bedroom.
Room dimensions: 4m long x 3.5m wide x 2.4m ceiling height
Openings: 2 doors, 1 window
Step 1: Total wall area
(4 + 3.5) x 2 x 2.4 = 7.5 x 2 x 2.4 = 36 m²
Step 2: Subtract openings
2 doors x 2 m² = 4 m² 1 window x 1.5 m² = 1.5 m² Total to subtract: 5.5 m²
Adjusted wall area: 36 - 5.5 = 30.5 m²
Step 3: Divide by coverage rate
Using 12 m² per litre (smooth, previously painted walls): 30.5 / 12 = 2.54 litres per coat
Step 4: Multiply by two coats
2.54 x 2 = 5.08 litres total
So for this bedroom, you need just over 5 litres of wall paint. The practical answer is one 5-litre tin plus one 1-litre tester pot for top-ups, or a single 5-litre tin if you're prepared to be careful.
Note that if the walls had a textured finish, the coverage rate would drop to 10 m² per litre, and the total would rise to around 6.1 litres -- enough to push you into buying a second 5-litre tin. That distinction alone can change how you shop.
What to Do With the Result
Once you have your litre figure, round up to the nearest tin size rather than buying the exact amount. Paint is sold in standard sizes: 1 litre, 2.5 litres, 5 litres, and sometimes 10 litres for trade quantities. Buying 5.08 litres means buying a 5-litre tin plus something else. In that case, a 1-litre tin of the same paint is usually the sensible top-up.
Keep any leftover paint. Seal the tin properly -- use a rubber mallet to close the lid fully, store it upright in a cool, dry place -- and it will last two to three years for touch-ups. This matters most if your paint is a custom mix. Touch-up paint from a new batch, even the same code, may not match the original finish once it dries.
If you're painting a feature wall in a bold or dark colour, I think it's worth buying a small extra amount regardless of the calculation. Feature walls attract attention, and patchy coverage shows more on saturated colours than on neutral tones.
For rooms with bare plaster -- new builds, recently plastered walls, or heavily patched areas -- apply a mist coat (heavily diluted emulsion, roughly 10:1 paint to water) before your first proper coat. Calculate the primer or mist coat separately using the same formula.
Common Mistakes to Avoid
Forgetting to account for two coats is the most common error. The coverage rate on the tin refers to one coat. If you calculate for one coat and buy accordingly, you will run out before you finish. Always multiply by two for standard interior work.
Some people measure the room perimeter instead of using the formula correctly. The formula accounts for this -- (length + width) x 2 -- but people sometimes measure all four walls and add them up incorrectly, especially in rooms where two walls are longer than the others. A 4m x 3.5m room has a perimeter of 15m, which at 2.4m height gives 36 m². That's correct. The formula is safer than trying to add wall by wall in your head.
Ignoring surface texture and condition is the third one. A smooth, sealed wall and a rough, textured wall consume very different amounts of paint for the same area. The coverage rate on the tin assumes best-case conditions. If your walls are anything other than smooth and previously sealed, use the lower end of the range (10 m² per litre) in your calculation.
Ready to Calculate?
Take five minutes before you go to the shop. Measure the room, note the doors and windows, and use the Paint Calculator to get an exact figure. It costs nothing, takes less time than queuing at the paint counter, and means you won't be back in two hours looking for a tin that matches.